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Work, Energy & Power — Homi Bhabha Exam Physics MCQs with Solutions
Free Homi Bhabha Exam Physics Work, Energy & Power MCQs with step-by-step solutions covering Work Done, Kinetic & Potential Energy, Conservation of Energy, Power & Its Units. Practise online on Prepizo — no login needed.
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Sample questions with solutions
Q1 — Work Done · hard · theory
A coolie carrying a load on his head walks a distance on a level platform. The work done by him against gravity is:
A. maximum
B. zero ✓ Correct
C. equal to the distance walked
D. equal to the weight of the load
Solution: The load moves horizontally while gravity acts vertically, so the displacement in the direction of the gravitational force is zero — work done against gravity is zero.
Q2 — Work Done · hard · numerical
A force of 10 N acts on a body but the body does not move. The work done is:
A. 0 J ✓ Correct
B. 1 J
C. 10 J
D. 100 J
Solution: Work $= F \times s$; with zero displacement the work done is zero, however large the force.
Q3 — Work Done · hard · numerical
A body is lifted vertically through 4 m by applying a force equal to its weight of 25 N. The work done against gravity is:
A. 29 J
B. 21 J
C. 100 J ✓ Correct
D. 6.25 J
Solution: Work $= F \times h = 25 \times 4 = 100$ J.
Q4 — Work Done · hard · theory
When friction acts on a body opposite to its motion, the work done by friction is:
A. zero
B. infinite
C. positive
D. negative ✓ Correct
Solution: Friction acts opposite to the displacement, so the work it does on the body is negative.
Q5 — Kinetic & Potential Energy · hard · numerical
If the speed of a moving body is doubled, its kinetic energy becomes:
A. 8 times
B. unchanged
C. 4 times ✓ Correct
D. 2 times
Solution: K.E. $\propto v^2$, so doubling the speed multiplies the kinetic energy by $2^2 = 4$.
Q6 — Kinetic & Potential Energy · hard · numerical
Two bodies of masses 2 kg and 4 kg move with the same kinetic energy. The ratio of their speeds (2 kg : 4 kg) is:
A. $1 : \sqrt{2}$
B. 1 : 2
C. 2 : 1
D. $\sqrt{2} : 1$ ✓ Correct
Solution: Equal K.E.: $\tfrac12 m_1 v_1^2 = \tfrac12 m_2 v_2^2 \Rightarrow \dfrac{v_1}{v_2} = \sqrt{\dfrac{m_2}{m_1}} = \sqrt{2}$.
Q7 — Kinetic & Potential Energy · hard · numerical
A body of mass 1 kg is dropped from a height of 5 m. Just before hitting the ground (taking $g = 10$ m/s²) its kinetic energy is:
A. 10 J
B. 25 J
C. 5 J
D. 50 J ✓ Correct
Solution: By energy conservation, K.E. at bottom = initial P.E. $= mgh = 1 \times 10 \times 5 = 50$ J.
Q8 — Kinetic & Potential Energy · hard · theory
The work done by the net force acting on a body is equal to the change in its:
A. power
B. momentum
C. potential energy
D. kinetic energy ✓ Correct
Solution: By the work–energy theorem, net work done = change in kinetic energy.
Q9 — Conservation of Energy · hard · theory
For a freely falling body, as it falls its potential energy decreases and its kinetic energy:
A. remains constant
B. becomes zero
C. increases by the same amount ✓ Correct
D. decreases
Solution: P.E. converts to K.E. so that their sum (total mechanical energy) stays constant.
Q10 — Conservation of Energy · hard · numerical
A stone of mass 1 kg is thrown up and reaches a height of 8 m. Its potential energy at the top (taking $g = 10$ m/s²) equals its initial kinetic energy of:
A. 8 J
B. 800 J
C. 80 J ✓ Correct
D. 40 J
Solution: At the top all K.E. becomes P.E.: initial K.E. $= mgh = 1 \times 10 \times 8 = 80$ J.
Q11 — Conservation of Energy · hard · theory
A body falls from a height. At the mid-point of its fall (ignoring air resistance):
A. it has only kinetic energy
B. its total energy is halved
C. it has only potential energy
D. its kinetic energy equals its potential energy ✓ Correct
Solution: At half the height, half the initial P.E. has become K.E., so K.E. = P.E. there (total energy unchanged).
Q12 — Power & Its Units · hard · numerical
A boy of weight 400 N climbs a staircase of height 5 m in 10 s. His power is:
A. 2000 W
B. 200 W ✓ Correct
C. 20 W
D. 4000 W
Solution: Work $= 400 \times 5 = 2000$ J; power $= \dfrac{2000}{10} = 200$ W.
Q13 — Power & Its Units · hard · numerical
The number of joules in 1 kilowatt-hour (kWh) is:
A. 1000 J
B. 3,600,000 J ✓ Correct
C. 60,000 J
D. 3600 J
Solution: 1 kWh $= 1000$ W $\times 3600$ s $= 3.6 \times 10^6$ J.
