Physics Practice Test: Work, Energy and Momentum
- Work, Energy and Momentum
- 15 questions
- 20 minutes
This test covers work, kinetic and potential energy, conservation of energy, power, efficiency, impulse and momentum in collisions. Use g = 9.8 m/s² and ignore friction unless a question mentions it. Momentum is conserved in every collision; kinetic energy is conserved only in elastic ones.
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Question 1
A worker pushes a box 8.0 m across a floor with a constant horizontal force of 50 N. How much work does the worker do on the box?
Correct answer: C
W = Fd = 50 N × 8.0 m = 400 J, since the force points in the direction of motion. Adding the numbers gives 58, and dividing gives 6.25.
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Question 2
A child pulls a sled 10 m along level snow with a rope that makes a 60° angle with the ground. The tension in the rope is 100 N. How much work does the rope do on the sled?
Correct answer: B
Only the component of force along the motion does work: W = Fd cos θ = 100 × 10 × cos 60° = 500 J. Using sin 60° gives 866 J, and ignoring the angle gives 1000 J.
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Question 3
What is the kinetic energy of a 1200 kg car moving at 15 m/s?
Correct answer: C
KE = ½mv² = ½ × 1200 × 15² = 135 000 J = 135 kJ. Forgetting to square the speed gives 9.0 kJ, and forgetting the ½ gives 270 kJ.
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Question 4
A cyclist doubles her speed. What happens to her kinetic energy?
Correct answer: D
Kinetic energy depends on the square of speed (½mv²), so doubling v multiplies KE by 2² = 4. Momentum, which depends on v itself, would only double.
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Question 5
A 2.0 kg book is lifted from the floor onto a shelf 1.5 m high. By how much does its gravitational potential energy increase?
Correct answer: D
ΔPE = mgh = 2.0 × 9.8 × 1.5 = 29.4 J. The value 19.6 is only the book's weight in newtons (mg), without the height.
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Question 6
A ball is dropped from rest from a height of 20 m. Ignoring air resistance, how fast is it moving just before it hits the ground?
Correct answer: B
All the potential energy becomes kinetic: mgh = ½mv², so v = √(2gh) = √(2 × 9.8 × 20) ≈ 19.8 m/s. Leaving out the 2 gives 14 m/s, and forgetting the square root gives 392.
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Question 7
A spring with spring constant 200 N/m is compressed by 0.10 m. How much elastic potential energy does it store?
Correct answer: A
PE = ½kx² = ½ × 200 × 0.10² = 1.0 J. Forgetting the ½ gives 2.0 J, and using kx (the force) gives 20.
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Question 8
A motor lifts a 50 kg crate 12 m straight up at constant speed in 8.0 s. What is the motor's power output?
Correct answer: B
Work = mgh = 50 × 9.8 × 12 = 5880 J, and P = W/t = 5880 ÷ 8.0 = 735 W. The value 5880 is the work, not the power, and 75 W leaves out g.
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Question 9
A 0.15 kg baseball moves at 40 m/s. What is the magnitude of its momentum?
Correct answer: A
p = mv = 0.15 × 40 = 6.0 kg·m/s. The value 120 is the kinetic energy in joules (½mv²), and 267 comes from dividing speed by mass.
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Question 10
A tennis racket exerts an average force of 300 N on a ball for 0.020 s. What is the change in the ball's momentum?
Correct answer: A
The change in momentum equals the impulse: FΔt = 300 N × 0.020 s = 6.0 N·s = 6.0 kg·m/s. Dividing the force by the time gives 15 000.
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Question 11
A 1500 kg car moving at 20 m/s hits a parked 1000 kg car, and the two cars lock together. What is their speed just after the collision?
Correct answer: C
Momentum is conserved: 1500 × 20 = (1500 + 1000)v, so v = 30 000 ÷ 2500 = 12 m/s. Simply halving the speed gives 10 m/s, which ignores the masses.
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Question 12
In the same kind of crash, a 1500 kg car at 20 m/s hits a parked 1000 kg car and the cars move off together at 12 m/s. How much kinetic energy is lost in the collision?
Correct answer: B
Before: ½ × 1500 × 20² = 300 kJ. After: ½ × 2500 × 12² = 180 kJ. The loss is 300 − 180 = 120 kJ, turned into heat, sound and bent metal. Kinetic energy is not conserved in a collision where objects stick together, so 0 is wrong.
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Question 13
A 4.0 kg rifle at rest fires a 0.010 kg bullet at 400 m/s. What is the rifle's recoil speed?
Correct answer: A
Total momentum is zero before and after: 0.010 × 400 = 4.0 × v, so v = 4.0 ÷ 4.0 = 1.0 m/s. The value 4.0 is the bullet's momentum in kg·m/s, not the rifle's speed.
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Question 14
Which quantities are conserved in a perfectly elastic collision between two balls on a frictionless surface?
Correct answer: D
Momentum is conserved in every collision without outside forces, and "elastic" means kinetic energy is conserved too. In an inelastic collision only momentum is conserved.
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Question 15
An electric motor uses 2000 J of electrical energy and does 1500 J of useful work. What is its efficiency?
Correct answer: C
Efficiency = useful output ÷ input = 1500 ÷ 2000 = 0.75 = 75%. Dividing the other way gives 133%, which is impossible because no machine puts out more energy than it takes in; 25% is the share that is wasted.
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