Board of Intermediate and Secondary Education Rawalpindi Grade 9 Physics notes chapter 3 short long questions, Mcqs, Problems, and Quiz Pdf Download.
Dynamics 9th Class all Notes Pdf download
Table of Contents
Q.2) Define the following terms:
i) Inertia
ii) Momentum
iii) Force
iv) Force of friction
v) Centripetal force
Answer:
i) INERTIA
The inertia of a body is its property due to which it resists any change in its state of rest or motion.
The inertia of a body depends on mass, the greater the mass of the body greater the inertia which means more ability to resist its state of rest or motion.
ii) MOMENTUM
The momentum of a body is the quantity of motion possessed due to its mass and velocity.
Mathematically it is defined as the product of mass and velocity.
Momentum= mass x velocity
p = mv
Momentum is a vector quantity. Its S.I unit is kg ms-1.
iii)FORCE
A force moves or tends to move, stop, or tends to stop the motion of a body.
It can change the direction of motion of a body and also can change the shape and size of the body on which it acts.
Force = mass x acceleration
F = m a
iv)FORCE OF FRICTION
Friction is a force that is created whenever two surfaces move or try to move across each other.
Friction depends on the texture of both surfaces.
v) CENTRIPETAL FORCE
A centripetal force is a force that keeps a body moving in a circle. Centripetal force is a center-seeking force which means that the force is always directed toward the center of the circle and is always perpendicular to the motion of the body.
Mathematically;
Fc = mv2/r
Q.3) What is the difference between
i) Mass and weight
ii) Action and reaction
iii) Sliding friction and rolling friction
Answer:
i) MASS AND WEIGHT
| Mass | Weight |
| 1) The quantity of matter in a body is called it’s mass. | 1) Weight is the force by which the earth attracts a body towards its center. |
| 2) Mass is a scalar quantity. | 2) Weight is a vector quantity and is always directed towards the center of the earth. |
| 3) Mass of a body is always constant everywhere in the universe. | 3) Weight of a body varies from place to place and becomes zero on the center of the earth and far away from the surface of the earth. |
| 4) Mass can be determined by an ordinary balance. | 4) Weight of a body is measured by spring balance. |
| 5) Unit of mass in the S.I system is a kilogram (kg). | 5) Unit of weight in the S.I system is Newton (N). |
ii) ACTION AND REACTION
Forces always act in pairs and always act in opposite directions. When you push an object, the object pushes back with an equal force. Think of a book on a table. The weight of the book exerts a downward force on the table. This is the action force. The table exerts an equal upward force on the book. This is the reaction force. Note that the two forces act on different objects. The action force acts on the table, and the reaction force acts on the books.
iii) SLIDING AND ROLLING FRICTION
When a body slides (moves with all its surface area in contact) over another body, the resistance faced is due to sliding friction. On the other hand, if a body rolls (moves with just a single point of its surface in contact) over another body, the resistance, in this case, is rolling friction. The greater the surface in contact greater will be the friction, hence sliding friction is always greater than rolling friction.
Q.4) What is the law of Inertia?
Answer:
LAW OF INERTIA
Inertia is the property of a body that keeps it in a state of rest or uniform motion until acted upon by an unbalanced force. The inertia of a body depends on its mass. Greater the mass of a body is the inertia or its tendency to resist change in its state of rest or motion.
Experiment:
When the cardboard is jerked quickly, the coin will fall into the glass. The inertia of the coin resists the change of its initial state, which is stationary. As a result, the coin does not move with the cardboard and falls into the glass because of gravity.
Q.5) Why is it dangerous to travel on the roof of a bus?
Answer:
If a person is traveling on the roof of a bus and suddenly brakes are applied, the moving bus quickly starts slowing down. But the inertia of the body of the person on the roof tries to maintain the momentum. As a result, the person would be thrown forward with tremendous force and may fall in front of the bus and get run over. Even if the person does not fall off the bus, he would hit the roof with enough force to injure him fatally. Similarly, if the bus takes a turn, the person would fly off the roof due to centrifugal force.
Q.6) Why does a passenger move outward when a bus takes a turn?
Answer:
When the bus takes a turn, the passengers are in the motion of a straight line and due to inertia the passengers want to keep their motion that’s why they fall in the opposite direction. Also, if the bus is taking a sharp turn, the centrifugal force acting on the passengers would push them outward, away from the center of the circular path of the bus.
Q.7) How can you relate a force with the change of momentum of a body?
Answer:
Consider a body of mass m, initially moving with a velocity of magnitude vi. A force F acts on the body and causes it to accelerate to a final velocity of magnitude vf.
So the initial and final momentum of the body;
The rate of change in momentum is given by;
So from the above equation, we can define force and states Newton’s second law of motions as;
“When a force acts on a body, it produces acceleration in the body and will be equal to the rate of change of momentum of the body.”
Q.8) What will be the tension in a rope that is pulled from its ends by two opposite forces 100 N each ?
Answer:
If the system is in equilibrium, the tension in the rope would be 100 N.
Q.9) Action and reaction are always equal and opposite. Then how does a body move?
Answer:
Action and reaction forces act on different bodies and not on the same bodies. Therefore, they cannot cancel each other out.
