Energy, Work And PowerCopy
- A 10 kg object is raised vertically at constant speed through a height of 5 m in 2 s. What is the minimum power required?
- A) 25 W
- B) 100 W
- C) 250 W
- D) 500 W
- An object is projected upwards with speed vv. At its highest point, its kinetic energy is:
- A) maximum
- B) zero
- C) equal to potential energy
- D) greater than its potential energy
- The efficiency of a device is 40%. If it receives 200 J of energy, how much is wasted?
- A) 80 J
- B) 120 J
- C) 160 J
- D) 40 J
- Which of the following expressions is not dimensionally equivalent to energy?
- A) FdFd
- B) 12mv2frac{1}{2}mv^2
- C) mghmgh
- D) Pt2Pt^2
- A ball of mass 2 kg is dropped from a height of 20 m. Assuming no air resistance, its speed just before hitting the ground is:
- A) 19.8 m/s
- B) 9.8 m/s
- C) 20 m/s
- D) 14 m/s
- An engine of power 5 kW lifts a 100 kg object vertically. How high can it lift the object in 10 s?
- A) 5 m
- B) 50 m
- C) 100 m
- D) 500 m
- A machine does 500 J of useful work while consuming 2000 J. Its efficiency is:
- A) 25%
- B) 50%
- C) 75%
- D) 20%
- A car of mass 1000 kg increases its speed from 10 m/s to 20 m/s. The work done by the engine is:
- A) 100,000 J
- B) 150,000 J
- C) 200,000 J
- D) 300,000 J
- The gravitational potential energy of an object depends on:
- A) speed and acceleration
- B) mass and velocity
- C) mass and height
- D) height and volume
- A motor lifts a 500 N weight through 2 m in 4 s. What is the average power output?
- A) 1000 W
- B) 250 W
- C) 500 W
- D) 2000 W
- Which of the following is not a unit of power?
- A) Watt
- B) Joule/second
- C) Newton/second
- D) kg·m²/s³
- If a man runs up a flight of stairs in half the time, his work done:
- A) doubles
- B) remains the same
- C) halves
- D) quadruples
- The total mechanical energy of a projectile (ignoring air resistance) is:
- A) constantly increasing
- B) constantly decreasing
- C) zero
- D) constant
- What is the energy transferred when a force of 10 N moves an object through 3 m at an angle of 60° to the direction of motion?
- A) 30 J
- B) 15 J
- C) 5 J
- D) 25 J
- A 2 kg object slides down a frictionless ramp of height 10 m. Its speed at the bottom is:
- A) 14 m/s
- B) 9.8 m/s
- C) 10 m/s
- D) 20 m/s
- A machine with 30% efficiency uses 1000 J of energy. Output energy is:
- A) 300 J
- B) 700 J
- C) 1000 J
- D) 1300 J
- Which of the following best describes a situation of zero work done?
- A) Holding a bag motionless
- B) Pushing a trolley
- C) Lifting a book
- D) Pulling a sledge
- Which graph represents the relationship between power and time if work is constant?
- A) Increasing curve
- B) Horizontal line
- C) Decreasing curve
- D) Vertical line
- When a car brakes, its kinetic energy is converted to:
- A) sound energy
- B) potential energy
- C) heat energy
- D) chemical energy
- If the force doubles and distance halves, work done:
- A) remains same
- B) doubles
- C) halves
- D) quadruples
⚡CONTINUATION OF MCQs: Chapter 1.7 – Energy, Work and Power
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Biology Full Scale Course
- A pulley system is 60% efficient. If 300 J of work is output, input work is:
- A) 180 J
- B) 500 J
- C) 200 J
- D) 300 J
- In an ideal pendulum, when is the kinetic energy maximum?
- A) At the topmost point
- B) At the lowest point
- C) At the start
- D) Midway
- A force of 50 N acts at 30° to the direction of motion for 4 m. Work done is:
- A) 100 J
- B) 150 J
- C) 173.2 J
- D) 200 J
- A heater rated 2.5 kW runs for 3 hours. Energy used is:
- A) 2.5 kWh
- B) 7.5 MJ
- C) 3.0 kWh
- D) 7.5 kWh
- Which factor does not affect power?
- A) Force applied
- B) Time taken
- C) Distance moved
- D) Acceleration
- A spring-loaded toy converts elastic potential energy into:
- A) kinetic energy
- B) chemical energy
- C) gravitational potential energy
- D) nuclear energy
- Which energy transformation occurs in a falling apple?
- A) KE → PE
- B) PE → KE
- C) PE → heat
- D) KE → light
- What is the SI unit of work?
