Chapter 13: WORK AND ENERGY
Chapter 13. Work and Energy Class 10 Science and Technology (212) NIOS Textbook Solutions
Chapter 13. WORK AND ENERGY
INTEXT QUESTIONS 13.1
Choose the correct option
1. (i) Work done is zero:
(a) When force and displacement are in the same direction.
(b) When force and displacement of the body are in opposite directions
(c) When force acting on the body is perpendicular to the direction of the displacement of the body.
(d) When force makes an angle with displacement
Answer: (c) When force acting on the body is perpendicular to the direction of displacement of the body.
[ Work is done only when force causes displacement in its own direction. If force acts perpendicular to displacement, it does not move the body in that direction, so work done is zero.]
(ii) 1 J of work is done when a force of 0.01 N moves a body through a distance of :
(a) 0.01 m (b) 0.1 m (c) 1 m (d) 10 m (e) 100 m
Answer: (e) 100 m
[ We know that, work done = force × distance
1 J = 0.01 N × Distance
Distance 
Therefore, 1 J of work is done when a force of 0.01 N moves a body through 100 m distance.
(iii) In which of the following situations work is done?
(a) A person is climbing up a stair case.
(b) A satellite revolving around the earth in closed circular orbit
(c) Two teams play a tug of war and both pull with equal force
(d) A person is standing with heavy load on his head.
(a) A person is climbing up a staircase.
[ Work is done when force causes displacement. While climbing stairs, the person applies force and moves upward, so work is done. In the other cases, there is no displacement in the direction of force.]
2. A car of mass 500 kg is moving with a constant speed of 10 m/s on a rough horizontal road. Force expended by the engine of the car is 1000 N. Calculate work done in 10 s by:
(a) net force on the car (b) gravitational force
(c) the engine (d) frictional force
Answer: Given, Mass of car = 500 kg , Speed = 10 m/s , Engine force = 1000 N and Time = 10 s
Distance = Speed × Time = 10 × 10 = 100 m
(a) Since the car moves with constant speed, acceleration = 0, so net force = 0.
Work done = 0 J
(b) Gravitational force acts vertically downward, but displacement is horizontal.
Work done = 0 J
(c) Work = Force × Distance
= 1000 × 100
= 100000 J
Work done = 100000 J=105 
J
(d) Since speed is constant, frictional force = engine force = 1000 N and acts opposite to motion.
Work = – Force × Distance
= –1000 × 100
= –100000 J=-105 J 
Work done 
INTEXT QUESTIONS 13.2
1. Explain the terms work and energy with one example each.
Answer: Work is said to be done when force is applied on a body and the body moves through some distance in the direction of force. Example: A person climbing up a staircase.
Energy: The ability to do work is called energy. Example: A weightlifter lifts the weight.
2. The ability to do work is called ……………
Answer: The ability to do work is called Energy.
3. The SI unit of all forms of energy is ……………
Answer: The SI unit of all forms of energy is Joule (J).
4. Energy possessed by a spring is …………… energy.
Answer: Energy possessed by a spring is Potential energy.
5. The energy possessed by a body due to its position is called …………… energy.
Answer: The energy possessed by a body due to its position is called Potential energy.
6. The energy possessed by a body due to its motion is called …………… energy.
Answer: The energy possessed by a body due to its motion is called Kinetic energy.
7. At height h the potential energy is EP
at height h/2 the potential energy would be ……………
Answer: At height h the potential energy is Eₚ, at height h/2 the potential energy would be Eₚ/2.
8. At height h the potential energy of a body of mass m is Ep
. At the same height the potential energy of a body of mass m/2 would be ……………
Answer: At height h the potential energy of a body of mass m is Eₚ. At the same height the potential energy of a body of mass m/2 would be Eₚ/2.
9. A body of mass m moving with a speed v has kinetic energy, Ek
. The body if moves with speed 2v, will have kinetic energy equal to ……………
Answer: A body of mass m moving with speed v has kinetic energy Eₖ. If it moves with speed 2v, kinetic energy becomes 4Eₖ.
10. A body of mass m moving with a speed v has kinetic energy Ek . A body of mass 2m moving with the same speed will have a kinetic energy………….
Answer: A body of mass m moving with speed v has kinetic energy Eₖ. A body of mass 2m moving with the same speed will have kinetic energy 2Eₖ.
INTEXT QUESTIONS 13.3
Give one example each of the following energy transformations.
1. (i) Light energy into chemical energy.
(ii) Chemical energy into heat.
(iii) Chemical energy into electrical energy.
(iv) Mechanical energy into electrical energy.
(v) Thermal energy into electrical energy.
(vi) Light energy into electrical energy.
Answer: (i) In photosynthesis green plants transform light energy into chemical energy of carbohydrates.
(ii) In digestion of food chemical energy of food is converted into heat.
(iii) Electrical cells convert chemical energy into electrical energy.
(iv) In electric generators mechanical energy is converted into electrical energy.
(v) In thermal power plants heat energy is converted into electrical energy.
