# Work & Energy (SA)

Short Answer Type Questions-I (2 Marks)

1. 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. [2011 (T-II)]

2. 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 longer arrow. Observe the diagrams carefully and state whether work done by force is –ve, +ve or 0. [2011 (T-II)] 3. (a) Define 1 watt.

(b) An electric bulb of 60 W (sixty watt) is used for 6 (six) hours per day. Calculate the
units of energy consumed in one day by the bulb.

4. (a) Is it possible that a force is acting on a body but still work done is zero? Explain giving one example. [2011 (T-II)]

(b) Two bodies of equal masses move with uniform velocities of v and 3v respectively.
Find the ratio of their kinetic energies.

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

(b) Is it possible that a force is acting on a body but still the work done is zero? Explain giving one example. [2011 (T-II)]

6. (a) What is meant by potential energy? Is potential energy a vector or scalar quantity? (b) Given one example of a body having potential energy. [2011 (T-II)]

OTHER IMPORTANT QUESTIONS

1. A sparrow and a crow are having the same kinetic energy during a flight. Which of the two is moving fast and why?

2. Why does a satellite going around the Earth in a circular path does not work?

3. A locomotive exerts a force of 7500 N and pulls a train by 1.5 km. How much work is done by the locomotive in mega joules?

4. Why is no work done, when a force acts at right angles to the direction of displacement?

5. The energy of a torch cell is converted into two other forms of energy in a flashlight bulb. Name the energy conversions.

6. A man climbs a slope and another walks the same distance on a level road. Which of the two expends more energy and why?

7. A nail becomes hot when hammered into a plank. Explain, why?

8. If you apply 1 J of energy to lift a book of. 0.5 kg, how high will it rise? [ g = 10 ms^–2]

9. What force will cause a displacement of 2 m, while doing a work of 60 J?

10. Calculate the work done by a machine of 50 W power rating in 30 s.

## Short Answer Type Questions-II (3 Marks)

1. A mass of 10 kg is dropped from a height of 50 cm. Find its : (a) potential energy just before dropping (b) kinetic energy just on touching the ground (c) velocity with which it hits the ground [Given g = 10 ms–2] [2011 (T-II)]

2. (i) Define the term potential energy. Write the S.I. unit of potential energy.

(ii) A body of mass 50 kg is situated at a height of 10 m. What is its potential energy (Given g = 10 ms–2) [2011 (T-II)]

3. (i) Define power. Mention its S.I. unit. (ii) A body of mass 50 kg runs up a staircase of 40 steps in 8 s. If the height of each step is 15 cm, find his power. (Given, g = 10 ms–2) [2011 (T-II)]

4. A light and heavy object have the same momentum, find out the ratio of their kinetic
energies. Which one has a larger kinetic energy? [2011 (T-II)]

5. (a) Can any object have momentum even if its mechanical energy is zero? Explain why?

(b) A ball is dropped from a height of 10 m. If energy of the ball reduces by 40% after
striking the ground, how high can the ball bounce back? (g = 10 m/s2). [2011 (T-II)]

6. (a) Define potential energy. Write an expression for potential energy of an object of mass m raised through a height h.

(b) Find the energy possessed by an object of mass 10 kg when it is raised to a height of six metre above the ground given g = 9.8 ms–2. [2011 (T-II)]

7. Define power. A boy of mass 45 kg climbs up 20 steps in 20 sec. If each step is 25 cm high, calculate the power of the boy used in climbing. (Take g = 10 m/s2) [2011 (T-II)]

8. (a) Write the relation between commercial unit of electricity and the S.I. unit of energy. [2011 (T-II)] (b) A boy of mass 45 kg climbs up 20 steps in 20 second. If each step is 25 cm high, calculate the power used in climbing.

9. (a) When is work done by a force negative? [2011 (T-II)] (b) Two bodies have their masses m1/m2 = 3 and their kinetic energies E1/E2 = 1/3. What will be the ratio of their velocities?

10. (a) Define one Watt.

