Lakhmir singh solutions physics class 9 Solutions for Chapter 4 work and energy

How much work is done when a body of mass m is raised to a height h above the ground?

State the SI unit of work.

Is work a scalar or a vector quantity?

Define 1 joule of work.

What is the condition for a force to do work on a body?

Is energy a vector quantity?

What are the units of a) work b) energy

What is the work done against gravity when a body is moved horizontally along a frictionless surface?

By how much will the kinetic energy of a body increase if its speed is doubled?

Write an expression for the kinetic energy of a body of mass m moving with a velocity v.

If the speed of a body is halved, what will be the change in its kinetic energy?

On what factors does the kinetic energy of a body depend?

Which would have a greater effect on the kinetic energy of an object: doubling the mass or doubling the velocity?

How fast should a man of 50 kg run so that his kinetic energy will be 625J?

State whether the following objects possess kinetic energy, potential energy, or both:

a) A man climbing a hill

b) A flying aeroplane

c) A bird running on the ground

d) A ceiling fan in the off position

e) A stretched spring lying on the ground

Two bodies A and B of equal masses are kept at heights of h and 2h respectively. What will be the ratio of their potential energies?

What is the kinetic energy of a body of mass 1 kg moving with a speed of 2 m/s?

Is potential energy a vector or a scalar quantity?

A load of 100 kg is pulled up by 5 m. Calculate the work done.

State whether the following statement is true or false: The potential energy of a body of mass 1 kg kept at a height of 1m is 1J

What happens to the potential energy of a body when its height is doubled?

What kind of energy is possessed by the following:

a) A stone kept on roof-top

b) A running car

c) Water stored in the reservoir of a dam

d) A compressed spring

e) A stretched rubber band.

Fill in the blanks with suitable words:

a) Work is measured as a product of _______ and ________

b) The work done on a body moving in a circular path is __________

c) 1 joule is the work done when a force of one ________ moves an object through a distance of one ______ in the direction of _______

d) The ability of a body to do work is called _________. The ability of a body to do work because of its motion is called _______

e) The sum of the potential and kinetic energies of a body is called ______ energy.

What are the quantities on which the amount of work done depends? How are they related to work?

Is it possible that a force is acting on a body but still the work done is zero? Explain giving one

A boy throws a rubber ball vertically upwards. What type of work, positive or negative is done:

a) by the force applied by the boy?

b) by the gravitational force of earth?

Write the formula for work done on a body when the body moves at an angle to the direction of the force. Give the meaning of each symbol used.

How does the kinetic energy of a moving body depend on its

a) speed

b) mass

Give one example each in which a force does

a) positive work

b) negative work

c) zero work

A ball of mass 200g falls from a height of 5 meters. What is its kinetic energy when it just reaches the ground?

Find the momentum of a body of mass 100g having a kinetic energy of 20J.

Two objects having equal masses are moving with uniform velocities of 2 m/s and 6 m/s respectively. Calculate the ratio of their kinetic energies.

A body of 2 kg falls from rest. What will be its kinetic energy during the fall at the end of 2s?

On a level road, a scooterist applies brakes to slow down from a speed of 10 m/s to 5 m/s. If the mass of the scooterist and the scooter be 150kg, calculate the work done by the brakes.

A man drops a 10 kg rock from the top of a 5m ladder. What is its speed just before it hits the ground? What is its kinetic energy when it reaches the ground?

Calculate the work done by the brakes of a car of mass 1000kg when its speed is reduced from 20 m/s to 10 m/s.

A body of mass 100kg is lifted up by 10m. Find:

a) the amount of work done

b) potential energy of the body at that height

A boy weighing 50kg climbs up a vertical height of 100m. Calculate the amount of work done by him. How much potential energy does he gain?

To what height should a box of mass 150kg be lifted, so that its potential energy may become 7350 joules?

A body of mass 2kg is thrown vertically upwards with an initial velocity of 20 m/s. What will be its potential energy at the end of 2s?

How much work is done when a force of 1 N moves a body through a distance of 1m in its own direction?

A car is being driven by a force of 2.5 1010 N. Travelling at a constant speed of 5 m/s, it takes 2 minutes to reach a certain place. Calculate the work done.

Explain by an example that a body may possess energy even when it is not in motion.

a) On what factors does the gravitational potential energy of a body depend?

b) Give one example each of a body possesses both, i) kinetic energy ii) potential energy

Give two examples where a body possesses both kinetic energy as well as potential energy.

How much is the mass of a man if he has to do 2500 joules of work in climbing a tree 5m tall?

If the work done by a force in moving an object through a distance of 20 cm is 24.2J, what is the magnitude of the force?

A boy weighing 40kg makes a high jump of 1.5m

a) What is his kinetic energy at the highest point?

b) What is his potential energy at the highest point?

