Learn the Turning Effect of Forces of Physics class 9th! Chapter 4 Summary, Conceptual Questions, and Comprehensive Questions.

Table of Contents

## Physics Class 9 Chapter 4 Turning Effect of Forces Conceptual Questions

### Q.1) Can the rectangular components of the vector be greater than the vector itself? Explain.

**Answer:**

No, the rectangular components cannot be greater than the resultant vector.

The resultant of a vector i.e. force vector ‘F’ is given by the equation;

F = √(F_{x}^{2} + F_{y}^{2})

or F^{2} = F_{x}^{2} + F_{y}^{2}

where F is the resultant vector

and F_{x }and F_{y }are the components of that vector.

It is clear from the equation that the sum of the square of rectangular components is equal to the square of the magnitude of the vector. Therefore, the rectangular components of a vector cannot be greater than the vector itself.

### Q.2) Explain why door handles are not put near hinges?

**Answer:**

The door rotates about the hinges due to torque. Mathematically, torque is defined as

**Torque** =** perpendicular distance from the axis of rotation **× **force applied**

It can be seen from above formula that, torque depends upon the**1) Applied force.2) The moment arm of the force.**

If the door handle is put near the hinges then the distance from the hinges to the handle decreases (moment arm of force become very small) hence a large amount of force is required to produce a torque to open/close the door, therefore.

If the door handle is put far from hinges, little force is required to produce a large amount of torque and it open/close the door easily.

### Q.3) Can a small force ever exert a greater torque than a larger force? Explain.

**Answer:**

Yes, a small force may exert a greater torque than a large force. Mathematically, torque is defined as

**Torque** =** perpendicular distance from the axis of rotation **× **force applied**

It can be seen from above formula that, torque depends upon the**1) Applied force.2) The moment arm of the force.**

So, if the moment arm is greater then less force will be required to produce a large torque. Similarly, if the moment arm is very small then a very large amount of force is required to produce the same torque.

### Q.4) Why it is better to use a long spanner rather than a short one to tighten a nut on a bolt?

**Answer:**

We know the equation of torque** Torque** =** perpendicular distance from the axis of rotation **× **force applied**

It can be seen from above formula that, torque depends upon the**1) Applied force.2) The moment arm of the force.**

It is clear that it is better to use a long spanner rather a short one to tight a nut. In the long spanner, the moment arm for the rotation of the nut increases, which decreases the required force to tighten the nut. Therefore, it is better to use a long spanner than a short one to open or tighten a nut.

### Q.5) The gravitational force acting on a satellite is always directed towards the centre of the earth: Does this force exert a torque on satellite?

**Answer:**

According to the definition of torque;** Torque** =** perpendicular distance from the axis of rotation **× **force applied**

The gravitational force acting on a satellite is always directed towards the center of the earth, so this is a central force (centripetal force). For the central force, torque is zero because the applied force is passing through axis of rotation, has zero moment arm zero.

### Q.6) Can we have situations in which an object is not in equilibrium, even though the net force on it is zero? Give two examples.

**Answer:**

Yes, we can have situations in which an object is not in equilibrium, even though the net force is not zero.

1) Turning a spanner requires two hands to move, so an opposite torque acts on it making a couple. The net force, in this case, is zero, but the net torques are not zero so it is not in equilibrium.

2) Turning a steering also produce couple so the net force is zero but the net torques are not.

### Q.7) Why do tightrope walkers carry a long, narrow rod?

**Answer:**

A tightrope walker uses a long rod to balance its centre of mass. If the centre of mass shift to one side, gravity will exert a torque on the acrobat, tending to cause a rotation about the rope and consequently fall.

To stay balanced, the acrobat must create a counter-torque having equal magnitude and opposite sign. That is why tightrope walkers carry a long pole.

### Q.8) Why does wearing high-heeled shoes sometimes cause lower back pain?

**Answer:**

Wearing heels causes lower back to lean forward more than normal because when a person wears high heels, the center of gravity shifts from the previous position to a new forward position. As a result of which the person loses the balance. In order to maintain the balance that person has to exert force on his lower back which causes lower back pain.

### Q.9) Why is it more difficult to lean backwards. Explain?

**Answer:**

The balance of our body is maintained by our brain. When someone leans forward, the visual senses see the whereabouts of the forward side and the person’s brain work out to shift the centre of gravity to keep the person balanced. But when someone leans backwards the visual senses cannot see the whereabouts of the backward side hence it becomes difficult for the brain to shift the centre of gravity to keep the person balanced.

