Grade 10 physics – Energy, Work, Power and Machines Quiz
1. What is the SI unit of work?
Work is measured in joules (J). One joule equals one newton of force causing a displacement of one metre in the direction of the force.
2. A constant force F acts in the same direction as displacement s. Which formula gives the work done by the force?
When force and displacement are parallel, work done is the product of the force and the displacement: W = F × s.
3. If a force acts perpendicular to the displacement of an object, the work done by that force is:
Work is F·s·cosθ. If the force is perpendicular to displacement, θ = 90° and cos90° = 0, so work = 0.
4. Which expression defines power?
Power is the rate at which energy is transferred or work is done: P = ΔE/Δt (work or energy divided by time).
5. What is the SI unit of power?
The SI unit of power is the watt (W), defined as one joule per second (1 W = 1 J/s).
6. What is the formula for kinetic energy of a moving object?
Kinetic energy of an object of mass m moving at speed v is KE = 1/2 m v^2.
7. What expression gives the gravitational potential energy near Earth's surface?
Gravitational potential energy relative to a reference height is PE = m g h, where g is acceleration due to gravity and h is height above the reference.
8. In the absence of friction and other non-conservative forces, what happens to the total mechanical energy of a system?
When no non-conservative forces (like friction) do work, mechanical energy (kinetic + potential) is conserved and remains constant.
9. How is efficiency of a machine calculated (in percent)?
Efficiency = (useful energy output ÷ total energy input) × 100%; it measures the fraction of input energy converted to useful output.
10. What is the mechanical advantage (MA) of a machine?
Mechanical advantage is the ratio of load (output force) to effort (input force): MA = load / effort.
11. For a lever in equilibrium, the mechanical advantage equals:
From moments: effort × effort arm = load × load arm, so MA (load/effort) = effort arm / load arm.
12. A single movable pulley (one pulley attached to the load) ideally provides what mechanical advantage?
An ideal single movable pulley shares the load between two rope segments, so the effort required is half the load: MA = 2.
13. What is the ideal mechanical advantage (IMA) of an inclined plane?
For an inclined plane, IMA = distance along the slope ÷ vertical rise (length ÷ height); a longer slope for same height reduces effort.
14. A 2 kg stone falls freely through 3 m. Using g = 10 m/s², what is the work done by gravity on the stone?
Work by gravity = weight × vertical displacement = m g h = 2 × 10 × 3 = 60 J. It is positive because displacement is in the direction of the gravitational force.
15. A student lifts a 60 kg crate through 3 m in 4 s. Using g = 10 m/s², what is the average power output?
Work = m g h = 60×10×3 = 1800 J. Power = work / time = 1800 ÷ 4 = 450 W.
16. When an object moves around a closed path and returns to its starting point under only a conservative force (e.g. gravity), the net work done by that force is:
Conservative forces do net zero work over a closed path because potential energy depends only on position, so total change in potential is zero.
17. Which simple machine changes the direction of an applied force without ideal mechanical advantage greater than 1?
A fixed pulley changes the direction of the applied force (e.g., pulling down to lift up) but does not multiply the force ideally (MA = 1).
18. Approximately how many watts are in one horsepower?
One mechanical horsepower is defined as about 746 watts.
19. If a machine has an efficiency of 60% and you input 500 J of energy, how much useful energy is produced?
Useful output = efficiency × input = 0.60 × 500 J = 300 J.
20. An object moves at constant velocity on a horizontal surface. What is the net work done on the object?
At constant velocity, the net force is zero, so the net work done on the object is zero (no change in kinetic energy).
21. Which expression gives instantaneous power when a constant force F moves an object with velocity v in its direction?
Instantaneous power delivered by a force is the dot product of force and velocity. For force and velocity in same direction, P = F × v.
22. Which sequence best describes energy transformation in a hydroelectric power station?
Water stored at height has gravitational potential energy; as it falls the energy becomes kinetic and is converted by turbines and generators into electrical energy.
23. A machine has mechanical advantage 4. If the effort applied is 50 N, what load can the machine ideally lift?
MA = load / effort = 4, so load = MA × effort = 4 × 50 N = 200 N (ideal, ignoring friction).
24. A lever has an effort arm of 2.0 m and a load arm of 0.5 m. What is the mechanical advantage?
MA = effort arm ÷ load arm = 2.0 ÷ 0.5 = 4.0; the lever multiplies the applied force four times ideally.
25. Why is the actual efficiency of real machines always less than 100%?
Real machines suffer energy losses (e.g., friction, air resistance, deformation) that convert useful energy into unusable heat, so efficiency is below 100%.
26. What is the SI unit of energy?
The SI unit of energy is the joule (J). A joule is the work done when a force of one newton moves an object one metre.
27. A student lifts a 2 kg textbook vertically upward by 0.5 m at constant speed. What is the work done against gravity? (Take g = 9.8 m/s^2)
Work against gravity is W = m g h = 2 × 9.8 × 0.5 = 9.8 J.
28. A machine delivers 2000 J of useful energy from 5000 J supplied. What is its efficiency?
Efficiency = (useful energy output / energy input) × 100% = (2000 / 5000) × 100% = 40%.
29. If a constant force of 10 N acts on an object and moves it 3 m in the direction of the force, how much work is done?
Work = force × distance = 10 N × 3 m = 30 J, since force and displacement are in the same direction.
30. Which of the following is an example of energy being converted from chemical energy to kinetic energy?
In a bus, the chemical energy in fuel is converted by the engine into kinetic energy of motion when the bus accelerates.
31. Power is defined as:
Power is the rate of doing work or transferring energy, measured in watts (1 W = 1 J/s).
32. A motor lifts a 100 kg load 10 m in 20 s. What is the average power output? (Take g = 9.8 m/s^2)
Work = m g h = 100 × 9.8 × 10 = 9800 J. Power = work / time = 9800 / 20 = 490 W. (Wait) Correction: 9800/20 = 490, so correct answer is 490 W. However, among these choices 4900 W is incorrect. The correct choice should be 490 W.
33. Which simple machine is used when you use a crowbar to lift a heavy stone?
A crowbar acts as a lever, using a pivot (fulcrum) to multiply the input force to lift a heavy object.
34. A block is pulled along a horizontal surface with force 50 N at an angle of 60° above the horizontal for 4 m. What is the work done by the force in the horizontal direction?
Work = F cosθ × d = 50 × cos60° × 4 = 50 × 0.5 × 4 = 100 J.
35. What is mechanical advantage (MA) of a simple machine?
Mechanical advantage is defined as output (load) force divided by input (effort) force, indicating how much the machine multiplies force.
36. A 2 m long inclined plane raises a crate 0.5 m vertically. What is the ideal mechanical advantage (IMA) of the ramp?
IMA for an inclined plane = length of slope / vertical rise = 2 / 0.5 = 4.
37. Which of these best describes 'work done' when there is no displacement?
Work requires displacement. If there is no displacement in the direction of the force, the work done is zero, even if effort is felt.
38. An electric kettle rated 2000 W is used for 3 minutes. How much energy does it use? (1 minute = 60 s)
Energy = power × time = 2000 W × (3 × 60 s) = 2000 × 180 = 360000 J = 360 kJ.
39. Which type of energy is stored in a stretched spring?
A stretched or compressed spring stores elastic potential energy, which can be released as kinetic energy when the spring returns to equilibrium.
40. A child on a swing reaches maximum height of 1.2 m above the lowest point. What form of energy is maximum at that point?
At maximum height the speed is momentarily zero so kinetic energy is minimal and gravitational potential energy is maximal.
41. If a motor does 1500 J of work in 5 seconds, what is its power output?
Power = work / time = 1500 J / 5 s = 300 W.
42. A fixed pulley changes the direction of the force but does not change the magnitude. What is the ideal mechanical advantage of a single fixed pulley?
A single fixed pulley only redirects force; the ideal mechanical advantage is 1 because the output force equals the input force (ignoring friction).
43. Which statement best describes conservation of energy in a closed system?
The law of conservation of energy states that in a closed system energy is neither created nor destroyed but can be transformed from one form to another.
44. A crane applies an upward force of 2000 N to lift a load 2 m in 10 s. What is the power developed by the crane (ignore weight other than the lifting force)?
Work = force × distance = 2000 × 2 = 4000 J. Power = 4000 J / 10 s = 400 W.
45. Which of the following increases the work done by a force?
Work = F cosθ × d, so increasing the component of force in the direction of motion (increasing F cosθ) increases work for the same displacement.
46. Which machine would most reduce the effort needed to raise a heavy bucket from a well?
A system of movable pulleys provides a greater mechanical advantage, allowing a smaller effort to lift a given load compared with a single fixed pulley.
47. A 60 W bulb is left on for 2 hours. How much energy does it consume in kilowatt-hours (kWh)?
Energy (kWh) = power (kW) × time (h). Power = 60 W = 0.06 kW. Time = 2 h. Energy = 0.06 × 2 = 0.12 kWh.
48. What is the useful purpose of a second-class lever (like a wheelbarrow)?
A second-class lever places the load between the fulcrum and effort, giving a mechanical advantage so a smaller effort can lift a larger load.
49. Which quantity is zero when a force acts perpendicular to the displacement?
If force is perpendicular to displacement, the component of force along the displacement is zero, so work = F cos90° × d = 0.
50. A cyclist pedals such that the bicycle does 1200 J of work in 30 s. What is the cyclist's power output?
Power = work / time = 1200 J / 30 s = 40 W.
51. Which of these increases the efficiency of a machine in practice?
Reducing friction reduces energy losses as heat, increasing the fraction of input energy converted to useful work and thus improving efficiency.