For example, the action force exerted on the board by the diver causes a reaction force by the board on the diver. The force of the diver on the board is equal and opposite to the force exerted by the diving board. The greater the force exerted upon the diving board, the higher the dive will be.
Q.10) A horse pulls the cart. If the action and reaction are equal and opposite then how does the cart move?
Answer:
When the horse pushed the ground back as an action, the ground pushed him in the forward direction as a reaction. The other force on the horse is the pull from the cart in the backward direction. If the force because of the ground is more than the backward force because of the cart, then the cart will move.
Q.11) What is the law of conservation of momentum?
Answer:
LAW OF CONSERVATION OF MOMENTUM:
“The total momentum of an isolated system of interacting bodies remains constant.”
OR
“Total momentum of an isolated system before the collision is always equal to the total momentum after the collision.”
Consider two bodies of mass m1 and m2 moving initially with velocities u1 and u2.
Total momentum before the collision = m1u1 + m2u2
Let after collision their velocities become v1 and v2.
Total momentum after collision = m1v1 + m2v2
According to the law of conservation of momentum
Momentum before collision = Momentum after collision
m1u1 + m2u2 = m1v1 + m2v2
Q.12) Why is the law of conservation of momentum important ?
Answer:
Law of conservation of momentum is very important in daily life. For example, it enables machines such as rockets and jet engines to work. In these machines, hot gasses produced by the burning of fuel rush out with large momentum. In order to conserve momentum, the machines gain an equal and opposite momentum. This enables them to move with a very high velocity.
Q.13) When a gun is fired, it recoils. Why ?
Answer:
When a gun is fired, it recoils in order to conserve momentum. Before firing, the gun and the bullet are at rest so the total momentum before firing is zero. When a gun is fired the bullet gains momentum and moves in the forward direction. In order to conserve momentum the gun recoils.
If the bullet has mass mb and velocity after the shot is vb out of the gun, it has momentum pb given simply by
pb = mbvb,
and mass of the gun mg and velocity vg then it has momentum pg.
pg = mgvg
According to the law of conservation of momentum;
Momentum before collision = Momentum after collision
As the momentum before the collision is zero because the gun and the bullet are at rest before shot, so
0= pb+ pg
pg = -pb,
or
mgvg = -mbvb
“Minus” sign shows that both have opposite directions of motion. Although the bullet’s mass is small, its speed is quite large, so it is released with large momentum. The gun has a much larger mass, so the recoil speed is much smaller, but still large enough to give a serious kick against the shooter’s shoulder.
Q.14) Describe two situations in which force of friction is needed.
Answer:
Friction plays a vital role in our daily life because
1) It becomes difficult to walk on a slippery road due to low friction. When we move on the ice, it becomes difficult to walk due to the low friction of ice.
2) We cannot fix nails in the wood or wall if there is no friction. It is friction that holds the nail.
3) A horse can not pull a cart unless friction furnishes him a secure foothold.
Q.15) How does oiling the moving parts of a machine lowers friction?
Answer:
If two objects cannot be pressed together, surface to surface, so as to rub together, then no friction is created. Oil gives a layer of protection between touching parts of machines to reduce friction, heat, and damage. Though oil only reduces friction and does not eliminate it entirely.
Q.16) Describe ways to reduce friction.
Answer:
Methods of reducing friction are:
Oil is used as a lubricant and provides a thin layer of liquid that separates the moving surfaces.
Ball bearings reduce frictions by making the moving surface roll rather than slide.
By designing the object to allow the easy flow of fluid around it (streamlining) as is done for rockets and racing cars the drag can be reduced considerably.
By polishing the contact surfaces smooth to eliminate crests and troughs which cause friction.
Magnetic ball bearings totally eliminate friction in sensitive instruments by eliminating physical contact.
Maglev trains totally avoid physical contact with track and completely reduce surface to surface friction.
Q.17) Why is rolling friction less than sliding friction?
Answer:
When an object slides across the ground, it has much more surface area in direct contact with the ground, which means that the amount of friction is significantly higher because the amount of friction depends on the area of contact. When an object rolls along the ground, only at some point on the object contacts the ground at any point in time, making the stopping force (friction) much weaker.
Q.18) What do you know about the following:
i) Tension in a string
ii) Limiting force of friction
iii) Braking force
iv) Skidding of vehicles
v) Seatbelts
vi) Banking of roads
vii) Cream separator
Answer:
i) Tension in the string:
The force exerted by a string when it is subjected to pull is called tension in the string.
If a person is holding a block of weight W attached to the end of a string, a force is experienced by him. This force is known as Tension. When the body is at rest, the magnitude of tension is equal to the weight of the body suspended by the string. Tension and the weight act in the opposite direction. Tension is a vector quantity, which has both magnitude and direction. Its magnitude remains constant at all points of the string.
The S.I unit of the tension is Newton (N).
ii) Limiting force of friction:
The maximum value of friction is known as the force of limiting friction (Fs). It depends on the normal reaction between the two surfaces in contact.
iii) Braking force:
Friction between a rotating component (e.g drum and disc) and stationary component (e.g the brake shoe or pad) causes the drum or disc to slow down such a force is known as braking force.
