Energy, Work, Power and Efficiency
Chapter 9 MCQs
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
1
A car of mass 1200 kg travels at 20 m/s.
What is the kinetic energy of the car?
A 12 kJ
B 240 kJ
C 480 kJ
D 4.8 MJ
2
An object of mass 10 kg has kinetic energy 320 J.
What is its speed?
A 4.0 m/s
B 5.7 m/s
C 8.0 m/s
D 64 m/s
3
A box of mass 4.0 kg is lifted vertically through a height of 2.5 m.
Take g = 9.8 N/kg.
What is the increase in gravitational potential energy?
A 16 J
B 98 J
C 157 J
D 392 J
4
A load gains 588 J of gravitational potential energy when lifted through 6.0 m.
Take g = 9.8 N/kg.
What is the mass of the load?
A 6.0 kg
B 10 kg
C 60 kg
D 98 kg
5
A crate is pulled across a horizontal floor by a force of 50 N. The horizontal component of the force is 40 N. The crate moves 8.0 m horizontally.
How much work is done by the pulling force on the crate?
A 250 J
B 320 J
C 400 J
D 720 J
6
A force–distance graph for a moving object is described as follows.
-
From 0 m to 3.0 m, the force increases uniformly from 0 N to 60 N.
-
From 3.0 m to 7.0 m, the force remains 60 N.
-
From 7.0 m to 10.0 m, the force decreases uniformly to 0 N.
How much work is done?
A 180 J
B 240 J
C 330 J
D 420 J
7
A machine transfers 75 kJ of energy in 2.5 minutes.
What is the power of the machine?
A 30 W
B 300 W
C 500 W
D 187 500 W
8
A lift raises a passenger of mass 80 kg through 12 m in 24 s.
Take g = 9.8 N/kg.
What is the useful power output?
A 39 W
B 98 W
C 392 W
D 9400 W
9
A motor receives 1800 J of energy. It transfers 450 J usefully.
What is the efficiency of the motor?
A 4.0%
B 25%
C 40%
D 75%
10
A device is 35% efficient. It produces 420 J of useful energy.
How much total energy is supplied to the device?
A 147 J
B 555 J
C 1200 J
D 14 700 J
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
11
A motor has an input power of 900 W and is 60% efficient.
How much useful energy does it transfer in 25 s?
A 540 J
B 13 500 J
C 22 500 J
D 37 500 J
12
A machine has an input power of 2.5 kW and a useful output power of 2.0 kW.
What is the rate of wasted energy transfer?
A 0.50 kW
B 1.25 kW
C 2.25 kW
D 4.50 kW
13
A lamp has an input power of 60 W. It transfers 9.0 W usefully as light.
What is the efficiency of the lamp?
A 6.7%
B 15%
C 51%
D 667%
14
An electric heater is rated at 1.8 kW and is used for 40 minutes.
How much energy is transferred?
A 0.045 kWh
B 1.2 kWh
C 72 kWh
D 108 kWh
15
Electricity costs Rs 65 per kWh. A 1.8 kW heater is used for 40 minutes.
What is the cost?
A Rs 39
B Rs 65
C Rs 78
D Rs 117
16
A pump raises 500 kg of water through a height of 6.0 m. The pump is 75% efficient.
Take g = 9.8 N/kg.
How much electrical energy is supplied to the pump?
A 22 050 J
B 29 400 J
C 39 200 J
D 147 000 J
17
A car of mass 1200 kg increases its speed from 30 m/s to 60 m/s.
What is the increase in kinetic energy?
A 540 kJ
B 1.08 MJ
C 1.62 MJ
D 2.16 MJ
18
A roller coaster car starts from rest at a height of 45 m. 20% of its initial gravitational potential energy is transferred to thermal stores before reaching the bottom.
Take g = 9.8 N/kg.
What is its speed at the bottom?
A 21 m/s
B 27 m/s
C 30 m/s
D 42 m/s
19
A force of 30 N acts at 60° to the direction of motion of an object. The object moves 5.0 m.
How much work is done by the force?
A 75 J
B 130 J
C 150 J
D 260 J
20
A crane lifts a load of weight 200 N vertically through 3.0 m. It then moves the load horizontally through 4.0 m at constant height.
How much work is done by the crane against gravity?
A 200 J
B 600 J
C 800 J
D 1400 J
21
A spring is stretched by a force that increases uniformly from 0 N to 40 N. The extension is 0.20 m.
How much energy is stored elastically in the spring?
A 2.0 J
B 4.0 J
C 8.0 J
D 40 J
22
A vehicle engine provides a driving force of 500 N while the vehicle moves at a constant speed of 12 m/s.