What type of energy is possessed:

a) By the stretched rubber strings of a catapult?

b) By the piece of stone which is thrown away on releasing the stretched rubber strings of catapult?

A weightlifter is lifting weights of mass 200kg up to a height of 2 meters. If g = 9.8 m/s^2, calculate:

a) potential energy acquired by the weight

b) work done by the weightlifter

a) Define the term ‘work’. Write the formula for the work done on a body when a force acts on the body in the direction of its displacement. Give the meaning of each symbol that occurs in the formula.

b) A person of mass 50kg climbs a tower of height 72 meters. Calculate the work done.

a) When do we say that work is done? Write the formula for the work done by a body in moving up against gravity. Give the meaning of each symbol that occurs in it.

b) How much work is done when a force of 2N moves a body through a distance of 10cm in the direction of force?

a) What happens to the work done when the displacement of a body is at right angles to the direction of force acting on it? Explain your answer. b) A force of 50N acts on a body and moves it a distance of 4m on a horizontal surface. Calculate the work done if the direction of force is at an angle of 60 degrees to the horizontal surface.

a) Define the term ‘energy’ of a body. What is the SI unit of energy.

b) What are the various forms of energy?

c) Two bodies having equal masses are moving with uniform speeds of v and 2v respectively. Find the ratio of their kinetic energies.

a) What do you understand by the kinetic energy of a body?

b) A body is thrown vertically upwards. Its velocity goes on decreasing. What happens to its kinetic energy as its velocity becomes zero?

c) A horse and a dog are running with the same speed. If the weight of the horse is ten times that of the dog, what is the ratio of their kinetic energies.

a) Explain by an example of what is meant by potential energy. Write down the expression for the gravitational potential energy of a body of mass m placed at a height h above the surface of the earth.

b) What is the difference between potential energy and kinetic energy?

c) A ball of mass 0.5 kg slows down from a speed of 5 m/s to that of 3 m/s. Calculate the change in the kinetic energy of the ball. State your answer giving proper units.

a) What is the difference between gravitational potential energy and elastic potential energy? Give one example of a body having gravitational potential energy and another having elastic potential energy.

b) If 784 J of work was done in lifting a 20 kg mass, calculate the height through which it was lifted.

A car is accelerated on a leveled road and acquires a velocity 4 times of its initial velocity. During this process, the potential energy of the car:

a) Does not change

b) Becomes twice that of initial potential energy

c) Becomes 4 times that of initial potential energy

d) Becomes 16 times that of initial potential energy

A car is accelerated on a leveled road and attains a speed of 4 times its initial speed. In this process, the kinetic energy of the car:

a) Does not change

b) Becomes 4 times that of initial kinetic energy

c) Becomes 8 times that of initial kinetic energy

d) Becomes 16 times that of initial kinetic energy

In the case of negative work, the angle between the force and displacement is:

a) 0°

b) 45°

c) 90°

d) 180°

An iron sphere of mass 10 kg has the same diameter as an aluminium sphere of mass 3.5kg, Both the spheres are dropped simultaneously from a tower. When they are 10m above the ground, they have the same:

a) Acceleration

b) Momentum

c) Potential energy

d) Kinetic energy

A girl is carrying a school bag of 3 kg mass on her back and moves 200 m on a levelled road. If the value of g is 10 m/s2, the work done by the girl against the gravitational force will be:

a) 6000 J

b) 0.6 J

c) 0 J

d) 6 J

The work done on an object does not depend on the:

a) Displacement

b) The angle between force and displacement

c) Force applied

d) The initial velocity of the object

Water stored in a dam possesses:

a) No energy

b) Electrical energy

c) Kinetic energy

d) Potential energy

The momentum of a bullet of mass 20g fired from a gun is 10 kg.m/s. The kinetic energy of this bulletin kJ will be:

a) 5

b) 1.5

c) 2.5

d) 25

Each of the following statements describes a force acting. Which force is causing work to be done?

a) the weight of a book at rest on a table

b) the pull of a moving railway engine on its coaches

c) the tension in an elastic band wrapped around a parcel

d) the push of a person’s feet when standing on the floor

A girl weighing 400 N climbs a vertical ladder. If the value of g be 10 m/s2, the work done by her after climbing 2 m will be:

a) 200 J

b) 800 J

c) 8000 J

d) 2000 J

Which of the following does not possess the ability to do work not because of motion?

a) a sparrow flying in the sky

b) a sparrow moving slowly on the ground

c) a sparrow in the nest on a tree

d) a squirrel going up a tree

A stone is thrown upwards as shown in the diagram. When it reaches P, which of the following has the greatest value for the stone?

a) its acceleration

b) its kinetic energy

c) its potential energy

d) its weight