Braking force depends on the following factors:
The diameter of the disc
The frictional material.
The size of the pad, friction face.
The force is used to clamp the pads onto the disc.
iv) Skidding of vehicles:
When brakes apply too strongly for a fast-moving car, the wheels of the car will lock up and the car will skid due to its large momentum. it will lose its directional control which may result in an accident. In order to reduce the chance of skidding, it is advisable not to apply the brakes too hard that lock up the rolling motion, especially at high speed.
v) Seat belts:
In case of an accident, a person not wearing a seat belt will continue moving until stopped suddenly by something before him. This something may be a windscreen, another passenger, or the back of the seat in front of them. Seat belts are useful in two ways,
They provide an external force to a person wearing a seatbelt.
The additional time is required for stretching seat belts. This increases the stopping time for momentum to change and reduces the effect of the collision.
vi) Banking of roads:
If a car is on a level (unbanked) surface, the forces acting on the car are its weight W, pulling the car down and the normal force N due to the road, which pushes the car upward. Both of these forces act in the vertical direction and have no horizontal component. If there is no friction, there is no force that can supply the centripetal force required to make the car move in a circular path; there is no way that the car can turn.
On the other hand, if the car is on a banked turn, the normal force (which is always perpendicular to the road’s surface) is no longer vertical. The normal force now has a horizontal component, and this component can act as the centripetal force on the car.
vii) Cream separator:
A separator is a high-speed spinner. It acts on the same principle as in a centrifuge machine. The bowl spins at a very high speed causing the heavier contents of milk to move outward in the bowl, pushing the lighter contents inward towards the spinning axis. The cream and butter are lighter than other components in milk. Therefore, skim milk, which is denser than cream, is collected in the outer wall of the bowl. The lighter part (cream) is pushed towards the center from where it is collected through a pipe.
Q.19) What would happen if all friction suddenly disappears?
Answer:
If there is no friction then we could not walk, we would keep slipping. Nothing would be steady on the ground, many things would just slide. Most of the conventional motion would stop and jet propulsion would be the dominant mode of starting and stopping motion.
Q.20) Why is the spinner of a washing machine made to spin at a very high speed?
Answer:
The dryer of a washing machine is a basket spinner. They have a perforated wall having large numbers of fine holes in the cylindrical rotor. The lid of the cylindrical container is closed after putting wet clothes in it. When it spins at high speed, the water from wet clothes is forced out through these holes due to centrifugal force.
Q1. Define dynamics?
Ans: Dynamics:
The branch of mechanics that deals with the study of the motion of an object and the cause of its motion is called dynamics.
Q2. Define Force.
Ans: Force:
A force moves or tends to move stops or tends to stop the motion of a body. The force can also change the direction of motion of a body
F = ma
SI unit of force is newton
Note: A force can also change the shape or size of a body in which it acts.
Q3. Define inertia. Experiment to understand inertia?
OR
What is the law of inertia?
Ans: See Q # 3.4 from Exercise.
Q4. Put a one-rupee coin over a piece of card paper placed on an empty glass. Push the card with a sudden stroke of a finger. The card will move ahead while the coin falls into the glass. Why does it do that?
Ans: Due to inertia, the card will continue its motion in the forward direction while the coin will remain at rest and will fall into the glass.
Q5. Define Momentum.
Ans: Momentum:
The momentum of a body is the quantity of motion it possesses due to its mass and velocity.
The momentum P of a body is given by the product of its mass m and velocity v. Thus
P = mv
Momentum is a vector quantity.
The momentum of a system depends on its mass and velocity.
Q6. A bullet has very small inertia due to its small mass. But why is its impact so strong when it is fired from the gun?
Ans: According to the law of conservation of momentum, the mass of the bullet is much smaller than the gun therefore the recoil is much greater than the velocity of the gun. Therefore, the impact of the bullet is very strong.
Q7. The impact of the loaded truck on a body coming it’s way is very large even if the truck is moving slowly.
Ans: Greater is the mass of the truck, and the greater will be its momentum. Therefore, the loaded truck has a large impact.
Q8. Explain Newton’s first law of motion by a practical example of daily life?
Ans: Newton’s laws of motion:
A body continues its state of rest or uniform motion in a straight line provided no net force acts on it.
Examples:
Case I (Part I):
According to Newton’s first law of motion, a body at rest remains at rest provided no net force acts on it. This part of the law is true as we observe that objects do not move by themselves unless someone moves them. For example, a book lying on a table remains at rest as long as no net force acts on it.
Case II (Part II):
Similarly, a moving object does not stop moving by itself. A ball rolled on the rough ground stops earlier than that rolled on smooth ground. It is because rough surfaces offer greater friction. If there would be no force to oppose the motion of a body then the moving body would never stop.
Q9. Why is Newton’s first law of motion also known as the law of inertia?
Ans: Since Newton’s first law of motion deals with the inertial property of matter, therefore, Newton’s first law of motion is also known as the law of inertia.
Q10. Why do the passengers standing in a bus fall forward when the driver applies brakes suddenly?
