CBSE Class 9 Science Work and Energy Worksheet Set B

Question. Which of the following statements is true about work done?
(i) Work done by a force is always positive.
(ii) SI unit of work is joule.
(iii) Work has both, magnitude and direction.
(iv) Work is said to be done if an object is displaced when a force acts on it.
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (ii) and (iv)
(d) (i), (ii), (iii) and (iv)
Answer. C

Question. A man of weight 60 kg wt takes a body of mass 15 kg at a height 10 m on a building in 3 minutes. The efficiency of man is
(a) 10%
(b) 20%
(c) 30%
(d) 40%
Answer. B

Question. When an ideal simple pendulum oscillates between the extreme points P and Q, there is continuous (i) of potential energy and kinetic energy. The potential energy depends on the choice of (ii). Force acting on the bob of the pendulum is maximum at (iii) , and minimum at (iv).
(i) (ii) (iii) (iv)
(a) Dissipation reference mid point extremes level
(b) Dissipation mass mid point extremes
(c) Exchange reference extremes mid point level
(d) Exchange velocity extremes mid point
Answer. C

Question. Which of the following statements is/are incorrect?
(i) Work and energy have different units.
(ii) Two bodies of unequal masses have equal acceleration at any instant of time when they are dropped from a cliff.
(iii) When an aeroplane takes off, the work done by its weight is positive.
(iv) When the speed of a particle is doubled, the ratio of its momentum and kinetic energy gets halved.
(a) (ii) only
(b) (i) and (iii) only
(c) (i), (ii) and (iv) only
(d) (i), (ii), (iii) and (iv)
Answer. B

Question. Column-I Column-II
(P) Force                  1. kg m²s^–2
(Q) Work                 2. kg m s^–1
(R) Momentum         3. kg m² s^–3
(S) Power                4. kg m s^–2
(a) P-4, Q-1, R-2, S-3
(b) P-3, Q-2, R-4, S-1
(c) P-3, Q-4, R-2, S-1
(d) P-1, Q-4, R-3, S-2
Answer. A

Question. A body rolls down on inclined plane, it has
(a) only kinetic energy
(b) only potential energy
(c) both kinetic energy and potential energy
(d) neither kinetic energy nor potential energy
Answer. C

Question. When the momentum of a body is increased by 100%, its K.E. increases by
(a) 100%
(b) 200%
(c) 300%
(d) 400%
Answer. C

Question. A rocket rises up in air due to the force generated by the fuel. The work done by the
(a) fuel is negative work and by force of gravity is positive work
(b) fuel is positive work and by force of gravity is negative work
(c) both fuel and force of gravity do positive work
(d) both fuel and force of gravity do negative work.
Answer. B

Question. Two masses of 1 g and 4 g are moving with equal linear momenta. The ratio of their kinetic energies is :
(a) 4 : 1
(b) 2 : 1
(c) 1 : 2
(d) 1 : 16
Answer. A

Question.At sea level, a nitrogen molecule in air has an average translational kinetic energy of 6.2 × 10^–21 J. Its mass is 4.7 × 10^–26 kg. If the molecule shoots up straight without resistance,it will rise to a height of
(a) 1.35 km
(b) 13.5 km
(c) 135 km
(d) 1350 km
Answer. B

Question. A rod of mass m and length l is lying on a horizontal table. Work done in making it stand on one end will be
(a) mgl
(b) mgl/2
(c) mgl/4
(d) 2mgl
Answer. B

Question. The power of a water pump is 2 kW. If g = 10 m/s², the amount of water it can raise in 1 min to a height of 10 m is
(a) 2000 litre
(b) 1000 litre
(c) 100 litre
(d) 1200 litre
Answer. D

Question. In which of the following cases is the potential energy of a spring minimum?
(a) When it is compressed.
(b) When it is extended.
(c) When it is at its natural length.
(d) When it is at its natural length but is kept at a height h above the ground.
Answer. C

Question. Column-I            Column-II
(P) Kinetic energy             1. mgh
(Q) Power                        2. 1/2 mv²
(R) Potential energy         3. mv
(S) Momentum                 4. W/t
(a) P-2, Q-4, R-1, S-3
(b) P-3, Q-2, R-4, S-1
(c) P-3, Q-4, R-2, S-1
(d) P-1, Q-4, R-3, S-2
Answer. A

