Work and Energy Physics class 11 notes for kpk 2021, khan academy, short question, long question, numerical problems, and solved exercises.
Biology Class 11 Notes for kpk
Yes, a bucket does possess the potential energy.
When we lower the bucket to the bottom of the well, water exerts a thrust on the bucket in the upward direction. So, we have to do some work against that upthrust of water to take the bucket to the bottom of the well. This work done is stored in form of potential energy in the bucket. So, we can say that bucket possess negative potential energy if we take the ground level as a reference level.
When an arrow is shot from its bow, it has k.E. From where does it get the k.E?
When we stretch the string of the bow to shot the arrow, some work is done on the bow. This work done is stored in the bow in form of elastic potential energy.
When we release the string of the bow and shot the arrow, the stored P.E of the bow will convert in form of K.E of arrow making it capable of moving in the forward direction.
So, we can say that the arrow gets its K.E from the P.E of the bow.
Does a hydrogen-filled balloon possess any P.E? Explain?
Yes, a hydrogen-filled balloon does possess P.E.
Hydrogen is a lighter gas than air, so when we fill a balloon with hydrogen gas and release it in the air, it starts moving upward against the gravitational field.
So, due to the upthrust of air on the balloon, some work is done on the balloon which will store in form of P.E in a balloon.
Is K.E a vector quantity?
No, K.E is not a vector quantity. It is a scalar quantity.
As K.E is given by,
Where “m” represents the mass of an object which is a scalar quantity and “v2” represents the magnitude of the velocity of the object which is also a scalar quantity. That’s why K.E is also a scalar quantity.
What happens to K.E of a bullet when it penetrates into a target?
K.E of the bullet becomes smaller and smaller as it penetrates the target.
When a bullet penetrates a target, the target exerts a resisting force on the bullet. Some of the K.E of the bullet is used to overcome that resisting force offered by the target. While some part of K.E converts into other forms of energy such as sound energy.
Does the tension in the string of a swinging pendulum do any work?
Work done is defined as the product of the force applied to a body and the displacement covered in the direction of the force; i.e.
W = F.d = Fdcosθ
In the case of a swinging pendulum, the tension in the string is directed upward which the bob swings in the direction perpendicular to the tension i.e the angle between them is 900.
Thus eq(i) becomes;
W = Fdcos(90) = Fd(0) = 0
This shows that there is no work done in case of string swinging pendulum.
Q.7 A meteor when enters into the earth’s atmosphere burns. What happens to its energy?
There are particles in the air. When a meteor enters into the atmosphere of earth, it burns due to friction between air particles and meteor.
When a meteor collides with air particles some of its energy is transferred to air particles. Some of the energy possessed by the meteor is used to overcome the friction offered by the air particles while a part of it is also converted into heat energy and light energy which results in the loss of its energy.
Q.8 What type of energy is stored in the spring of watch?
Elastic potential energy is stored in spring of watch and is given by
Where “k” is the spring constant and “x” is displacement.
Q.9 A man drops a cup from a certain height, which breaks into pieces. What energy changes are involved?
When a man drops a cup from a certain height, its P.E decreases and it converts into K.E.
When it strikes to the ground it breaks into pieces and produces noise. Hence some of its K.E converts into sound energy and some into heat energy while the remaining will get distributed into K.E of pieces of the cup.
Q.10 A man rowing boat upstream is at rest with respect to shore, is he doing work?
No, there is no work done by the man while he is at rest with respect to the shore.
According to the definition of work done,
W = Force.displacement
W = F.d
As the man is at rest so there is no displacement i.e. d = 0,
W = F.0 = 0
⇒ W = 0
This shows that there is no work done by the man rowing boat but at rest with respect to shore.
Q.11 Why energy savers are used instead of normal bulbs?
We use energy savers instead of normal bulbs due to the owing reasons:
1. Energy savers consume less energy as compared to normal bulbs.
2. They produce a very small amount of heat than normal bulbs
3. Light produced by energy savers is of high intensity and power as compared to normal bulbs.
4. These are long-lasting than normal bulbs.
Comprehensive Questions Biology Notes
Q.1 Define work and show that it is the dot product of force and displacement. At what conditions work done will be maximum or minimum?
“Work is scalar product of force and displacement”
It can also be defined as;
“The product of the displacement and the components of the force in the direction of displacement”
Work is done by both constant force as well as variable force.
Consider a force ‘F’ applied on a body, making an angle ‘θ’ with the horizontal as shown in the figure. Then by resolving force into its components, we can see from the figure that the body is displaced by force Fx, so work is done by Fx.
W = F.d
W = Fxd = (Fcosθ) d = Fd cosθ
The above equation can also be obtained by resolving d into its components keeping F and θ constant such that;
W = Fdx = F(dcosθ) = Fdcosθ
So, we get work done as
Work Done = (magnitude of force)(magnitude of displacement)(cosine of the angle between F and d)
Conditions for maximum and minimum work done:
Work equation tells us that,
1. Greater the applied force F, greater will be the work done provided “d” and “θ” are kept constant.
2. Greater the distanced, greater will be the work done provided force F and θ are kept constant.
3. Maximum Work: When θ = 0o, then W = Fdcos θ = Fdcos0o = Fd. So, work done will be maximum for θ = 0o
4. Minimum Work: When θ = 90o, the W = Fdcos θ = Fdcos90o = 0. So work done will be minimum for θ = 90o.
5. Negative Work: When θ = 180o, then W = Fdcos180o = -Fd. So work done will be negative for θ = 180o.
Q.2 Define power and show that power is the dot product of force and velocity. What are the different units of power used in our daily life?
Power is defined as;
“The amount of work done by a body in one second is called power”
So we can redefine power as,
“The dot product of force and velocity is called power”
SI unit of power is called watt (W).
“Power is said to be one watt if one joule of work is done by a body in one second”
Watt is a very small unit so we usually measure it in kilowatt given by,
1 kilowatt = 1kW = 1000 watts
In British Engineering system, the unit of power is called horsepower and is given by
1 horse power = 1hp = 746 watts
Q.3 Explain the work energy principle in the cases of the change in k.E of body.
Q.4 Prove that Absolute P.E = GmMe / Re.
Absolute P.E is defined as;
“Absolute potential energy of a body on the surface of the earth is the amount of work done on a body to displace it from surface of earth to infinity”
Divide the whole path between the surface of earth and infinity in n small patches i.e. 1, 2, 3, 4, ……., n.
Where n is infinity. Let r1, r2, r3, ……… rn be the distances from the centre of the earth to these patches
Now consider a body of mass “m” is placed at point 1 at a distance r1 from the centre of the earth, then work done in lifting the body from point 1 to point 2 is given by,
W1→2 = Fav.Δr
Here F is gravitational force given by,
As we know, Δr = r2 – r1, so we can write r2 = Δr+r1
Putting this value, we get
Value of Δr is much much smaller, so neglecting its higher values, we get
Putting this value in equation (1), we get
Q.5 Calculate the values of the escape velocity of a body and show that it is equal to 11.2 km s-1.
“The initial velocity, which a projectile must have at the earth’s surface in order to get out of earth’s gravitational field, is known as escape velocity”
As work done in lifting a body from earth’s surface to infinity is given by,
where Me is the mass of the earth and Re is the radius of the earth.
If a projectile is given an initial K.E equal to GmMe/Re, it will reach an infinite distance from the earth.
Such a projectile will have escape velocity which can be calculated by equating the initial kinetic energy (K.E) with absolute potential energy.
ves = 11.2 kms-1.
Q.6 Describe briefly various non-conventional sources of energy.
“Non-conventional energy sources are those sources, which are ecologically safe and renewable.”
Some of the conventional energy sources are the sun, the winds, tides, geothermal energy, biomass, farm, and animal waste etc.
1.Energy from Biomass:
Biomass is usually referred to garbage, some of these are usually found around us as dead trees, tree branches, left-over crops, wood chips, and bark, used tires etc.
Trucks bring garbage to the power plant where they fed to the furnace to burn. The heat produced is then used to boil water and energy of steam to run turbines and generators. In another procedure, Garbage is decomposed to produce methane, which can be burned right at the form to make an rgy to run farms.
2.Energy from Waves:
The ocean is a very good source of a large amount of energy produced by sun and wind. This makes wave energy a potential power platform. Waves of larger amplitude have a larger amount of energy, which is a stored form of solar energy. Winds that produce waves are caused by the pressure differences in atmosphere arising due to solar heating.
Source of geothermal energy is a result of sun rays striking to the ace of the earth. Another source is the centre of the earth, which is almost 4000 degree Celsius. This large amount of energy has a capability to burn rocks so this warm surface of the earth as well. Volcanic areas are specifically are a very significant source of energy.
Sun and stars are tireless sources of energy. This energy result due to nuclear reactions that take place in stars and sun, converting matter into energy. At present, nuclear energy is covering 16% of world’s electricity and scientists are working to make a fusion reactor, which will be a perfect source of an immense amount of energy with lesser disadvantages as compared to a fission reactor.
It is a form of energy, obtained by changing sea level. It only takes energy to change the tides. It is a very reliable form of energy source due to its predictability. Unlike solar or wind energy it does not depend on seasons instead, it purely depends on gravitational energy.
It is the radiant light and heat from the sun serving as a heat source to human from ancient times It is considered as the most available source of energy on earth. There are usually two technologies to utilize this form of energy, passive and active.
The active solar technique includes the use of photovoltaic panels and solar thermal collectors to convert sunlight into other energy forms.
The passive solar technique includes orienting a building to earth, using materials of favourable thermal mass or light dispersing properties.
Non-uniform distribution of heat due to solar energy in different regions cause pressure difference, winds the blow from an area of high pressure to an area of low pressure. At sea, winds are stronger than on the land surface. Almost 100 billion watts per year of power in the shape of wind energy is available on earth. In wind areas, windmills are used to produce mechanical energy, which may be used in tube wells or flour mills.