Ans: The passengers standing in a bus fall forward when its driver applies brakes suddenly. It is because the upper parts of their bodies tend to continue their motion, while lower parts of their bodies in contact with the bus stop with it. Hence, they fall forward.
Q11. When a bus takes a sharp turn, passengers fall in an outward direction. Why?
Ans: When a bus takes a sharp turn, passengers fall in the outward direction. It is due to inertia that they want to continue their motion in a straight line and thus fall outwards.
Q12. Define net force?
Ans: Net force:
The net force is the resultant of all the forces acting on a body.
Q13. State and prove Newton’s second law of motion.
Or
Show that F = ma.
Ans: Newton’s second law of motion:
When a net force acts on a body, it produces acceleration in the body in the direction of the net force. The magnitude of this acceleration is directly proportional to the net force acting on the body and inversely proportional to its mass.
Derivation of formula F = ma:
If a force produces an acceleration ‘a’ in a body of mass ‘m’, then we can state mathematically that
a ∝ F………..(i)
and a ∝ ……….(ii)
By combining (i) and (ii)
Or a ∝
Or F ∝ ma
Putting k as proportionality constant, we get
F = kma ………(iii)
In SI units, the value of k comes out to be 1. Thus Eq. (iii)
F = ma
Q14. Define SI units of force?
Ans: SI units of force:
SI unit of force is newton (N).
Newton (1 N):
One newton (1 N) is the force that produces an acceleration of 1 ms – 2 in a body of mass of 1 kg.
Thus, a force of one newton can be expressed as
1 N = 1 kg x 1 ms – 2
Or 1 N = 1 kg ms – 2
Q15. Differentiate between mass and weight.
Ans: See Q # 3.3(i) from Exercise
DO YOU KNOW?
When a bus takes a sharp turn, passengers fall in the outward direction. It is due to inertia that they want to continue their motion in a straight line and thus fall outwards.
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Q16.Explain Newton’s third law of motion by practical examples of daily life?
Ans: Newton’s third law of motion:
To every action, there is always an equal but opposite reaction.
Explanation:
Newton’s third law of motion deals with the reaction of a body when a force acts on it. Let body A exerts a force on another body B, body B reacts against this force and exerts a force on body A. The force exerted by body A on B is the action force whereas the force exerted by body B on A is called the reaction force.
Note that action and reaction forces act on different bodies.
Examples:
A book lying on a table:
Consider a book lying on a table. The weight of the book is acting on the table in a downward direction. This is the action. The reaction of the table acts on the book in the upward direction.
An air-filled balloon:
Take an air-filled balloon. When the balloon is set free, the air inside it rushes out and the balloon moves forward. In this example, the action is by the balloon that pushes the air out of it when set free. The reaction of the air which escapes out from the balloon acts on the balloon. It is due to this reaction of the escaping air that moves the balloon forward.
Taking off a rocket:
A rocket moves on the same principle. When its fuel burns, hot gasses escape out from its tail at a very high speed. The reaction of these gasses on the rocket causes it to move opposite to the gasses rushing out of its tail.
- How much force do you need to prevent the book from falling?
Ans: The force which is needed to prevent the book from falling is equal to the weight of the book, i.e. R = W
- Which is the action?
Ans: The weight of the book acting in a downward direction is called action.
- Is there any reaction? If yes, then what is its direction?
Ans: Yes, the force applied to prevent the book from falling is called the reaction force. The reactional force is acting in the upward direction. Opposite to weight of the book i.e. R = W
Q17. What do you know about tension and acceleration in a string?
Ans: Tension and acceleration in a string:
Consider a block supported by a string. The upper end of the string is fixed on a stand. Let w be the weight of the block. The block pulls the string downwards by its weight. This causes a tension T in the string. The tension T in the string is acting upwards at the block. As the block is at rest, therefore, the weight of the block acting downwards must be balanced by the upwards tension T in the string. Thus, the tension T in the string must be equal and opposite to the weight w of the block.
Q18. Calculate the tension and acceleration in the string during the motion of bodies connected by the string and passing over friction pulleys using the second law of motion?
Ans: Vertical motion of two bodies attached to the ends of a string that passes over the frictionless pulley:
Suppose two bodies A and B having masses m1 and m2 respectively are connected to two ends of an inextensible string that passes over a frictionless pulley. If m1 is greater than m2, then body A will move downward and body B will move upward.
Two forces are acting on the body A:
- Its weight w1 = m1g
- Tension of string T1 acting upward.
Since body A will move downward, hence its weight m1g is greater than the tension T in the string.
The net force acting on body A = m1g – T
According to Newton’s second law of motion:
m1g – T = m1a ……….. (i)
Two forces are acting on the body B:
- Its weight w2 = m2g, acting downward.
- The tension of string T acts upward.
As the body moves upward, hence its weight m2g is less than the tension T in the
string.
