WTMA Energy, Work, and Power 5 — Questions and Answers
Question 1: A spring-loaded device stores 500 joules of energy when compressed. If it releases all that energy in 0.1 seconds, what is the average power output?
- 50 watts
- 500 watts
- 5,000 watts (Correct answer)
- 0.05 watts
Correct answer: 5,000 watts
Power = Energy / Time = 500 J / 0.1 s = 5,000 watts.
Question 2: On a construction site, a crane lifts a steel beam weighing 2,000 lbs to a height of 30 feet. How much work is done?
- 2,000 ft-lb
- 30 ft-lb
- 60,000 ft-lb (Correct answer)
- 6,000 ft-lb
Correct answer: 60,000 ft-lb
Work = Force × Distance = 2,000 lbs × 30 ft = 60,000 ft-lb.
Question 3: Two identical balls are dropped from different heights. Ball A is dropped from 10 feet; Ball B from 20 feet. Compare their speeds just before hitting the ground.
- Ball B is twice as fast as Ball A
- Ball B is about 1.4 times faster than Ball A (Correct answer)
- Both hit at the same speed
- Ball A is faster
Correct answer: Ball B is about 1.4 times faster than Ball A
Speed is proportional to the square root of height (v = √(2gh)), so doubling height increases speed by √2 ≈ 1.4 times.
Question 4: Which of the following machines can have an efficiency greater than 100%?
- A frictionless pulley
- A perfectly lubricated gear train
- No machine can exceed 100% efficiency (Correct answer)
- A hydraulic press
Correct answer: No machine can exceed 100% efficiency
No machine can exceed 100% efficiency because energy cannot be created; some energy is always lost to friction or heat.
Question 5: A worker applies 50 lbs of force at an angle of 90° to the direction of motion. How much work is done?
- 50 ft-lb per foot traveled
- 0 ft-lb regardless of distance (Correct answer)
- 25 ft-lb per foot traveled
- 100 ft-lb per foot traveled
Correct answer: 0 ft-lb regardless of distance
Work = Force × Distance × cos(90°) = F × d × 0 = 0; a perpendicular force does no work.
Question 6: A bicycle rider coasts down a hill and gains speed. Which energy conversion is occurring?
- Kinetic to potential
- Chemical to thermal
- Gravitational potential to kinetic (Correct answer)
- Thermal to mechanical
Correct answer: Gravitational potential to kinetic
As the rider descends, gravitational potential energy converts into kinetic energy, increasing speed.
Question 7: A pump moves 200 gallons of water per minute to a height of 50 feet. If water weighs 8.34 lbs/gallon, approximately how much power is needed (in ft-lb/min)?
- 83,400 ft-lb/min (Correct answer)
- 41,700 ft-lb/min
- 1,668 ft-lb/min
- 167,000 ft-lb/min
Correct answer: 83,400 ft-lb/min
Power = (200 gal/min × 8.34 lb/gal × 50 ft) = 83,400 ft-lb/min.
WTMA Energy, Work, and Power 5
A spring-loaded device stores 500 joules of energy when compressed.
If it releases all that energy in 0.1 seconds, what is the average power output?