### Q.10) Can a single force applied to a body change both it’s translational and rotational? Explain.

**Answer:**

Yes, a single force may produce both rotational and translation motion in the body.

When the line of action of single force passes through the center of gravity of the body, it produces acceleration.

Whereas when the line of action of a single force is not passing through the center of gravity, it produces rotation.

### Q.11) Two forces produce the same torque. Does it follow that they have the same magnitude? Explain.

**Answer:**

No, because torque depends on both force and moment arm. So for same torque, we can have different forces with different moment arms.

For Example: Suppose a 5 N force is applied to a nut by a spanner of length 2 m, the torque produced on the nut will be 10 Nm.

Similarly, for a spanner of length 4 m, a force of 2.5 N is required to produce a torque of 10 Nm.

### Q.12) Describe the path of the brick attached to a rope moving in a circle after you suddenly let go of the rope.

** Note: This question is mixed with Q11 in the book, and is written incompletely.**** Answer:**

A brick moving in a circle, when we let it go, it will take the tangential path of the circle at that instant. As the centripetal force is removed, which maintains the circular path of the brick, so the brick will take the linear path perpendicular to the rope.

## Comprehensive Questions Physics Class 9th Notes

### Q.1) What are force diagrams? Define like and unlike parallel forces with example.

**Answer:**__Force diagrams:__* “Since force is a vector quantity so the effect of all forces acting on the object is usually represented by drawing arrows. Such diagrams are known as force diagrams”.*

In force diagrams, the objects on which forces are shown is reduced to a dot at its center and the forces acting on the object are represented by arrows pointing away from it.__Example:__

Consider the figure in which a brick is lying on the table. The forces acting on the brick are represented by arrows. There are two forces acting on the brick; the weight of the brick that is acting downward and the reaction force i.e. normal force due to the table which is pushing the brick upward.

These forces are equal in magnitude and opposite in direction. Such forces are called balanced forces. When an object is acted on by balanced forces, the forces cancel each other’s effect and the object behaves as no force is acting on it.**Like Parallel Forces:**

**“The forces that are parallel to each other and have the same direction are known as like parallel forces”.****Example:**

When we lift a box with double support we are applying like parallel forces from each support. The force from one support can be greater than the other.

**Unlike Parallel Forces:**

*“The forces that are parallel to each other but have opposite directions are known as unlike parallel forces”.***Example:**

While driving a car the force acting on the steering wheel by both hands are unlike parallel forces as these are parallel but acting in opposite direction.

### Q.2) Explain the addition of forces, in connection with head to tail rule.

**Answer:**__Addition of Forces:__

* “The process of obtaining a single force which produces the same effect as produced by a number of forces acting together is known as the addition of forces”.*__Rules for the addition of forces:__

In the case of parallel forces the vectors are added as;

– Add the magnitude of force vectors in case of like parallel forces.

– Subtract the magnitude of force vectors in case of unlike parallel forces.**Example:**

**Head to Tail Rule for Addition of forces:**

If the forces acting on a body are not parallel to each to other but are making an angle, the forces cannot be simply added as described above. In this case, the forces are added by using head to tail rule of vector addition.**Example:**

Consider two forces ‘**F**_{1}‘and ‘**F**_{2}‘ that are acting on a body such that the force ‘**F**_{1}‘ is making an angle θ_{1} with the x-axis and the force **F**_{2}‘ is making an angle θ_{2} with x-axis as shown in figure;

In order to add these vectors head to tail rule is followed that is;

1) Sketch the first force ‘**F**_{1}‘ using the same scale according to selected scale in a given direction.

2) Now place the tail of the second force vector ‘**F**_{2}‘on the head of the first force vector ‘**F**_{1}‘in the given direction.

3) Now the resultant force vector ‘**F**_{R}‘ can be obtained by joining the tail of first force vector ‘**F**_{1}‘to the head of second force vector ‘**F**_{2}‘.

4) To determine the magnitude of resultant measure the length of ‘**F**_{R}‘ and convert it back according to given scale. To determine the direction of the resultant measure the angle of resultant θ_{R}with the x-axis.

** F**_{R =} **F**_{1} +**F**_{2}

Read more: Physics Class 9 Chapter 3 Dynamics questions, answers Mardan board

**Q.3) Define moment of a force. Give its mathematical description and elaborate the factors on which it depends?**

**Answer:**__Moment of force__

** “The turning effect produced in a body about a fixed point due to the applied force is called torque or moment of force”**.

Mathematically,

*Torque = force applied × moment arm*

*τ = F**×*

*d*Torque is a vector quantity and its unit is Newton-meter or Nm.

__Factors affecting Torque:__Torque depends upon two factors:

**1) Magnitude of applied force (F)**

Greater the magnitude of force greater will be torque.

**2) Magnitude of moment arm (d)**

“

**.**

*The perpendicular distance between the axis of rotation and the line of the action of force is called the moment arm of the force”*The longer the moment arm of the force, the greater will be torque.

**Example**

When we open or close the door, Force ‘

**F**‘ is applied at perpendicular distance ‘

**d**‘ from the axis of rotation as shown in the figure. Increasing the applied force ‘

**F**‘ or the moment arm ‘

**d**‘ increases the tarque ‘

**. Reducing applied force ‘**

*τ’***F**‘ or moment arm ‘

**d**‘ decreases torque

**Q.4) What is resolution of forces? Explain with an example how forces can be resolved into rectangular components.**

**Answer:**__Resolution of Forces:__** “The process of splitting a force vector into two or more force vectors (component) is called resolution of forces”**.

__Explanation:__For Example, an ice block is being pulled by a boy using rope. We can think of force as tension in the rope. This single force F can be resolved into two components- one directed upwards rightwards along x-axis (F

_{x}) and the other directed upwards along y-axis (F

_{y}) as shown.

Consider the force ‘**F**‘ in the Cartesian coordinate system represented by line ‘OA’ making an angle θ with the x-axis below.

Draw a perpendicular ‘AB’ on x-axis from ‘A’. According to head to tail rule, ‘OA’ is the resultant vector of OB and BA.

Thus

OA = OB + BA

From the figure

F = F_{x} + F_{y}

F_{x} and F_{y} can be calculated by

**Q.5) What is couple? explain with examples.**

**Answer:**__Couple:__

**“Two equal and opposite parallel forces acting along different lines on a body constitute a couple”.**

The couple does not produce any translation, but only rotation. The resultant force of couple is zero but resultant of a couple is not zero; it is a pure moment. The shortest distance between two couple force is called **couple arm**__Examples:__

Forces applied on the steering wheelby both handsis an example of couple. Each hand grips the wheel at points on opposite sides of the shaft. When they apply a force that is equal in magnitude yet opposite in direction the wheel rotates.

Similarly exerting the force on the bicycle pedals, winding up the spring of a toy car, opening and closing the cap of a bottle and turning of a water tap are the examples of couple.

**Q.6) Define equilibrium. Explain its types and State the two conditions of equilibrium.**

**Answer:**__Equilibrium__*“The state of a body in which under the action of several forces acting together there is no change in translation motion, as well as rotational motion, is called equilibrium”*__Types of Equilibrium:__

The effect of force is to produce change in translational motion and effect of torque is to produce change in rotational motion. Thus the equilibrium is divided into two types. **1) Static equilibrium:***“When a body at rest under the action of several forces acting together and several torques acting on the body is said to be in static equilibrium.”*

If there is no change in the state of motion of a body, w.r.t it’s surrounding then it is called static.

When a body is in state of rest and the sum of all forces acting upon it is zero then it is said to be in dynamic equilibrium.

For example, all stationary bodies are in static equilibrium.**2) Dynamic equilibrium:**** “When a body is moving at uniform velocity under the action of several forces acting together the body is said to be dynamic equilibrium.”**If a body is changing its position w.r.t surrounding then it is called dynamic e.g. moving birds, movement of the earth around the sun, running cat and playing football etc.

Dynamic equilibrium, when a body is in state of uniform motion and the resultant of all forces acting upon it is zero then it is said to be in dynamic equilibrium.

For example, Jump by using parachute.

Conditions of equilibrium

There are two conditions of equilibrium which are given below.

1) First condition of equilibrium

2) Second condition of equilibrium

Read more: Physics class 9 Chapter 1 questions and answers Mardan board**First condition of equilibrium:**__Definition__

“A body will be in equilibrium if the sum of all the forces acting on the body is zero.”

Mathematically, sum of all forces acting along x-axis should be zero.