Q14 — Power & Its Units · hard · numerical
A pump raises 100 kg of water to a height of 10 m in 20 s (taking $g = 10$ m/s²). Its power is:
A. 500 W ✓ Correct
B. 50 W
C. 250 W
D. 1000 W
Solution: Work $= mgh = 100 \times 10 \times 10 = 10000$ J; power $= \dfrac{10000}{20} = 500$ W.
Q15 — Work Done · medium · numerical
A force of 20 N moves a body 5 m in the direction of the force. The work done is:
A. 25 J
B. 4 J
C. 15 J
D. 100 J ✓ Correct
Solution: Work $= F \times s = 20 \times 5 = 100$ J.
Q16 — Work Done · medium · theory
Work is done on a body only when:
A. a force acts and the body moves in the direction of (a component of) the force ✓ Correct
B. the body is at rest
C. the body has energy
D. a force acts on the body
Solution: Work requires both a force and a displacement along the force; either alone gives zero work.
Q17 — Work Done · medium · theory
The SI unit of work is the joule, which is equivalent to:
A. newton per metre
B. newton·metre (N·m) ✓ Correct
C. kilogram·metre
D. watt·metre
Solution: 1 joule = 1 newton × 1 metre (N·m).
Q18 — Kinetic & Potential Energy · medium · numerical
The kinetic energy of a 2 kg body moving at 10 m/s is:
A. 20 J
B. 10 J
C. 100 J ✓ Correct
D. 200 J
Solution: K.E. $= \tfrac12 m v^2 = \tfrac12 (2)(10^2) = 100$ J.
Q19 — Kinetic & Potential Energy · medium · numerical
The potential energy of a 5 kg body raised to a height of 4 m (taking $g = 10$ m/s²) is:
A. 100 J
B. 20 J
C. 9 J
D. 200 J ✓ Correct
Solution: P.E. $= mgh = 5 \times 10 \times 4 = 200$ J.
Q20 — Kinetic & Potential Energy · medium · theory
A stretched bow (about to shoot an arrow) possesses:
A. potential energy ✓ Correct
B. sound energy
C. no energy
D. only kinetic energy
Solution: A stretched bow stores elastic potential energy, which converts to the arrow's kinetic energy on release.
Q21 — Kinetic & Potential Energy · medium · numerical
If the mass of a moving body is doubled without changing its speed, its kinetic energy becomes:
A. 4 times
B. half
C. unchanged
D. 2 times ✓ Correct
Solution: K.E. $= \tfrac12 m v^2 \propto m$; doubling the mass doubles the kinetic energy.
Q22 — Conservation of Energy · medium · theory
The law of conservation of energy states that energy:
A. can be created but not destroyed
B. is lost during every change
C. can neither be created nor destroyed, only changed from one form to another ✓ Correct
D. can be destroyed but not created
Solution: The total energy of an isolated system stays constant; it only transforms between forms.
Q23 — Conservation of Energy · medium · theory
In a simple pendulum swinging back and forth, the energy continuously changes between:
A. sound and kinetic
B. heat and light
C. kinetic and potential energy ✓ Correct
D. chemical and potential
Solution: At the extremes energy is potential; at the mean position it is kinetic — the two interchange.
Q24 — Conservation of Energy · medium · theory
When a ball is dropped and bounces to a lower height each time, the "lost" mechanical energy is mainly converted into:
A. nuclear energy
B. chemical energy
C. heat and sound ✓ Correct
D. light and electricity
Solution: Energy is not destroyed; it is dissipated as heat and sound during each bounce.
Q25 — Conservation of Energy · medium · theory
In a hydroelectric power plant, the energy of falling water is converted mainly into:
A. electrical energy ✓ Correct
B. nuclear energy
C. chemical energy
D. sound energy
Solution: The potential/kinetic energy of falling water spins turbines and generators to produce electrical energy.
Q26 — Power & Its Units · medium · numerical
A machine does 600 J of work in 4 s. Its power is:
A. 604 W
B. 2400 W
C. 150 W ✓ Correct
D. 0.0067 W
Solution: Power $= \dfrac{\text{work}}{\text{time}} = \dfrac{600}{4} = 150$ W.
Q27 — Power & Its Units · medium · theory
One watt is equal to:
A. 1 joule × 1 second
B. 1 joule per second ✓ Correct
C. 1 joule per metre
D. 1 newton per second
Solution: Power is the rate of doing work: 1 W = 1 J/s.
Q28 — Power & Its Units · medium · theory
Two machines do the same amount of work, but machine A takes less time than machine B. Then:
A. machine B has greater power
B. power cannot be compared
C. machine A has greater power ✓ Correct
D. both have equal power
Solution: For the same work, less time means greater power ($P = W/t$).
Q29 — Power & Its Units · medium · numerical
An electric bulb rated 60 W is used for 1 minute. The electrical energy it consumes is:
A. 216000 J
B. 360 J
C. 60 J
D. 3600 J ✓ Correct
Solution: Energy $= P \times t = 60 \times 60 = 3600$ J.
Q30 — Power & Its Units · medium · theory
1 horsepower (hp) is approximately equal to:
A. 746 W ✓ Correct
B. 1000 W
C. 100 W
D. 1 W
Solution: 1 hp $\approx 746$ watts.