Question. The speed of a motor bike decreases by 4 times. Its kinetic energy will decrease by
(a) four times
(b) eight times
(c) sixteen times
(d) thirty two times.
Answer. C

Question.  The work required to be done to stop a car of 2500 kg moving at a velocity of 90 km h^–1 is
(a) –781.25 kJ
(b) 781.25 kJ
(c) 2250 kJ
(d) –2250 kJ.
Answer. A

Question. A rocket rises up vertically. What happens to its potential energy?
(a) It increases.
(b) It initially increases then decreases.
(c) It initially decreases then increases.
(d) It increases, till it becomes maximum.
Answer. D

Assertion & Reasoning Based Class 9 Science Chapter 11 Work and Energy

For question numbers 41-50, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(a) Both assertion and reason are true, and reason is correct explanation of the assertion.
(b) Both assertion and reason are true, but reason is not the correct explanation of the assertion.
(c) Assertion is true, but reason is false.
(d) Assertion is false, but reason is true.

Question. Assertion : Work done by or against gravitational force in moving a body from one point to another is independent of the actual path followed between the two points.
Reason : Gravitational forces are conservative forces.
Answer. A

Question. Assertion : A crane P lifts a car upto a certain height in 1 min. Another crane Q lifts the same car upto the same height in 2 min.
Then crane P consumes two times more fuel than crane Q.
Reason : Crane P supplies two times more power than crane Q.
Answer. A

Question. Assertion : No work is done when a woman carrying a load on her head, walks on a level road with a uniform velocity.
Reason : No work is done if force is perpendicular to the direction of displacement.
Answer. A

Question. Assertion : Distance covered in stopping a moving body is equal to kinetic energy/force applied.
Reason : Work done in brining a body to just stop must be equal to the kinetic energy of the body.
Answer. A

Question. Assertion : Work done by the gravitational force through a certain distance is constant irrespective of the fact that the body has a uniform or accelerated motion.
Reason : Gravitational force is a conservative force.
Answer. B

Question. Assertion : According to law of conservation of mechanical energy, change in potential energy is equal and opposite to the change in kinetic energy.
Reason : Mechanical energy is not a conserved quantity.
Answer. C

Question. Assertion : Graph between potential energy of a spring versus the extension or compression of the spring is a straight line.
Reason : Potential energy of a stretched or compressed spring, is directly proportional to square of extension or compression.
Answer. D

Question. Assertion : The kinetic energy, with any reference, must be positive.
Reason : It is because, in the expression for kinetic energy, the velocity appears with power 2 and mass is a scalar quantity.
Answer. A

Question. Assertion : The change in kinetic energy of a particle is equal to the work done on it by the net force.
Reason : Change in kinetic energy of particle is equal to the work done only in case of a system of one particle.
Answer. C

Question. Assertion : Work done against or by the force of friction in moving a body around a closed loop is zero.
Reason : Work done depends upon the nature of force.
Answer. D

Very Short Answer Class 9 Science Chapter 11 Work and Energy

Question. What is the unit of energy? 
Answer. The unit of energy is joules.

Question. What do you mean by kilo joule? 
Answer. A larger unit of energy is called kilo joule. 1 KJ equals to 1000J.

Question. Define energy?
Answer. The capacity of a body to do work is called the energy of the body.

Question. Wha is the unit of power? 
Answer. The unit of power is watt.

Question. What do you understand by 1kWh?
Answer. The energy used in one hour at the rate of 1KW is called 1KWh.

Short Answer Class 9 Science Chapter 11 Work and Energy

Question. How do we know that some entity is a form of energy 
Answer. If something is capable of doing work then that entity is assumed to have energy be it in any form. For example: a heat is a form of energy which makes the air particles move,sound is a form of energy which makes them vibrate.

Question. Distinguish between power and average power? 
Answer. Power is the rate (energy amount per time period) at which work is done or energy converted whereas the average is the average amount of work done or energy transferred per unit time.

