WORK AND ENERGY

Define watt. Express kilowatt in terms of joule per second. A 150 kg car
engine develops 500 W for each kg. What force does it exert in moving the
car at a speed of 20 m s–1?

How is the power related to the speed at which a body can be lifted? How
many kilograms will a man working at the power of 100 W, be able to lift at
constant speed of 1 m s–1 vertically?

What is power? How do you differentiate kilowatt from kilowatt hour? The
Jog Falls in Karnataka state are nearly 20 m high. 2000 tonnes of water
falls from it in a minute. Calculate the equivalent power if all this energy
can be utilized?

Four men lift a 250 kg box to a height of 1 m and hold it without raising or
lowering it. (a) How much work is done by the men in lifting the box?
(b) How much work do they do in just holding it? (c) Why do they get tired
while holding it?

girl having mass of 35 kg sits on a trolley of mass 5 kg. The trolley is
given an initial velocity of 4 m s–1 by applying a force. The trolley comes to
rest after traversing a distance of 16 m. (a) How much work is done on the
trolley? (b) How much work is done by the girl?

A light and a heavy object have the same momentum. Find out the ratio of
their kinetic energies. Which one has a larger kinetic energy?

If an electric iron of 1200 W is used for 30 minutes everyday, find electric
energy consumed in the month of April

A ball is dropped from a height of 10 m. If the energy of the ball reduces by 40% after
striking the ground, how much high can the ball bounce back?

Is it possible that an object is in the state of accelerated motion due to
external force acting on it, but no work is being done by the force. Explain
it with an example.

The velocity of a body moving in a straight line is increased by applying a
constant force F, for some distance in the direction of the motion. Prove
that the increase in the kinetic energy of the body is equal to the work done
by the force on the body.

The weight of a person on a planet A is about half that on the earth. He can
jump upto 0.4 m height on the surface of the earth. How high he can jump
on the planet A?

The power of a motor pump is 2 kW. How much water per minute the
pump can raise to a height of 10 m?

Can any object have momentum even if its mechanical energy is zero?
Explain.

Can any object have mechanical energy even if its momentum is zero?
Explain.

A rocket is moving up with a velocity v. If the velocity of this rocket is suddenly
tripled, what will be the ratio of two kinetic energies?

A freely falling object eventually stops on reaching the ground. What happens to its kinetic energy?

Find the energy in kW h consumed in 10 hours by four devices of power 500 W each.

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?

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

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

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?

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?

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

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.

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.

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

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

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?

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

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

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.

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

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

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?

Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term 'work'.

ӢSuma is swimming in a pond.

ӢA donkey is carrying a load on its back.

ӢA wind mill is lifting water from a well.

ӢA green plant is carrying out photosynthesis.

ӢAn engine is pulling a train.

ӢFood grains are getting dried in the sun.

ӢA sailboat is moving due to wind energy.

Define average power.

A lamp consumes 1000 J of electrical energy in 10 s. What is its power?

Define 1 watt of power:

What is power?

The kinetic energy of an object of mass,m moving with a velocity of 5 m s-1 is 25 J. What will be its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times?

Write an expression for the kinetic energy of an object.

What is the kinetic energy of an object?

A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field?

Define 1 J of work.

Write an expression for the work done when a force is acting on an object in the direction of its displacement.

When do we say that work is done?

A force of 7 N acts on an object. The displacement is, say 8 m, in the direction of the force (Fig. 11.3). Let us take it that the force acts on the object through the displacement. What is the work done in this case?