Net force acting on body B = T – m2g
According to Newton’s second law of motion:
T – m2g = m2a ……….. (ii)
Adding Eq. (i) and Eq. (ii), we get acceleration a.
m1g – m2g = m1a + m2a
(m1 – m2)g = (m1 + m2)a
Magnitude of acceleration = a = g…………(iii)
Putting the value of a in Eq. (ii), we get
T – m2g = m2( g)
T = m2g + m2( g)
T = m2g
T = m2g
Magnitude of tension = T = g
Note:
Atwood machine:
The above arrangement is also known as the Atwood machine. It can be used to find the acceleration g due to gravity using Eq. (iii)
g = a
DO YOU KNOW
An Atwood machine is an arrangement of two objects of unequal masses. Both the objects are attached to the ends of a string. The string passes over a frictionless pulley. This arrangement is sometimes used to find the acceleration due to gravity.
Q19. Calculate the tension and acceleration in a string during the motion of two bodies attached to the ends of a string that passes over a frictionless pulley such that one body moves vertically and the other moves on a smooth horizontal surface:
Ans: Motion of two bodies attached to the ends of a string that passes over a frictionless pulley such that one body moves vertically and the other moves on a smooth horizontal surface:
Consider two bodies A and B of masses m1 and m2 respectively attached to the ends of an inextensible string
.
Let body A move downwards with an acceleration a. Since the string is inextensible, therefore, body B also moves over the horizontal surface with the same acceleration a. As the pulley is frictionless, hence tension T will be the same throughout the string. Since body A moves downwards, therefore, its weight m1g is greater than the tension T in the string.
\ Net force acting on body A = m1g – T
According to Newton’s second law of motion:
m1g – T = m1a ……….. (i)
The forces acting on body B are
- Weight m2g of the body B acting downward.
- Reaction R of the horizontal surface acting on body B in the upwards direction.
- Tension T in the string pulls body B horizontally over the smooth surface.
As body B has no vertical motion, the resultant vertical forces (m2g and R) must be zero. Thus, the net force acting on body B is T.
According to Newton’s second law of motion;
T = m2a …………..(ii)
Adding Eq. (i) and (ii), we get acceleration a as
m1g – m2a = m1a
m1g = m1a + m2a
m1g = a (m1 + m2)
a = g……….(iii)
Putting the value of a in equation (ii) to get tension T as
T = g
Q20. Show the relationship between momentum and force OR Derive Newton’s Second Law of motion with the help of momentum.
OR
Prove that F = ΔP/t.
OR
How can you relate a force with the change of momentum of a body?
Ans. See Q # 3. 7 from Exercise
Q21. Show that Ns = Kg m/s OR Ns = Kg ms – 1
Ans: L.H.S = N s = (1kg x 1 ms – 2 ) x s = Kg x ms – 2 + 1 = Kg x ms – 1 = Kg ms – 1
USEFUL INFORMATION
Fragile objects such as glasswares etc. are packed with suitable materials such as Styrofoam rings, balls, polythene sheets with airbags etc.
The air enclosed in the cavities of these materials makes them flexible and soft. During any mishap, they increase the impact time on fragile objects. An increase in impact time lowers the rate of change of momentum and hence lessens the impact of force. This lowers the possible damage due to an accident.
USEFUL INFORMATION
In an accident at high speed, the impact force is very large due to the extremely short stopping time. For safety purposes, vehicles have rigid cages for passengers with crumple zones at their front and rear ends.
During an accident, crumple zones collapse. This increases the impact time by providing extra time for crumpling. The impact of force is highly reduced and saves passengers from severe injuries.
USEFUL INFORMATION
In case of an accident, a person not wearing a seatbelt will continue moving until stopped suddenly by something before him. This something may be a windscreen, another passenger, or the back of the seat in front of him/her. Seat Belts are useful in two ways:
- They provide an external force to a person wearing a seatbelt.
- The additional time is required for stretching seat belts. This prolongs the stopping time for momentum to change and reduces the effect of a collision.
Q22. Define System and Isolated system?
Ans: System:
A system is a group of bodies within certain boundaries.
Isolated system:
An isolated system is a group of interacting bodies on which no external force is acting. The momentum of an isolated system is always conserved. This is the Law of Conservation of Momentum.
Q23: What is the law of conservation of momentum?
OR
State and explain the law of conservation of momentum?
Ans: See Q # 3.11 from Exercise.
Q24. When a gun is fired, it recoils. Why?
Ans: See Q # 3.13 from Exercise.
Q25. Under which principle do rockets and jet engines work?
Ans: Working of rockets and jet engines:
Rockets and jet engines also work on the same principle. In these machines, hot gasses produced by the burning fuel rush out with large momentum. The machines gain equal and opposite momentum. This enables them to move with very high velocities.
Q26. What do you know about friction?
Ans. Friction:
The force that opposes the motion of moving objects is called friction.
Friction is a force that comes into action as soon as a body is pushed or pulled over a surface.
Factors on which friction depends:
In the case of solids, the force of friction between two bodies depends upon many factors such as the nature of the two surfaces in contact and the pressing force between them.
Examples:
Rub your palm over different surfaces such as tables, carpets, polished marble surfaces, brick, etc. You will find the smoother the surface, the easier it is to move over the surface. Moreover, the harder you press your palm over the surface, the more difficult it would be to move.
Q27: Have you noticed why a moving ball stops?
Ans: Moving ball stops due to the force of friction.
Q28. Why does a bicycle stop when the cyclist stops pedaling?
Ans: Bicycle stops due to the force of friction.
