Chapter 5: Work, Energy and PowerMCQs with Answers
All 24 MCQs of 9th Class Physics Chapter 5, Work, Energy and Power, with answers and the reason each option is right or wrong: the 9 textbook exercise MCQs plus 15 more board-style MCQs. Punjab Board new syllabus.
Try each one first: tap an option to check it, then read the reasoning and why the others are wrong.
A note about the questions
The wording follows published solved exercises and every answer was re-checked by hand, so the wording or the order of options may differ slightly from your print.
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Set A: Textbook exercise 9 MCQs
The multiple-choice questions at the end of Chapter 5 in your book, in the book's option order.
Q5.1Work
Work done is maximum when the angle between the force F and the displacement d is:
Answer(a) 0°
Step by step
Work at an angle: W = Fd cos θ.
cos 0° = 1, cos 30° = 0.866, cos 60° = 0.5, cos 90° = 0.
So the work is largest at 0°.
Why the others are wrong
(b) at 30° only 0.866 of the force does work.
(c) at 60° only half the force does work.
(d) at 90° no work is done (cos 90° = 0).
Remember
Push along the motion for the most work.
Q5.2Work
A joule can also be written as:
Answer(d) kg m² s⁻²
Step by step
W = F × S, so J = N × m.
N = kg m s⁻².
J = kg m s⁻² × m = kg m² s⁻².
Why the others are wrong
(a) kg m s⁻² is the newton, a unit of force.
(b) kg m s⁻¹ is the unit of momentum.
(c) kg m² s⁻³ is the watt (J s⁻¹).
Remember
Joule = newton × metre: add one more m to the newton.
Q5.3Power
The SI unit of power is:
Answer(c) watt
Step by step
P = W ÷ t.
Unit: J ÷ s = J s⁻¹, named the watt.
Why the others are wrong
(a) the joule is the unit of work and energy.
(b) the newton is the unit of force.
(d) the second is the unit of time.
Remember
Bulbs and motors are rated in watts.
Q5.4Power
The power of a water pump is 2 kW. The amount of water it can raise in one minute to a height of 5 m is:
Answer(d) 2400 litres
Step by step
Energy given in 60 s: W = P × t = 2000 × 60 = 120 000 J.
This raises water: W = mgh, so m = W ÷ (gh) = 120 000 ÷ 50 = 2400 kg.
1 kg of water is 1 litre, so 2400 litres (taking g = 10 m s⁻²).
Why the others are wrong
(a) 1000 litres would need only 50 000 J.
(b) 1200 litres is only half; that is what a 1 kW pump would raise.
(c) 2000 litres just copies the 2000 W; the time (60 s) and height (5 m) must be used.
Remember
m = Pt ÷ (gh); 1 kg of water = 1 litre.
Q5.5Kinetic Energy
A bullet of mass 0.05 kg has a speed of 300 m s⁻¹. Its kinetic energy will be:
Answer(a) 2250 J
Step by step
Ek = ½mv².
v² = 300² = 90 000.
Ek = 0.5 × 0.05 × 90 000 = 2250 J.
Why the others are wrong
(b) 4500 J forgets the ½.
(c) 1500 J does not come from ½mv².
(d) 1125 J halves the answer twice.
Remember
Square the speed first.
Q5.6Kinetic Energy
If a car doubles its speed, its kinetic energy will be:
Answer(d) increased to four times
Step by step
Ek ∝ v² for a fixed mass.
New Ek = ½m(2v)² = 4 × ½mv².
Why the others are wrong
(a) kinetic energy depends on speed.
(b) doubling would be true only if Ek ∝ v.
(c) 3 does not come from 2².
Remember
Speed squared: double the speed, four times the energy.
Q5.7Potential Energy
The energy possessed by a body by virtue of its position is:
Answer(b) potential energy
Step by step
Kinetic energy is due to motion.
Energy due to position, such as height above the ground, is potential energy (mgh).
Why the others are wrong
(a) kinetic energy is due to motion.
(c) chemical energy is stored in the bonds of substances.
(d) solar energy is energy from the Sun.
Remember
Position → potential; motion → kinetic.
Q5.8Kinetic Energy
The magnitude of momentum of an object is doubled. The kinetic energy of the object will:
Answer(b) increase to four times
Step by step
p = mv; with m fixed, doubling p doubles v.
Ek = ½mv² ∝ v², so it becomes 2² = 4 times.
Or: Ek = p² ÷ 2m, so Ek ∝ p².
Why the others are wrong
(a) Ek depends on the square of the momentum.
(c) Ek increases, it does not fall.
(d) a faster body has more kinetic energy.
Remember
Ek = p² ÷ 2m.
Q5.9Sources of Energy
Which of the following is not a renewable energy source?
Answer(b) Fossil fuels
Step by step
Renewable sources are replaced naturally: rain, wind, sunlight.
Fossil fuels are limited and take millions of years to form.
Why the others are wrong
(a) rain keeps filling the dams, so hydroelectric energy is renewable.
(c) wind keeps blowing.
(d) the Sun keeps shining.
Remember
Fossil fuels run out.
Set B: More MCQs from this chapter 15 MCQs
Extra questions on the same ideas, of the kind set in board papers and class tests.
QB1Work
Work is done when:
Answer(a) a force moves a body through a distance in its direction
Step by step
W = FS cos θ.
No displacement (wall, holding still) means S = 0, so W = 0.
At right angles, cos 90° = 0, so W = 0.
Why the others are wrong
(b) the wall does not move, so no work is done on it.
(c) the box does not move, so no work is done on it.