- A) Newton
- B) Joule
- C) Watt
- D) Calorie
- A box is dragged 10 m by a 20 N force. If friction opposes with 5 N, work done by applied force is:
- A) 150 J
- B) 200 J
- C) 100 J
- D) 250 J
- Which of the following processes is most efficient in energy conversion?
- A) Human muscle
- B) Electric motor
- C) Petrol engine
- D) Coal power plant
- Two objects of equal mass fall from different heights. The ratio of their final kinetic energies equals the ratio of:
- A) Their masses
- B) Their speeds
- C) Their heights
- D) Their powers
- A boy lifts a weight using a ramp. The work done:
- A) is less
- B) is more
- C) is same
- D) depends on slope angle
- If velocity of a moving car is halved, kinetic energy becomes:
- A) Doubled
- B) Halved
- C) Quartered
- D) Remains same
- An object moving with constant speed on a level road has:
- A) zero kinetic energy
- B) constant kinetic energy
- C) increasing work done
- D) zero net energy
- A crane does 15000 J of work in 30 seconds. Its power is:
- A) 50 W
- B) 150 W
- C) 500 W
- D) 750 W
ANSWER KEY & EXPLANATIONS
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Physics Full Scale Course
| Q# | Answer | Option | Explanation |
|---|---|---|---|
| 1 | 250 W | C | Work = mgh = 10×10×5 = 500 J. Power = 500 J / 2 s = 250 W |
| 2 | zero | B | At the peak, speed = 0 ⇒ KE = 0 |
| 3 | 120 J | B | 40% of 200 J = 80 J used ⇒ 200 – 80 = 120 J wasted |
| 4 | Pt² | D | Power × time² = J/s × s² = J·s ≠ energy |
| 5 | 19.8 m/s | A | Use v=2gh=2×10×20=400=20v = sqrt{2gh} = sqrt{2×10×20} = sqrt{400} = 20 m/s |
| 6 | 50 m | B | Power = mgh/t ⇒ h = Pt/mg = 5000×10/1000×10 = 50 m |
| 7 | 25% | A | Efficiency = 500 / 2000 = 0.25 = 25% |
| 8 | 150,000 J | B | ΔKE = ½m(v² – u²) = 500×(400-100) = 150,000 J |
| 9 | mass and height | C | GPE = mgh |
| 10 | 250 W | B | Power = work/time = 500×2 / 4 = 250 W |
| 11 | Newton/second | C | Not a unit of power |
| 12 | remains the same | B | Work = force × distance; time doesn’t change work |
| 13 | constant | D | No air resistance ⇒ KE + PE = constant |
| 14 | 15 J | B | Work = Fdcosθ = 10×3×0.5 = 15 J |
| 15 | 14 m/s | A | Use KE = PE ⇒ ½mv² = mgh ⇒ v = √2gh = √200 = ~14.1 |
| 16 | 300 J | A | 30% of 1000 = 300 J |
| 17 | Holding a bag motionless | A | No displacement ⇒ zero work |
| 18 | Decreasing curve | C | P = W/t ⇒ as t ↑, P ↓ |
| 19 | heat energy | C | Brakes convert KE → heat |
| 20 | remains same | A | Work = force × distance ⇒ 2F × 0.5d = same |
| 21 | 500 J | B | Efficiency = output/input ⇒ input = 300 / 0.6 = 500 |
| 22 | At the lowest point | B | KE max when speed max (bottom of swing) |
| 23 | 173.2 J | C | Work = Fdcosθ = 50×4×cos30 = ~173.2 |
| 24 | 7.5 kWh | D | Power × time = 2.5 × 3 = 7.5 kWh |
| 25 | Acceleration | D | Power depends on force, distance, and time—not acceleration |
| 26 | kinetic energy | A | Elastic PE becomes KE in motion |
| 27 | PE → KE | B | Falling converts potential to kinetic |
| 28 | Joule | B | SI unit of work = J |
| 29 | 200 J | B | 20 N × 10 m = 200 J (friction doesn’t affect this value) |
| 30 | Electric motor | B | Highest efficiency among listed options |
| 31 | Their heights | C | KE final = PE initial = mgh |
| 32 | is same | C | Work = mgh, ramp doesn’t affect amount |
| 33 | Quartered | C | KE ∝ v² ⇒ (½v)² = ¼KE |
| 34 | constant kinetic energy | B | Constant speed ⇒ constant KE |
| 35 | 500 W | C | Power = 15000 / 30 = 500 W |
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Physics Full Scale Course