Q29. Why does friction oppose motion?
OR
Give the microscopic concept of friction?
Ans: Microscopic concept of friction:
No surface is perfectly smooth. A surface that appears smooth has pits and bumps that can be seen under a microscope. The figure shows two wooden blocks with their polished surfaces in contact.
A magnified view of two smooth surfaces in contact shows the gaps and contacts between them. The contact points between the two surfaces form a sort of cold welds. These cold welds resist the surfaces from sliding over each other. Adding weight over the upper block increases the force pressing the surfaces together and hence, increases the resistance. Thus, the greater the pressing force greater will be the friction between the sliding surfaces.
Tidbits
Pushing the opposite walls by palms and feet increases friction. This enables the boy to move up the walls
Q30. What do you know about limiting the force of friction?
Ans: See Q # 3.18 (ii) from Exercises
Q31. What do you know about the coefficient of friction?
Ans: Coefficient of friction (µ):
The ratio between the force of limiting friction Fs and the normal reaction R is constant. This constant is called the coefficient of friction and is represented by µ
Thus µ = (i)
Or = µ R (ii)
If m is the mass of the block, then for a horizontal surface,
R = mg (iii)
Hence = µ mg (iv)
Q32. Describe the situations in which force of friction is needed.
Ans: See Q # 3.14 from Exercise.
Q33. Describe the coefficient of friction ( µ ) between some common materials?
Ans: Coefficient of friction ( µ )between some common materials:
| Materials | µs |
| Glass and Glass | 0.9 |
| Glass and Metal | 0.5 – 0.7 |
| Ice and Wood | 0.05 |
| Iron and Iron | 1.0 |
| Rubber and Concrete | 0.6 |
| Steel and Steel | 0.8 |
| Tyre and Road, dry | 1 |
| Tyre and Road, wet | 0.2 |
| Wood and Wood | 0.25 – 0.6 |
| Wood and Metal | 0.2 – 0.6 |
| Wood and Concrete | 0.62 |
QUICK QUIZ
- Which shoes offer less friction?
Ans: The smooth surface shoe offers less friction.
- Which shoe is better for walking on a dry track?
Ans: The smooth surface shoe is better for walking on a dry track.
- Which shoes are better for jogging?
Ans: The rough surface shoe is better for jogging.
- Which sole will wear out early?
Ans: The smooth surface sole will wear out early.
Q34. Why is rolling friction less than sliding friction?
OR
Demonstrate that rolling friction is less than sliding friction?
Ans: See Q # 3.17 from Exercise.
Q35. The first thing about a wheel is that it rolls as it moves rather than slides. This greatly reduces friction. Why?
Ans: When the axle of a wheel is pushed, the force of friction between the wheel and the ground at the point of contact provides the reaction force. The reaction force acts at the contact points of the wheel in a direction opposite to the applied force. The wheel rolls without rupturing the cold welds. That is why the rolling friction is extremely small than sliding friction.
Q36. Why are ball bearings or roller bearings used to reduce friction?
Ans: The fact that rolling friction is less than sliding friction is applied in ball bearings or roller bearings to reduce losses due to friction.
Q37.Friction is a ‘necessary evil’. Comment.
Ans: In certain cases, we need friction while in certain others we have to reduce friction. It means friction has some advantages as well as disadvantages. It is difficult to walk on ice because of less friction. A nail stays in the wood because of friction. Similarly, we can tie the knot because of this force. A horse will not be able to pull a wagon unless friction furnishes him a secure foot-hold.
Friction causes energy loss and reduces the efficiency of machines. Friction causes rapid wear and tear.
In short, we can say that friction is necessary for everyday activities. Thus, the statement that “friction is necessary evil” is correct.
The wheel would not recoil on pushing it if there would be no friction between the wheel and the ground. Thus, friction is desirable for wheels to roll over a surface.
Q38. Why is it dangerous to drive on a wet road?
Ans: It is dangerous to drive on a wet road because the friction between the road and the tires is very small. This increases the chance of slipping the tires on the road. The threading on tires is designed to increase friction. Thus, threading improves road grip and makes it safer to drive even on a wet road.
Q39. Why does a cyclist apply brakes to stop his/her bicycle?
Ans: A cyclist applies brakes to stop his/her bicycle. As soon as brakes are applied, the wheels stop rolling and begin to slide over the road. Since sliding friction is much greater than rolling friction. Therefore, the cycle stops very quickly.
QUICK QUIZ
- Why is it easy to roll a cylindrical eraser on a paper sheet than to slide it?
Ans: The fact that rolling friction is less than sliding friction. A cylindrical eraser is a rolling body and can easily roll on a paper sheet than slide it. Because rolling friction is the force of friction between a rolling body and a surface over which it rolls.
- Do we roll or slide the eraser to remove the pencil work from our notebook?
Ans: The fact that sliding friction is greater than rolling friction. We slide the eraser to remove the pencil work from our notebook.
Q40. Explain breaking force and skidding of vehicles?
Ans: See Q # 3.18 (iii & iv) from Exercise.
Mini Exercise
- In which case do you need a smaller force and why?
Ans: In the case of rolling friction we need a smaller force. Because the rolling friction is lesser than sliding friction.
- In which case it is easy for the tire to roll over?
- Rough ground
- Smooth ground
Ans: On the smooth ground, it is easy for the tire to roll over due to less friction.
Q41. Describe the situation in daily life in which friction is most desirable?
Ans: Sometimes friction is most desirable. We cannot write if there would be no friction between paper and pencil. Friction enables us to walk on the ground. We cannot run on slippery ground.
A slippery ground offers very little friction. Hence, anybody who tries to run on the slippery ground may meet with an accident. Similarly, it is dangerous to apply brakes with full force to stop a fast-moving vehicle on a slippery road.
Birds could not fly if there was no air resistance. The reaction of pushed air enables birds to fly. Thus, in many situations, we need friction while in other situations we need to reduce it as much as possible.
Q42. Write a dream during which you are driving a car and suddenly the friction disappears. What happened next…?
Ans: If suddenly the friction disappears then we cannot stop the car. Nothing would be steady on the ground, the car would be just sliding and sliding.
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Q43. Describe the advantages of friction?
Ans: Advantages of friction in daily life:
- It enables animals to walk or crawl without slipping.
- It stops cars, trains, bicycles, MRT trains, etc.
- It enables us to hold things firmly with our hands.
- It prevents objects from sliding down a slope.
- It allows nails to hold things.
Q44. What are the disadvantages of friction?
Ans: Problems caused by friction:
- It causes energy loss and reduces the efficiency of machines.
- It causes rapid wear and tear of the moving parts of machines.
Most of our useful energy is lost as heat and sound due to friction between various moving parts of machines. In machines, friction also causes wear and tear on their moving parts.
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Q45. What are the methods to reduce friction?
Ans: See Q # 3.16 from Exercise.
DO YOU KNOW
Friction is highly desirable when climbing up a hill.
Q46. Define uniform circular motion?
Ans: Uniform circular motion:
The motion of an object in a circular path is known as circular motion. The motion of the moon around the Earth is circular motion.
Q47. Define the centripetal force. Derive the relation of centripetal force on a body moving in a circle?
Ans: Centripetal Force:
Centripetal force is a force that keeps a body moving in a circle. The center-seeking force is called the centripetal force. It keeps the body moving in a circle. Centripetal force always acts perpendicular to the motion of the body.
Examples:
Let us study the centripetal forces in the following examples:
- A stone tied to one end of a string rotating in a circle. The tension in the string provides the necessary centripetal force. It keeps the stone remaining in the circle. If the string is not strong enough to provide the necessary tension, it breaks and the stone moves away along a tangent to the circle
- The moon revolves around the Earth. The gravitational force of the Earth provides the necessary centripetal force.
Derivation of centripetal Force:
Let a body of mass m move with uniform speed v in a circle of radius r. The acceleration ac produced by the centripetal force Fc is given by
Centripetal acceleration ac = ……..(i)
According to Newton’s second law of motion, the centripetal force Fc is given by
Fc = m ac …………….(ii)
Fc = ……………(iii)
Factors on which centripetal force depends:
Equation Fc = shows that the centripetal force needed by a body moving in a circle depends on:
- The mass m of the body
- Square of its velocity v
- Reciprocal to the radius r of the circle.
Q48. What is the centrifugal force? Explain.
Ans: Centrifugal force:
The centripetal force pulls the object towards the center of the circle. As a reaction, another force appears at the center of the circle which is equal in magnitude to the centripetal force but opposite in direction. This outward reaction is called centrifugal force
According to Newton’s third law of motion, there exists a reaction to this centripetal force. The centripetal reaction that pulls the string outward is sometimes called the centrifugal force.
For Example:
If we whirl a piece of stone tied to one end of a string, the stone is pulled inwards due to centripetal force transmitted to it through the string. As a reaction, the stone pulls our hand outwards, and this outward reaction force on our hand is the centrifugal force.
DO YOU KNOW?
While the coaster cars move around the loop, the track provides centripetal force preventing them from moving away from the circle.
Q49. Identify the use of centripetal force in (i) safe driving by banking roads (ii) washing machine dryers (iii) cream separator.
Ans: i. Banking of the roads:
The curvature of the road must be inclined to control the centrifugal force of the vehicle.
Banking of roads means to make the road slide towards the center of curvature with an angle. It is helpful because if the velocity of the car is more or there is less friction between the tires and the road it reduces the danger of the car moving out of the circular track.
Explanation:
When a car takes a turn, the centripetal force is needed to keep it in its curved track. The friction between the tires and the road provides the necessary centripetal force. The car would skid if the force of friction between the tires and the road is not sufficient enough particularly when the roads are wet. This problem is solved by the banking of curved roads.
- Washing machine dryer:
The dryer of a washing machine is a basket spinner. They have a perforated wall having large numbers of fine holes in the cylindrical rotor. The lid of the cylindrical container is closed after putting wet clothes in it. When it spins at high speed, the water from wet clothes is forced out through these holes due to a lack of centripetal force.
iii. Cream Separator:
Most modern plants use a separator to control the fat contents of various products. A separator is a high-speed spinner. It acts on the same principle of centrifuge machines. The bowl spins at a very high speed causing the heavier contents of milk to move outward in the bowl pushing the lighter contents inward towards the spinning axis.