Question. Define kinetic energy and mechanical energy? 
Answer. Kinetic energy is the energy possessed by a body due to its motion. Kinetic energy of an object increases with its speed. Kinetic energy of body moving with a certain velocity =work done on it to make it acquire that velocity.
The energy acquired by the objects upon which work is done is known as mechanical energy. Mechanical energy is the energy that is possessed by an object due to its motion or due to its position.

Question. What will happen if nature will not allowed the transformation of energy?
Answer. If the nature will not allow the transformation of energy then many of the process will not take place like, cooking food which is transformation of electrical into thermal energy would not be possible, transformation of energy stored in food into energy required by growth of an individual will not take place.

Question. How much the energy owned by a moving body by virtue of its motion? 
Answer. If the mass is m of the moving body and υ is the velocity with which it moves, the energy possessed by it is given by 12mυ2. This is called the kinetic energy of the moving body.

Long Answer Class 9 Science Chapter 11 Work and Energy

Question. Explain potential energy and types of potential energy?
Answer. Potential energy is the energy possessed by a body by virtue of its position. The potential energy of an object at a height depends on the ground level or the zero level we choose.
An object in a given position can have a certain potential energy with respect to one level and a different value of potential energy with the respect to another level. The types of potential energy are:
1. Gravitational potential energy: Gravitational potential energy is energy an object possesses because of its position in a gravitational field. The use of gravitational potential energy is for an object near the surface of the Earth where the gravitational acceleration can be assumed to be constant at about 9.8 m/s2.
2. Elastic potential energy: Elastic potential energy is the potential energy stored as a result of deformation of an elastic object, like the stretching of a spring. It is equal to the work done to stretch the spring, which depends upon the spring constant k as well as the distance stretched.
3. Nuclear potential energy: Nuclear potential energy is the potential energy of the particles such as protons and neutrons that are present inside the nucleus of an atom. The process by which two or more nuclei of the atoms get combined to form a large nucleus is called nuclear fusion.
4. Chemical potential energy: Chemical potential energy is the energy stored in the chemical bonds of a substance. The various chemicals that make up gasoline contain a large amount of chemical potential energy that is released when the gasoline is burned in a controlled way in the engine of the car.

Question. Explain the law of conservation of energy? 
Answer. 
The law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be conserved over time. This law means that energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another. According to this law energy can only be converted from one form to another. The total energy before and after the transformation remains the same. For example: Water can produce electricity. Water falls from the sky, converting potential energy to kinetic energy. This energy is then used to rotate the turbine of a generator to produce electricity. In this process, the potential energy of water in a dam can be turned into kinetic energy which can then become electric energy. Another example for law of conservation is: two football players collided on the field, and both went flying backwards. Energy was transferred from each player to the other, sending them in the opposite direction from which they had been running.

Question. An electric bulb of 60 W is used for 7h per day. What are the units of energy consumed in one day by the bulb? 
Answer. 
The power of electric bulb is = 60 W
= 60 / 1000
= 0.06 kW
Time used t is = 7 h
Energy = power * time taken
= 0.06 kW * 7 h
= 0.42 kW h
= 0.42 units
So, the energy consumed by the bulb is 0.42 units.

Question. A boy of mass 60 kg runs up a stairs of 45 steps in 8s. If the height of ach step is 15cm. what is his power? Given g = 10 ms^-2? 
Answer. 
Weight of the boy is mg = 60 kg * 10 ms^-2
= 600N
Height of the staircase is h = 45 * 15 / 100m
= 675 / 100
= 6.75 m
Time taken to climb t is = 8 s
We know that,
Power (p) = work done / time taken
= mgh / t
= 600 N * 6.75 m / 8 s
= 4050 / 8
= 506 W
So the power is 506 W.

Question. An object of mass is 14 kg is at the certain height above the ground. If the potential energy of the object is 490J. What is the height of the object is with the respect to the ground. Given g = 10 ms^-2?
Answer. 
Mass of the object m = 14 kg
Potential energy Ep = 490 J
We know that,
Ep = mgh
490 J = 14 kg * 10 ms-2 * h
490 J = 140 kg ms-2 * h
So,
h = 490 J / 140 kg ms-2
= 3.5 m
So, the object is at the height of 3.5 m

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