What is the power output of the engine?
A 42 W
B 500 W
C 6000 W
D 12 000 W
23
A fuel releases 45 MJ of energy per kg. A machine burns 3.0 kg of the fuel and is 28% efficient.
How much useful energy is transferred?
A 12.6 MJ
B 37.8 MJ
C 135 MJ
D 482 MJ
24
Which row gives an advantage and a disadvantage of nuclear fuel for generating electricity?
| advantage | disadvantage | |
|---|---|---|
| A | renewable | produces carbon dioxide |
| B | low carbon dioxide emissions during operation | produces radioactive waste |
| C | only works in sunny weather | produces radioactive waste |
| D | renewable | only works in windy weather |
25
Water stored high behind a dam is used in a hydroelectric power station.
What is the main energy store of the water before it flows down?
A chemical
B elastic potential
C gravitational potential
D nuclear
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
26
A student lists six energy resources.
coal, geothermal, hydroelectric, natural gas, solar, wind
How many of these are renewable?
A 2
B 3
C 4
D 5
27
A Sankey diagram for a machine has:
-
input energy = 1000 J
-
useful output energy = 350 J
-
energy wasted thermally = 450 J
The remaining wasted energy is transferred as sound.
How much energy is transferred as sound?
A 100 J
B 200 J
C 550 J
D 800 J
28
In a Sankey diagram, the input arrow has width 60 mm and the useful output arrow has width 15 mm.
What is the efficiency?
A 15%
B 25%
C 40%
D 75%
29
A machine raises a load of weight 1000 N through 2.0 m. The total input energy supplied is 5000 J.
What is the efficiency?
A 20%
B 40%
C 50%
D 250%
30
A student of mass 70 kg climbs 40 stairs in 12 s. Each stair has a vertical height of 0.18 m.
Take g = 9.8 N/kg.
What is the student’s useful power output?
A 100 W
B 260 W
C 410 W
D 4900 W
31
A box is pulled across a rough floor at constant speed. The frictional force is 20 N and the box moves 15 m.
How much energy is transferred to thermal stores?
A 0 J
B 150 J
C 300 J
D 750 J
32
A 4.0 kg object moves up a rough slope. Its initial speed is 6.0 m/s. It comes to rest after gaining a vertical height of 1.0 m.
Take g = 9.8 N/kg.
How much energy is transferred to thermal stores?
A 33 J
B 39 J
C 72 J
D 111 J
33
A ball is thrown vertically upwards with a speed of 10 m/s. Ignore air resistance.
Take g = 9.8 N/kg.
What is the maximum height reached above the point of release?
A 2.6 m
B 5.1 m
C 9.8 m
D 20 m
34
A pendulum swings from one side to the other. Air resistance is negligible.
At the lowest point of the swing, which row is correct?
| kinetic energy | gravitational potential energy | |
|---|---|---|
| A | maximum | minimum |
| B | maximum | maximum |
| C | minimum | maximum |
| D | zero | maximum |
35
Which statement must be correct for any energy transfer device?
A Useful output energy is always greater than wasted energy.
B Efficiency can be greater than 100% if the device is powerful.
C Total energy output cannot exceed total energy input.
D Wasted energy is always transferred as sound.
36
Which list contains only non-renewable energy resources?
A coal, natural gas, nuclear fuel
B hydroelectric, coal, wind
C solar, geothermal, nuclear fuel
D tidal, oil, wave
37
Which energy resource uses thermal energy from inside the Earth?
A geothermal
B hydroelectric
C nuclear fission
D wind
38
A solar panel is charging a battery.
Which useful energy transfer occurs in the solar panel and battery system?
A chemical energy to light energy
B electrical energy to nuclear energy
C light energy to electrical energy, then chemical energy
D thermal energy to gravitational potential energy
39
Which unit is equivalent to the joule?
A N/kg
B N/m
C N m
D W/s
40
A force of 10 N acts opposite to the direction of motion of an object that moves 4.0 m.
How much energy is transferred from the object by this force?
A 2.5 J
B 14 J
C 40 J
D 400 J
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
41
A force decreases uniformly from 100 N to 40 N while an object moves 5.0 m in the direction of the force.
How much work is done?
A 200 J
B 250 J
C 350 J
D 500 J
42
A 2.0 kg mass is raised vertically by 5.0 m.
Take g = 9.8 N/kg.
What is the useful energy transferred?
A 9.8 J
B 49 J
C 98 J
D 196 J
43
A motor has useful output power 75 W and efficiency 30%.
What is its input power?
A 22.5 W
B 105 W
C 250 W
D 2500 W
44
A device transfers 18 kJ of useful energy at a useful power output of 60 W.
How long does this take?
A 0.30 s
B 30 s
C 300 s
D 1080 s
45
An object has momentum p and kinetic energy E.
Its momentum is doubled while its mass remains constant.
What is its new kinetic energy?
A E/4
B E/2
C 2E
D 4E
46
Two objects have the same momentum. Object X has mass m and object Y has mass 4m.
What is the ratio of kinetic energy of X to kinetic energy of Y?
A 1 : 4
B 1 : 2
C 2 : 1
D 4 : 1
47
A car of mass 1000 kg travelling at 20 m/s is stopped by a constant braking force of 4000 N.
What is the braking distance?
A 25 m
B 50 m
C 100 m
D 200 m
48
A box is pulled at constant speed across a rough floor by a horizontal force of 25 N. It moves 8.0 m.
How much energy is transferred to thermal stores?
A 3.1 J
B 25 J
C 100 J
D 200 J
49
An electric motor lifts a load. Some energy is wasted.
Which energy transfer is the main useful transfer?
A electrical store to nuclear store
B electrical store to gravitational potential store
C gravitational potential store to chemical store
D thermal store to electrical store
50
A generator is 80% efficient. It supplies energy to a motor that is 50% efficient. The generator receives 10 kJ of input energy.
How much useful energy does the motor produce?
A 2 kJ
B 4 kJ
C 5 kJ
D 8 kJ
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
Chapter 9 Answer Key
| Q | Ans | Q | Ans | Q | Ans | Q | Ans | Q | Ans |
|---|---|---|---|---|---|---|---|---|---|
| 1 | B | 11 | B | 21 | B | 31 | C | 41 | C |
| 2 | C | 12 | A | 22 | C | 32 | A | 42 | C |
| 3 | B | 13 | B | 23 | B | 33 | B | 43 | C |
| 4 | B | 14 | B | 24 | B | 34 | A | 44 | C |
| 5 | B | 15 | C | 25 | C | 35 | C | 45 | D |
| 6 | D | 16 | C | 26 | C | 36 | A | 46 | D |
| 7 | C | 17 | C | 27 | B | 37 | A | 47 | B |
| 8 | C | 18 | B | 28 | B | 38 | C | 48 | D |
| 9 | B | 19 | A | 29 | B | 39 | C | 49 | B |
| 10 | C | 20 | B | 30 | C | 40 | C | 50 | B |
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
Detailed Explanations
1. B
-
Kinetic energy = 1/2 mv²
-
KE = 1/2 × 1200 × 20²
-
KE = 600 × 400
-
KE = 240 000 J
-
KE = 240 kJ
-
C is the “forgot 1/2” trap.
2. C
-
KE = 1/2 mv²
-
320 = 1/2 × 10 × v²
-
320 = 5v²
-
v² = 64
-
v = 8.0 m/s
3. B
-
Increase in GPE = mgh
-
GPE = 4.0 × 9.8 × 2.5
-
GPE = 98 J
4. B
-
GPE = mgh
-
m = GPE / gh
-
m = 588 / (9.8 × 6.0)
-
m = 588 / 58.8
-
m = 10 kg
5. B
-
Work done = force in direction of motion × distance
-
Only the horizontal component does work in the horizontal direction.
-
Work done = 40 × 8.0
-
Work done = 320 J
6. D
-
Work done = area under force–distance graph.
-
First triangle:
-
1/2 × 3.0 × 60 = 90 J
-
-
Rectangle:
-
4.0 × 60 = 240 J
-
-
Final triangle:
-
1/2 × 3.0 × 60 = 90 J
-
-
Total work = 90 + 240 + 90
-
Total work = 420 J
7. C
-
Power = energy / time
-
Energy = 75 kJ = 75 000 J
-
Time = 2.5 minutes = 150 s
-
Power = 75 000 / 150
-
Power = 500 W
8. C
-
Useful energy = mgh
-
Useful energy = 80 × 9.8 × 12
-
Useful energy = 9408 J
-
Power = energy / time
-
Power = 9408 / 24
-
Power = 392 W
9. B
-
Efficiency = useful output / total input × 100
-
Efficiency = 450 / 1800 × 100
-
Efficiency = 25%
-
Answer = 25%
10. C
-
Efficiency = useful output / total input
-
0.35 = 420 / total input
-
Total input = 420 / 0.35
-
Total input = 1200 J
11. B
-
Input energy = input power × time
-
Input energy = 900 × 25 = 22 500 J
-
Useful energy = 60% of 22 500
-
Useful energy = 0.60 × 22 500
-
Useful energy = 13 500 J
12. A
-
Wasted power = input power − useful output power
-
Wasted power = 2.5 − 2.0
-
Wasted power = 0.50 kW
13. B
-
Efficiency = useful power / input power × 100
-
Efficiency = 9.0 / 60 × 100
-
Efficiency = 15%
14. B
-
Energy in kWh = power in kW × time in hours
-
Time = 40 minutes = 40/60 h = 2/3 h
-
Energy = 1.8 × 2/3
-
Energy = 1.2 kWh
15. C
-
Energy used = 1.2 kWh
-
Cost = 1.2 × 65
-
Cost = Rs 78
16. C
-
Useful energy = mgh
-
Useful energy = 500 × 9.8 × 6.0
-
Useful energy = 29 400 J
-
Efficiency = useful output / total input
-
0.75 = 29 400 / input
-
Input = 29 400 / 0.75
-
Input = 39 200 J
17. C
-
Increase in KE = 1/2 m(v² − u²)
-
Increase = 1/2 × 1200 × (60² − 30²)
-
Increase = 600 × (3600 − 900)
-
Increase = 600 × 2700
-
Increase = 1 620 000 J
-
Increase = 1.62 MJ
18. B
-
20% is wasted, so 80% becomes kinetic energy.