(d) a force at right angles to the motion does no work.
Remember
No movement, no work.
QB2Work
No work is done by gravity on a satellite moving in a circular orbit because:
Answer(c) the force is at right angles to the displacement
Step by step
In a circle, the force points to the centre and the motion is along the tangent.
The angle between them is 90°, so W = FS cos 90° = 0.
Why the others are wrong
(a) gravity is what keeps the satellite in orbit.
(b) the satellite has mass.
(d) speed does not decide whether work is done.
Remember
Centripetal force does no work.
QB3Work
1 kJ is equal to:
Answer(c) 1000 J
Step by step
kilo = 10³.
1 kJ = 10³ J = 1000 J. (1 MJ = 10⁶ J.)
Why the others are wrong
(a) 10 J is a dekajoule, a prefix not used here.
(b) 100 J is a hectojoule.
(d) 10⁶ J is 1 MJ.
Remember
kilo = thousand.
QB4Work
A force of 10 N moves a box 4 m in its own direction. The work done is:
Answer(d) 40 J
Step by step
Force and displacement are in the same direction, so W = FS.
W = 10 × 4 = 40 J.
Why the others are wrong
(a) 2.5 divides instead of multiplying.
(b) 14 adds the numbers.
(c) 6 subtracts them.
Remember
Work = force × distance.
QB5Kinetic Energy
The kinetic energy of a 2 kg ball moving at 3 m s⁻¹ is:
Answer(b) 9 J
Step by step
Ek = ½mv².
½ × 2 × 9 = 9 J.
Why the others are wrong
(a) 6 is mv, the momentum.
(c) 18 forgets the ½.
(d) 3 does not come from ½mv².
Remember
Square the speed, then take half of m × v².
QB6Potential Energy
The potential energy of a 2 kg book on a shelf 5 m above the floor is (g = 10 m s⁻²):
Answer(d) 100 J
Step by step
Ep = mgh.
2 × 10 × 5 = 100 J.
Why the others are wrong
(a) 10 J is mh, without g.
(b) 50 J is gh, without the mass.
(c) 25 J does not come from mgh.
Remember
m × g × h.
QB7Potential Energy
A stretched rubber band has:
Answer(a) elastic potential energy
Step by step
Stretching does work on the band.
That work is stored as elastic potential energy.
Why the others are wrong
(b) a band held stretched is not moving.
(c) nuclear energy is stored in the nucleus of atoms.
(d) it makes sound only when released.
Remember
Stretched or squeezed = elastic potential energy.
QB8Conservation of Energy
A ball is dropped from a height h. Ignoring air resistance, halfway down its energy is:
Answer(c) half potential and half kinetic
Step by step
At the top: Ep = mgh, Ek = 0.
Halfway: Ep = mg(h/2), so Ek = mgh − mg(h/2) = mg(h/2).
Total energy = mgh throughout.
Why the others are wrong
(a) it is all kinetic only just before hitting the ground.
(b) it is all potential only at the top.
(d) energy is conserved; the total stays mgh.
Remember
Lost Ep = gained Ek.
QB9Conservation of Energy
The energy changes in a battery-powered torch are:
Answer(a) chemical → electrical → light (and heat)
Step by step
Cells: chemical energy → electrical energy.
Bulb: electrical energy → light energy + heat.
Why the others are wrong
(b) a torch does not start with light.
(c) no nuclear energy is involved.
(d) a torch does not run on heat.
Remember
Battery = stored chemical energy.
QB10Sources of Energy
Solar cells convert:
Answer(b) light energy into electrical energy
Step by step
Light falls on the cell.
The cell produces a small voltage: light → electrical energy.
Many cells joined in series make a solar panel with a useful voltage.
Why the others are wrong
(a) solar cells use light, not heat.
(c) that is what a bulb does.
(d) that is what a torch does as a whole.
Remember
Photo = light, voltaic = voltage.
QB11Sources of Energy
Geothermal energy comes from:
Answer(a) hot rocks deep under the Earth's surface
Step by step
Water pumped down to hot rocks turns to steam.
The steam drives turbines to generate electricity.
Why the others are wrong
(b) that is wind energy.
(c) that is tidal energy.
(d) that is biomass energy.
Remember
Geo-thermal = Earth heat.
QB12Efficiency
A machine is given 500 J of energy and does 400 J of useful work. Its efficiency is:
Answer(c) 80%
Step by step
Efficiency = (useful output ÷ total input) × 100.
400 ÷ 500 = 0.8, so 80%.
The other 100 J is wasted, mostly as heat.
Why the others are wrong
(a) 125% puts input over output; no machine can exceed 100%.
(b) 40% does not come from these numbers.
(d) 20% is the wasted fraction, not the efficiency.
Remember
Output on top, input below.
QB13Efficiency
The efficiency of a real machine is always:
Answer(c) less than 100%
Step by step
Output energy = input energy − wasted energy.
Wasted energy is never zero, so efficiency < 100%.
Why the others are wrong
(a) that would need zero energy loss.
(b) that would create energy, breaking the conservation law.
(d) a working machine gives some useful output.
Remember
No perfect machines.
QB14Power
1 watt is equal to:
Answer(b) 1 J s⁻¹
Step by step
P = W ÷ t.
1 W = 1 J ÷ 1 s = 1 J s⁻¹.
Why the others are wrong
(a) J s multiplies by time instead of dividing.
(c) N s is the unit of impulse.
(d) N m⁻¹ is the unit of the spring constant.
Remember
Watt = joule per second.
QB15Power
A 600 W motor runs for 10 s. The energy it uses is:
