Physics Practice Test: Work, Energy and Power
- Work, Energy and Power
- 10 questions
- 15 minutes
These 10 questions practice work, kinetic and potential energy, conservation of energy and power. Use g = 9.8 m/s² and ignore air resistance unless the question mentions energy losses. When speed is asked, energy conservation is often faster than kinematics.
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Question 1
A 3.0 kg melon falls 4.0 m from a high shelf to the floor. How much work does gravity do on it during the fall?
Correct answer: A
W = mgh = 3.0 × 9.8 × 4.0 ≈ 118 J, positive because gravity points in the direction of motion. 12 J forgets g, and 29 J is just the melon's weight in newtons.
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Question 2
A student carries a backpack across a level hallway at constant velocity. How much work does the upward force of the student's hands do on the backpack?
Correct answer: C
The hands push straight up while the displacement is horizontal, so the angle is 90° and W = Fd cos 90° = 0. Lifting effort feels like work, but no energy is given to the backpack.
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Question 3
A 0.50 kg ball speeds up from 6.0 m/s to 10 m/s. How much net work was done on it?
Correct answer: B
Wnet = ΔKE = ½(0.50)(10² − 6.0²) = 0.25 × 64 = 16 J. 4.0 J squares the change in speed (4.0 m/s) instead of taking the change in v².
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Question 4
A roller coaster car starts from rest at a height of 45 m on a frictionless track. What is its speed when it is 25 m above the ground?
Correct answer: D
It has dropped 20 m, so v = √(2 × 9.8 × 20) ≈ 19.8 m/s. 22.1 m/s uses the 25 m height instead of the drop, and 29.7 m/s uses the full 45 m.
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Question 5
A pendulum bob passes its lowest point at 3.0 m/s. Ignoring friction, how much higher than the lowest point does it rise?
Correct answer: B
½mv² = mgh gives h = v²/(2g) = 9.0/19.6 ≈ 0.46 m, and the mass cancels. 0.92 m forgets the ½ in kinetic energy.
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Question 6
A spring with k = 400 N/m is compressed 0.15 m and launches a 0.20 kg ball across a frictionless table. What is the ball's launch speed?
Correct answer: D
The spring stores ½(400)(0.15)² = 4.5 J, so ½(0.20)v² = 4.5 and v = √45 ≈ 6.7 m/s. 45 m/s forgets the square root, and 9.5 m/s drops the ½ from the spring energy.
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Question 7
A 70 kg student runs up stairs that rise 4.0 m vertically in 5.0 s. What is the student's average power output against gravity?
Correct answer: A
P = mgh/t = 70 × 9.8 × 4.0/5.0 ≈ 549 W. 2700 W is the energy in joules without dividing by the time, and 56 W forgets g.
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Question 8
A car travels at a constant 25 m/s on a level highway. Air drag and rolling resistance together are 800 N. What power must the engine deliver to the wheels?
Correct answer: C
At constant speed the driving force equals the 800 N resistance, and P = Fv = 800 × 25 = 20,000 W = 20 kW. 32 W divides the force by the speed instead of multiplying.
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Question 9
A 40 kg child on a sled starts from rest at the top of a hill 10 m high and reaches the bottom at 12 m/s. How much mechanical energy was lost to friction?
Correct answer: C
The starting potential energy is 40 × 9.8 × 10 = 3920 J and the final kinetic energy is ½(40)(12²) = 2880 J, so 3920 − 2880 = 1040 J was lost. The other values are only one of the two energies, or their sum.
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Question 10
A 2.0 kW electric heater runs for 3.0 hours. How much energy does it use, in joules?
Correct answer: A
E = Pt = 2000 W × 10,800 s ≈ 2.2 × 107 J, which is 6.0 kWh. 7.2 × 106 J is the energy for only one hour.
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