(vi) In Solar Cells Light energy is converted into electrical energy.
2. (i) A motor converts electrical energy into .....................
(ii) An electric heater converts electrical energy into .....................
(iii) A microphone converts sound energy into .....................
(iv) A loudspeaker converts sound energy into .....................
(v) A heat engine converts heat energy into .....................
(vi) When we rub our hands together we change work into .....................
Answer: (i) mechanical energy
(ii) heat energy
(iii) electrical energy
(iv) electrical energy
(v) mechanical energy
(vi) heat energy
INTEXT QUESTIONS 13.4
1. Kamya climbs up a staircase in 5 minutes, Suraiya takes only 3 minutes in going up the same staircase. The weight of Kamya is equal to the weight of Suraiya.
(i) Which of the two does more work?
(ii) Which of the two spends more power?
Answer: (i) They both do work against gravity. Because both of them have equal weight and climb equal height they do equal work.
(ii) Because Suraiya takes lesser time in climbing up the staircase and power is inversely proportional to time so, Suraiya spends more power.
2. Express 1.5 H.P. in SI Unit of power.
Answer: SI unit of power is watt
and 1 H.P. = 746 watt
1.5 H.P. = 746 × 1.5 = 1119.0 watt = 1.12 kW
3. One cricket ball and One plastic ball are dropped from the same height. Which will reach the ground with
(a) more energy, (b) less power?
Answer: (a) More energy – Cricket ball
[ The cricket ball has greater mass, so it possesses more gravitational potential energy and reaches the ground with more energy.]
(b) Less power – Plastic ball
[ The plastic ball has less mass, so less energy is converted per unit time and its power is less.]
TERMINAL EXERCISE
1. Define the following terms and give their SI units. (a) Work (b) Power (c) Energy
Answer: (a) Work is said to be done when a force causes displacement of a body in the direction of the force. SI unit: Joule (J)
(b) The rate of doing work is called power. SI unit: Watt (W)
(c) Energy is the capacity (ability) to do work. SI unit: Joule (J)
2. List different forms of energy.
Answer: The different forms of energy are:
(i) Mechanical energy (Kinetic energy and Potential energy)
(ii) Heat (Thermal) energy
(iii) Light energy
(iv) Sound energy
(v) Electrical energy
(vi) Chemical energy
(vii) Nuclear energy
(viii) Solar energy
3. State Law of Conservation of Energy. Explain with the help of examples.
Answer: Energy can neither be created nor destroyed. It can only be converted from one form into another. Therefore, the total amount of energy remains constant.
Examples: (i) Electric bulb: Electrical energy is converted into light energy and heat energy.
(ii) Falling object: As a body falls, its potential energy changes into kinetic energy, but total energy remains constant.
(iii) Hydroelectric power plant: Potential energy of stored water → kinetic energy → electrical energy.
4. List the energy transformation taking place in a thermal power plant.
Answer: Energy transformations in a thermal power plant:
Chemical energy (fuel) → Heat energy → Mechanical energy → Electrical energy
[ Fuel is burned to produce heat, heat converts water into steam, steam rotates the turbine (mechanical energy), and the generator converts it into electrical energy.]
5. A ball of mass 0.5 kg has100 J of kinetic energy. What is the velocity of the ball?
Answer: Given, Mass of ball (m) = 0.5 kg and Kinetic energy (EK
) = 100 J
We have,
EK=12mv2
⇒100=12×0.5×v2
⇒v2=2000.5=400 
⇒v2=202
⇒v=20 m/s 
Therefore, the velocity of the ball is 20 m/s.
6. A body of mass 100 kg is lifted up by 10 m. Find
(a) The amount of work done.
(b) Potential energy of the body at that height (g =10 m/s² )
Answer: Given, Mass (m) = 100 kg , Height (h) = 10 m and Acceleration due to gravity (g) = 10 m/s²
(a) We have, W = mgh
W=100×10×10 J
W=10000 J
Therefore, the work done is 10000 J
(b) We have, PE = mgh
PE=100×10×10 =10000 J
Therefore, the potential energy is 10000 J
7. Why road accidents at high speeds are much worse than the accidents at low speeds?
Answer: Road accidents at high speeds are much worse because a moving vehicle possesses more kinetic energy at higher speed. During an accident, this large amount of energy is released, causing greater damage to the vehicle and more serious injuries.
8. Two bodies of equal mass move with uniform velocities u and 4 u respectively. Find the ratio of their kinetic energies.
Answer: Given, Mass of both bodies = equal and Velocities = u and 4u
Formula for kinetic energy, 
For first body: Here, v=u 
For second body: Here, 
Therefore, the ratio of their kinetic energies is 1 : 16.
9. What would you like to prefer a ramp or a staircase to reach at the third floor of your hospital? Justify.
Answer: I would prefer a ramp to reach the third floor of the hospital.
A ramp requires less force to move upward because the effort is applied over a longer distance. It is more convenient for patients, wheelchairs, stretchers, and elderly people. Therefore, a ramp makes movement easier and safer.
Posted 3 weeks ago