(b) A lamp consumes 1000 J electrical energy in 10 s. What is its power? [2011 (T-II)]

11. The kinetic energy of an object of mass m moving with a velocity of 5 m/s is 25 J. Calculate its kinetic energy when its velocity is doubled? What will be its kinetic energy when its velocity is increased three times? [2011 (T-II)]

12. (a) If the speed of a particle is doubled, what will be the new kinetic energy?

(b) A spring is compressed, what change is expected in the potential energy of the
spring? [2011 (T-II)]

13. Calculate the electricity bill amount for a month of April, if 4 bulbs of 40 W for 5 hrs, 4 tube lights of 60 W for 5 hrs, a T.V. of 100 W for 6 hrs, a washing machine of 400 W for 3 hrs are used per day. The cost per unit is Rs. 1.80. [2011 (T-II)]

14. A 5 kg ball is thrown upwards with a speed of 10 m/s. (take g = 10 m/s2)
(a) Calculate the maximum height attained by it.
(b) Find the potential energy when it reaches the highest point. [2011 (T-II)]

15. Calculate the potential energy of an object of mass 50 kg raised to a height of 4 m above the ground. If the object falls down, what is the kinetic energy when it has fallen through 2 m? Take g = 9.8 m/s2. [2011 (T-II)]

16. (a) Name the form of energy associate in each case. [2011 (T-II)]

(i) A flying bird.

(ii) A man climbing the stairs

(iii) A compressed watch spring

(iv) A fast moving object.

(b) What is the commerical unit of energy? And state its relation with S.I. unit of energy.

17. (a) Name the energy transformation taking place in the following devices : [2011 (T-II)]

(i) a motor

(ii) a car engine

(iv) a nuclear reactor

(b) How much the work is done, when Earth moves around the Sun in its orbit?

18. (a) A truck and a car are running with same velocity. Which of the two has less kinetic energy?

(b) Give an example of electrical energy converted into light energy. [2011 (T-II)]

19. (a) The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?      [2011 (T-II)]

(b) An object is dropped from a height h. When is its

(i) potential energy maximum

(ii) kinetic energy maximum

20. Four persons jointly lift a 350 kg box to a height of 1 m and hold it.

(a) Calculate the work done by the persons in lifting the box.

(b) How much work do they do in just holding it?
(c) Why do they get tired while holding it? (g = 10 ms–2) [2011 (T-II)]

21. State law of conservation of energy. Explain with the help of a simple pendulum. Illustrate if necessary. [2011 (T-II)]

22. A boy throws a rubber ball vertically upwards. What type of work is done; [2011 (T-II)]

(a) by the force applied by the boy?

23. (a) 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 kinetic energy of the ball.

(b) Which would have greater effect on the kinetic energy of an object-doubling the mass
or doubling the velocity? [2011 (T-II)]

## OTHER IMPORTANT QUESTIONS

1. Derive an expression for kinetic energy of a body moving with a uniform velocity ‘v’ and having a mass ‘m’.

2. Two bodies A and B of masses 4 kg and 16 kg respectively have the same kinetic energy. Calculate the ratio of their velocities.

3. A boy of mass of 50 kg runs up a flight of 120 stairs, each measuring 25 cm in one minute and 40 seconds. Calculate the power of the boy. [Take g = 10 ms–2]

4. A motor pump of power 400 W operates for 2 minutes and 40 seconds and in doing so
raises 200 kg of water to the top of a building. If g = 10 ms–2, calculate the height of the
building.

5. A truck and a car are running at the same speed. If the mass of the truck is 10 times that of the car, calculate the ratio of the kinetic energy of the truck with respect to the car.

6. A mass of 10 kg is dropped from a height of 50 cm.

Find : (a) kinetic energy and (b) velocity, just as it reaches the ground. Does the velocity depend on the mass of the particle? Explain.

7. Write the energy change taking place when the heat of the Sun evaporates water, till it starts raining.

8. (i) A man whose mass is 50 kg, climbs up 30 steps of stairs in 30 seconds. If each step
is 20 cm high, calculate the power used in climbing the stairs.

(ii) A boy pushes a box through 10 m across a floor, offering 50 N resistance. What is the work done by the resisting force?

CBSE Class 9 Science