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Work, Energy & Power — Homi Bhabha Class 9 Physics MCQs with Solutions
Free Homi Bhabha Class 9 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 · easy · theory
When a force acts on a body but the body moves in a direction perpendicular to the force, the work done by that force is...
A. Negative
B. Zero ✓ Correct
C. Equal to the force
D. Maximum
Solution: Work = force × distance × cos(angle). When the angle is 90 degrees, cos 90 = 0, so the work done is zero.
Q2 — Work Done · easy · numerical
A force of 10 N moves a box 5 m in the direction of the force. How much work is done?
A. 2 J
B. 50 J ✓ Correct
C. 15 J
D. 500 J
Solution: Work = force × distance = 10 N × 5 m = 50 J, since the force and displacement are in the same direction.
Q3 — Kinetic & Potential Energy · easy · theory
The kinetic energy of a moving body depends on...
A. Both its mass and its speed ✓ Correct
B. Only its shape
C. Only its height above the ground
D. Only its mass
Solution: Kinetic energy = 1/2 × mass × speed squared, so it depends on both the mass and the speed of the body.
Q4 — Conservation of Energy · easy · theory
According to the law of conservation of energy, energy can...
A. Be destroyed but not created
B. Keep increasing on its own
C. Be created but not destroyed
D. Neither be created nor destroyed, only change from one form to another ✓ Correct
Solution: Energy can only be transformed from one form into another. The total amount of energy in an isolated system stays constant.
Q5 — Conservation of Energy · easy · theory
As a ball falls freely toward the ground, its...
A. Kinetic and potential energy both decrease
B. Potential energy decreases and kinetic energy increases ✓ Correct
C. Kinetic and potential energy both stay the same
D. Potential energy increases and kinetic energy decreases
Solution: As the ball falls its height decreases, so its potential energy decreases, while its speed increases, so its kinetic energy increases. The total stays constant.
Q6 — Power & Its Units · easy · theory
The SI unit of power is the...
A. Watt ✓ Correct
B. Newton
C. Joule
D. Watt-hour
Solution: Power is measured in watts, where 1 watt equals 1 joule per second.
Q7 — Power & Its Units · easy · theory
Power is best defined as...
A. Force multiplied by time
B. The rate of doing work ✓ Correct
C. The energy stored in a body
D. The total work done
Solution: Power = work done ÷ time taken, which is the rate at which work is done.
Q8 — 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.
Q9 — 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.
Q10 — 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.
Q11 — 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.
Q12 — 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 $2^2 = 4$.
Q13 — 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.: $\tfrac12 m_1 v_1^2 = \tfrac12 m_2 v_2^2 \Rightarrow \dfrac{v_1}{v_2} = \sqrt{\dfrac{m_2}{m_1}} = \sqrt{2}$.
Q14 — 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.
Q15 — 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.
Q16 — 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.
Q17 — 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.
Q18 — 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).
Q19 — 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.
Q20 — 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.
Q21 — 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.
Q22 — Work Done · hard · numerical
A 2 kg block is first lifted vertically upward by 3 m and then carried horizontally by 4 m. Taking g = 10 m/s squared, the total work done against gravity is...
A. 80 J
B. 100 J
C. 140 J
D. 60 J ✓ Correct
Solution: Work against gravity is done only during the vertical lift: mgh = 2 × 10 × 3 = 60 J. Carrying the block horizontally is perpendicular to gravity and adds no work against it, so the total is 60 J.
Q23 — Kinetic & Potential Energy · hard · numerical
Two bodies A and B have masses in the ratio 1:2 and speeds in the ratio 2:1. The ratio of their kinetic energies (A:B) is...
A. 4:1
B. 1:1
C. 1:2
D. 2:1 ✓ Correct
Solution: KE ratio = (m of A × v of A squared) : (m of B × v of B squared) = (1 × 2 squared) : (2 × 1 squared) = 4 : 2 = 2 : 1.
Q24 — Kinetic & Potential Energy · hard · numerical
An object of mass 2 kg is dropped from rest. Just before it hits the ground its kinetic energy is 400 J. Taking g = 10 m/s squared, the height from which it was dropped is...
A. 200 m
B. 20 m ✓ Correct
C. 10 m
D. 40 m
Solution: At the ground all the potential energy has become kinetic energy: mgh = 400, so h = 400 ÷ (2 × 10) = 20 m.
Q25 — Conservation of Energy · hard · numerical
A pendulum bob of mass 0.2 kg is released from a point 0.45 m above its lowest position. Taking g = 10 m/s squared, its speed at the lowest point is...
A. 4.5 m/s
B. 9 m/s
C. 1.5 m/s
D. 3 m/s ✓ Correct
Solution: The potential energy at release becomes kinetic energy at the bottom: v = square root of (2 × g × h) = square root of (2 × 10 × 0.45) = square root of 9 = 3 m/s. The mass cancels out.
Q26 — Power & Its Units · hard · numerical
A pump lifts 200 kg of water to a height of 10 m in 20 s. Taking g = 10 m/s squared, the power of the pump is...
A. 2000 W
B. 100 W
C. 1000 W ✓ Correct
D. 400 W
Solution: Work done = mgh = 200 × 10 × 10 = 20000 J. Power = work ÷ time = 20000 J ÷ 20 s = 1000 W.
Q27 — Power & Its Units · hard · numerical
A 60 W bulb is kept switched on for 5 hours. The electrical energy it consumes, in kilowatt-hours, is...
A. 0.06 kWh
B. 0.3 kWh ✓ Correct
C. 3 kWh
D. 300 kWh
Solution: 60 W = 0.06 kW. Energy = power × time = 0.06 kW × 5 h = 0.3 kWh.
Q28 — 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.
Q29 — 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.
Q30 — 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).