-
0.80mgh = 1/2 mv²
-
Mass cancels:
-
0.80gh = 1/2 v²
-
v² = 1.6gh
-
-
v² = 1.6 × 9.8 × 45
-
v² = 705.6
-
v = 26.6 m/s
-
Closest answer = 27 m/s
19. A
-
Work done = force × distance in direction of force
-
Component of force along motion = F cos 60°
-
Component = 30 × 0.5 = 15 N
-
Work done = 15 × 5.0
-
Work done = 75 J
20. B
-
Work done against gravity only depends on vertical height gained.
-
Horizontal movement at constant height does not increase GPE.
-
Work against gravity = weight × height
-
Work = 200 × 3.0
-
Work = 600 J
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
21. B
-
Energy stored in spring = area under force–extension graph.
-
Force increases uniformly from 0 to 40 N, so graph is a triangle.
-
Energy = 1/2 × force × extension
-
Energy = 1/2 × 40 × 0.20
-
Energy = 4.0 J
22. C
-
Power = force × velocity
-
Power = 500 × 12
-
Power = 6000 W
23. B
-
Total energy released = 45 MJ/kg × 3.0 kg
-
Total energy = 135 MJ
-
Useful energy = 28% of 135
-
Useful energy = 0.28 × 135
-
Useful energy = 37.8 MJ
24. B
-
Nuclear power stations produce low carbon dioxide emissions during operation.
-
But they produce radioactive waste.
-
Nuclear fuel is not renewable.
-
It also does not depend on sunny or windy weather.
25. C
-
Water stored high above the ground has gravitational potential energy.
-
In a hydroelectric power station:
-
GPE of water → KE of water → electrical energy
-
26. C
-
Renewable resources in the list:
-
geothermal
-
hydroelectric
-
solar
-
wind
-
-
Non-renewable:
-
coal
-
natural gas
-
-
Total renewable = 4
27. B
-
Input energy = useful + wasted thermal + wasted sound
-
1000 = 350 + 450 + sound
-
Sound = 1000 − 800
-
Sound = 200 J
28. B
-
In a Sankey diagram, arrow width is proportional to energy.
-
Efficiency = useful width / input width × 100
-
Efficiency = 15 / 60 × 100
-
Efficiency = 25%
29. B
-
Useful output energy = weight × height
-
Useful output = 1000 × 2.0 = 2000 J
-
Efficiency = useful output / input × 100
-
Efficiency = 2000 / 5000 × 100
-
Efficiency = 40%
30. C
-
Total vertical height = 40 × 0.18
-
Height = 7.2 m
-
Useful energy = mgh
-
Energy = 70 × 9.8 × 7.2
-
Energy = 4939 J
-
Power = energy / time
-
Power = 4939 / 12
-
Power = 411 W
-
Closest answer = 410 W
31. C
-
Energy transferred to thermal stores = work done against friction
-
Work = friction × distance
-
Work = 20 × 15
-
Work = 300 J
32. A
-
Initial KE = 1/2 mv²
-
Initial KE = 1/2 × 4.0 × 6.0²
-
Initial KE = 2.0 × 36 = 72 J
-
GPE gained = mgh
-
GPE = 4.0 × 9.8 × 1.0 = 39.2 J
-
Energy to thermal stores = 72 − 39.2
-
Energy to thermal stores = 32.8 J
-
Closest answer = 33 J
33. B
-
Initial KE becomes GPE at maximum height.
-
1/2 mv² = mgh
-
Mass cancels.
-
h = v² / 2g
-
h = 10² / (2 × 9.8)
-
h = 100 / 19.6
-
h = 5.1 m
34. A
-
At the lowest point:
-
speed is maximum
-
kinetic energy is maximum
-
height is minimum
-
gravitational potential energy is minimum
-
35. C
-
Energy cannot be created.
-
Total energy output cannot exceed total energy input.
-
Efficiency cannot be greater than 100%.
-
Wasted energy is not always sound; often it is thermal energy.
36. A
-
Non-renewable energy resources:
-
coal
-
natural gas
-
nuclear fuel
-
-
Hydroelectric, wind, solar, geothermal, tidal and wave are renewable.
37. A
-
Geothermal energy uses thermal energy from inside the Earth.
-
“Geo” = Earth, “thermal” = heat. Nice rare moment where naming actually helps.
38. C
-
Solar panel:
-
light energy → electrical energy
-
-
Battery charging:
-
electrical energy → chemical energy
-
-
So the useful transfer is:
-
light energy to electrical energy, then chemical energy
-
39. C
-
Work done = force × distance
-
Unit = N × m
-
So 1 joule = 1 N m
40. C
-
Energy transferred by force = force × distance
-
Energy = 10 × 4.0
-
Energy = 40 J
-
Since the force opposes motion, energy is transferred from the object’s kinetic store.
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
41. C
-
Force decreases uniformly from 100 N to 40 N.
-
Average force = (100 + 40) / 2
-
Average force = 70 N
-
Work done = average force × distance
-
Work done = 70 × 5.0
-
Work done = 350 J
42. C
-
Useful energy = mgh
-
Energy = 2.0 × 9.8 × 5.0
-
Energy = 98 J
43. C
-
Efficiency = useful output power / input power
-
0.30 = 75 / input power
-
Input power = 75 / 0.30
-
Input power = 250 W
44. C
-
Power = energy / time
-
Time = energy / power
-
Energy = 18 kJ = 18 000 J
-
Time = 18 000 / 60
-
Time = 300 s
45. D
-
KE = p² / 2m
-
If momentum doubles:
-
new momentum = 2p
-
new KE = (2p)² / 2m
-
new KE = 4p² / 2m
-
-
New KE = 4E
46. D
-
KE = p² / 2m
-
Same momentum means kinetic energy is inversely proportional to mass.
-
X has mass m.
-
Y has mass 4m.
-
KE of X : KE of Y = 1/m : 1/4m
-
Ratio = 4 : 1
47. B
-
Initial KE = 1/2 mv²
-
KE = 1/2 × 1000 × 20²
-
KE = 500 × 400
-
KE = 200 000 J
-
Work done by braking force = braking force × braking distance
-
4000 × distance = 200 000
-
Distance = 200 000 / 4000
-
Distance = 50 m
48. D
-
Constant speed means pulling force = friction.
-
Energy transferred to thermal stores = work done against friction.
-
Work = force × distance
-
Work = 25 × 8.0
-
Work = 200 J
49. B
-
The motor uses electrical energy to lift the load.
-
The useful energy store increased is the gravitational potential store.
-
Main useful transfer:
-
electrical store to gravitational potential store
-
50. B
-
Generator efficiency = 80%
-
Energy leaving generator usefully:
-
0.80 × 10 kJ = 8 kJ
-
-
Motor efficiency = 50%
-
Useful motor output:
-
0.50 × 8 kJ = 4 kJ
-
-
Final useful energy = 4 kJ
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
Common Traps From This Chapter
| Trap | Correct Rule |
|---|---|
| Kinetic energy | KE = 1/2 mv² |
| Speed doubles | KE becomes 4 times |
| Momentum doubles | KE becomes 4 times if mass unchanged |
| Gravitational potential energy | GPE = mgh |
| Work done | force × distance in direction of force |
| Force at an angle | use component in direction of motion |
| Force–distance graph | area = work done |
| Power | energy / time |
| Power for moving force | P = Fv |
| Efficiency | useful output / total input × 100 |
| kWh | kW × hours |
| Sankey diagram efficiency | useful arrow width / input arrow width |
| Hydroelectric starting store | gravitational potential |
| Friction | transfers energy to thermal stores |
| Braking distance by energy | KE / braking force |
| Renewable resources | solar, wind, hydroelectric, geothermal, tidal, wave |
| Non-renewable resources | coal, oil, gas, nuclear fuel |
| Nuclear advantage | low CO₂ during operation |
| Nuclear disadvantage | radioactive waste |
| Joule equivalent | N m |
| Energy conservation | total output cannot exceed total input |