Cream or butterfat is lighter than other components in milk. Therefore, skim milk, which is denser than cream, is collected at the outer wall of the bowl. The lighter part (cream) is pushed towards the center from where it is collected through a pipe.
SUMMARY
- A force is a push or pull. It moves or tends to move, stops, or tends to stop the motion of a body.
- Inertia: Inertia of a body is its property due to which it resists any change in its state of rest or uniform motion in a straight line.
- Momentum: The momentum of a body is the quantity of motion possessed by the body. The momentum of a body is equal to the product of its mass and velocity.
- Friction: The force that opposes the motion of a body is called friction.
- Newton’s first law of motion: Newton’s first law of motion states that a body continues its state of rest or uniform motion in a straight line provided no net force acts on it.
- Newton’s second law of motion: Newton’s second law of motion states that when a net force acts on a body, it produces acceleration in the body in the direction of the net force. The magnitude of this acceleration is directly proportional to the net force acting on it and inversely proportional to its mass. Mathematically, F = ma.
- Unit of Force: SI unit of force is newton (N). It is defined as the force which produces an acceleration of 1 ms – 2 in a body of mass 1 kg
- Mass: Mass of a body is the quantity of matter possessed by it. It is a scalar quantity. SI unit of mass is the kilogram (kg).
- Weight: The weight of a body is the force of gravity acting on it. It is a vector quantity. SI unit of weight is Newton (N).
- Newton’s third law of motion: Newton’s third law of motion states that to every action there is always an equal and opposite reaction.
- The acceleration and tension in a system of two bodies attached to the ends of a string that passes over a frictionless pulley such that both move vertically is given by:
a = g T = g
- The acceleration and tension in a system of two bodies attached to the ends of a string that passes over a frictionless pulley such that one moves vertically and the moves on a smooth horizontal surface are given by:
a = g T = g
- Law of conservation of momentum: Law of conservation of momentum states that the momentum of an isolated system of two or more two interacting bodies remains constant.
- Friction: A force between the sliding objects which opposes the relative motion between them is called friction.
- Rolling Friction: Rolling friction is the force of friction between a rolling body and a surface over which it rolls. Rolling friction is lesser than the sliding friction.
- The friction causes a loss of energy in machines and much work has to be done in overcoming it. Moreover, friction leads to much wear and tear on the moving parts of the machine. The friction can be reduced by:
(i) Smoothing the sliding surfaces in contact.
(ii) Using lubricants between sliding surfaces.
(iii) Using ball bearings or roller bearings.
- Circular motion: The motion of a body moving along a circular path is called circular motion.
- Centripetal force: The force which keeps the body moving in a circular path is called the centripetal force and is given by:
Fc =
- Centrifugal force: According to Newton’s third law of motion, there exists a reaction to the centripetal force. The centripetal reaction that pulls the string outward is sometimes called the centrifugal force.
Problems
Q.1) A force of 20 N moves a body with an acceleration of 2 ms-2. What is its mass?
Answer:
Q.2) The weight of a body is 147 N.What is its mass? Take the value of g as 10 ms-2
Answer:
Q.3) How much force is needed to prevent a body of mass 10 kg from falling?
Answer:
Mass; m = 10 kg
Acceleration due to gravity; a = g = 10 ms-2
Force; F =?
According to Newton’s 2nd law of motion,
F = ma
As
a = g
So
F = mg
F = 10 × 10
F = 100 N
Q.4) Find the acceleration produced by a force of 100 N in a mass of 50KG.
Answer:
Q.5) A body has weight 20 N.How much force is required to move it vertically upward with an acceleration of 2 ms-2
Answer:
Weight; w = 20 N
Acceleration; a = 2 ms-2
Vertically upward force (Tension); T =?
As
w = mg
Net force is
Fnet = ma
Weight is working vertically downward so the net force Fnet required to produce acceleration is T-W
T – W = ma
T – 20 = 2 x 2
T = 4 + 20
T = 24 N
Q.6) Two masses 52 kg and 48 kg are attached to the ends of a string that passes over a frictionless pulley. Find the tension in the string and acceleration in the bodies when both the masses are moving vertically.
Answer:
Q.7) Two masses 26 kg and 24 kg are attached to the ends of a string which passes over a frictionless pulley. 26 kg is lying over a smooth horizontal table. 24 N mass is moving vertically downward. Find the tension in the string and the acceleration in the bodies.
Answer:
Q.8) How much time is required to change 22 Ns momentum by a force of 20 N ?
Answer:
Q.9) How much is the force of friction between a wooden block of mass 5 kg and the horizontal marble floor? The coefficient of friction between wood and marble is 0.6.
Answer:
Mass = m = 5 kg
Coefficient of friction = µf = 0.6
Force of friction = Ff =?
Using formula
Ff = µf N
Ff = µf mg (N = mg)
Ff = 0.6 × 5 × 10
Ff = 30 N
Q.10) How much centripetal force is needed to make a body of mass 0.5 kg to move in a circle of radius 50 cm with a speed 3ms-1 ?
Answer:
