Question 1

Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term 'work'.(A) Suma is swimg in a pond.(B) A donkey is carrying a load on its back.(C) A wind-mill is lifting water from a well.(D) A green plant is carrying out photosynthesis.(E) An engine is pulling a train.(F) Food grains are getting dried in the sun.(G) A sailboat is moving due to wind energy.

Accepted Answer

Work done            W=F.S  = FS cosθ    where   θ is the angle between F and S(A)  :     While swimg, Suma pushes the water in the backward direction and as a result of Newton's 3rd law of motion, the water applies an equal force on Suma in forward direction. Hence Suma swims in the forward direction.        ∴ In this case,   θ =0o         ⟹ W=FS cos0o= FS(B) :      Donkey applies a force in the vertical direction to carry the load and its displacement is in the horizontal direction, thus  θ=90o  in this case.       ⟹ W=FS cos90o= 0(C)  :     Wind-mill applies a force in the vertical direction to lift the water from well and the water also moves in the vertical direction.        ∴ In this case,   θ =0o         ⟹ W=FS cos0o= FS(D)   :    During photosysnthesis, all the matabolic reactions take place inside the leaf of the plant, but there is no displacement either of the plant or the leaves.     ∴   S=0                 ⟹ W=0(E)   :      While pulling, the engine applies a force in the forward direction on the train and the train also moves in the forward direction.       ∴ In this case,   θ =0o         ⟹ W=FS cos0o= FS(F)   :   When the food grains are getting dried in the sun, they remain still at their position i.e the food grains do not get displaced.     ∴   S=0                 ⟹ W=0(G)   :     Wind  applies a force on the sailboat in the forward direction making the boat to move in the direction of force.         ∴ In this case,   θ =0o         ⟹ W=FS cos0o= FS

Question 2

An object thrown at a certain angle to the ground moves in a curved path and falls back to the ground. The initial and the final points of the path of the object lie on the same horizontal line. What is the work done by the force of gravity on the object?

Accepted Answer

Work done by the force of gravity on the object is equal to the negative of the change of potential energy of the object.Wg​=−ΔP.EWg​=− (mgh2​−mgh1​)           OR            Wg​=−mg (h2​−h1​)But as the object lies on the same horizontal line of the ground      i.e    h1​=h2​=0⟹ Wg​=0Thus work done by gravity force on the object is zero.

Question 3

A battery lights a bulb. Describe the energy changes involved in the process.

Accepted Answer

While lighting a bulb through a battery, the chemical energy of the battery gets converted into the electric energy which further gets converted into light energy along with heat energy. That is why after lighting a bulb for some time, it gets heated up.One form of energy can be transformed to another form but it cannot be created or destroyed. Chemical energy →Electric energy→Light energy + Heat energy

Question 4

Certain force acting on a 20 kg mass changes its velocity from 5 ms−1 to 2 ms−1. Calculate the work done by the force.

Accepted Answer

Given :          Initial velocity     u=5  m/s                      Final velocity     v=2  m/sThe work done is equal to the
change in the kinetic energy of an object.W=21​m(v2−u2)W=21​×20×(22−52)⟹ W=−210 J

Question 5

A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal what is the work done on the object by the gravitational force? Explain your answer.

Accepted Answer

Force of gravity and displacement are perpendicular to each other. Hence, work done is zero.

Question 6

The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?

Accepted Answer

No, it does not violate the law of conservation of energy.According to conservation of energy,         ETotal​=P.E+K.EThus for a freely falling object, its potential energy decreases but kinetic energy increases in such a way that its total mechanical energy remains constant.Moreover, potential energy of a freely falling object is converted into its kinetic energy.

Question 7

What are the various energy transformations that occur when you are riding a bicycle?

Accepted Answer

When a person is riding a bicycle, the muscular energy  gets converted into heat energy of the body of the person as well as in kinetic energy of the bicycle. The kinetic energy makes the bicycle to move whereas the heat energy heats up the body of the person. Muscular energy →  Kinetic  energy  +   Heat energy

Question 8

Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?

Accepted Answer

When a person tries to push a huge rock but fails to move it, then no energy of the person is transferred to the rock as it remains still and thus do not gain any kinetic energy. Actually the muscular energy of person is converted into heat energy which heats up the person's body. Thus he gets exhausted.

Question 9

A certain household has consumed 250 units of energy during a month. How much energy is this in Joules?

Accepted Answer

Energy consumed by certain households in a month      E=250 units  =250  kWh∴ E=250×3.6× 106  J                               (∵ 1kWh=3.6×106 J)⟹ E=9×108 Joules

Question 10

An object of Mass 40 kg is raised to a height of 5m above the ground what is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half way down.

Accepted Answer

Given :          Mass of the object        M=40 kg Height upto which it is raised      h=5 mPotential energy        P.E=MghP.E=40×10×5 = 2000 JLet its kinetic energy be Ek​ when it is half way down   i.e  h′=2h​According to conservation of energy,            K.Ei​+P.Ei​=K.Ef​+P.Ef​0+2000 = Ek​+Mg2h​∴   2000 = Ek​+40×10×25​⟹ Ek​=1000 J

Question 11

What is the work done by the force of gravity on a satellite moving round the earth? Justify your answer.

Accepted Answer

When a satellite revolves around the earth in circular orbit, the earth applies a gravitational force on the satellite in direction towards the center of earth but the satellite moves in tangential direction to the orbit. Therefore force of gravity and displacement are perpendicular to each other.Hence, the work done by force of gravity on the satellite is Zero.W=0

Question 12

Can there be displacement of an object in the absence of any force acting on it? Think. Discuss this question with your friends and teacher.

Accepted Answer

When a object moves with a constant velocity (v), then it gets displaced in the direction of motion but the net external force experienced by the object is zero.Fexternal​=maIf object moves with constant velocity, acceleration is zero.Since, a=0  ⟹ Fexternal​=0Using        s=ut+21​at2⟹    Displacement    s=ut    (∵ a=0)Hence, an object can have a displacement in the absence of any external force acting on it.

Question 13

A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer.

Accepted Answer

The person does not move from his position (no displacement) in spite of applying force on the bundle. Therefore, no work is done.

Question 14

An electric heater is rated 1500 W. How much energy does it use in 10 hours?

Accepted Answer

Time for which the heater is used     t=10 hours = 10×3600 s =36000 sEnergy consumed by the heater      E=P×t               where  P is the power of the heater∴   E=(1500) W × 36000 s⟹ E=5.4×107   JE=15  kWh                                  (∵ 1 kWh=3.6×106 J)

Question 15

Illustrate the law of conservation of energy by discussing the energy changes which occur when we draw a pendulum bob to one side and allow it to oscillate. Why does the bob eventually come to rest? What happens to its energy eventually? Is it a violation of the law of conservation of energy?

Accepted Answer

Law of conservation of energy state that energy can neither be created nor be destroyed it can only be converted into one form to another form.Now, let us consider a pendulum in which to and fro motion will take place. When pendulum bob moves from its rest position (mean position) to another extreme position its kinetic energy will be converted into potential energy when it is raised at some height 'h'. Now when it come back to its mean position, then its potential energy will be converted into kinetic energy and this phenomenon will take place again and again.The bob will eventually come to rest due to the frictional resistance offered by air on the surface of bob and pendulum loses its kinetic energy to overcome this friction and finally comes to rest.The law of conservation of energy is not violated because the kinetic energy loss by pendulum to overcome the friction is gained by surrounding, So total energy of system will remain conserved.

Question 16

An object of mass, m is moving with a constant velocity, v. How much work should be done on the object in order to bring the object to rest?

Accepted Answer

Initial velocity of the object  =vFinal velocity of the object  =0The work done is equal to the
change in the kinetic energy of an object.W=21​m[(0)2−v2]⟹ W=−21​mv2Here, work done is negative because it is acting on the mass.

Question 17

Calculate the work required to be done to stop a car of 1500 kg moving at the velocity of 60 km/h?

Accepted Answer

When a object is moving with a constant velocity, it possess K.EK.E=21​×m×v2So, in order to bring the object to rest i.e its same magnitude of energy is required so, as the final final energy comes to zero.v=60km/hr=16.66m/sSo, magnitude of work that needs to be done =21​×m×v2Hence, work required to stop the car =21​×1500×16.662 =208166.7J=208.17kJ

Question 18

In each of the following a force, F is acting on an object of mass M. The direction of displacement is from west to east shown by the longer arrow. Observe the diagram carefully and state whether the work done by the force is negative, positive, or zero.

Accepted Answer

Workdone         W=F.S  =F S  cosθ           where  θ is the angle between F  and  SFor (a)  :       θ=90∘             ∴  W=F S  cos90∘ = 0    For (b)  :       θ=0∘             ∴  W=F S  cos0∘ = FS >0   For (c)  :       θ=180∘             ∴  W=F S  cos180∘ =−FS <0

Question 19

Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?

Accepted Answer

Yes, I agree with Soni that the acceleration in an object could be zero even when several forces are acting on it. Let  equal and opposite forces  (F) act on an object, they cancel out each other so that the net external force experienced by the object is zero which makes the object to move with zero acceleration.Fexternal​=ma∴  Fexternal​=0              ⟹ a =0 ∴     vf​=vi​Thus  an object will kept on moving with a constant velocity v  if it was also moving initially.

Question 20

Find the energy in kwh consumed in 10 hours by four devices of power 500 w each.

Accepted Answer

Power of each device, P=500 W=0.5 kWTime for which the devices are used, t=10 hours Thus energy consumed by 4 devices, E=4×P×tE=4×0.500×10  kWh⟹ E=20  kWh

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