WTMA Energy, Work, and Power 3 — Questions and Answers
Question 1: A ball rolling along a flat surface gradually slows down due to friction. Where does the kinetic energy go?
- It is destroyed
- It converts to thermal energy (heat) (Correct answer)
- It converts to gravitational potential energy
- It converts to chemical energy
Correct answer: It converts to thermal energy (heat)
Friction converts kinetic energy into thermal energy, which is why surfaces warm slightly when objects slide across them.
Question 2: Which unit is commonly used to measure electrical power?
- Joule
- Newton
- Watt (Correct answer)
- Pascal
Correct answer: Watt
The watt (W) is the SI unit of power, including electrical power, equal to one joule per second.
Question 3: A 10 lb object is dropped from a height of 20 feet. Just before it hits the ground, approximately how much kinetic energy does it have (ignoring air resistance)?
- 10 ft-lb
- 200 ft-lb (Correct answer)
- 20 ft-lb
- 2,000 ft-lb
Correct answer: 200 ft-lb
The potential energy at the top (10 × 20 = 200 ft-lb) converts entirely to kinetic energy just before impact.
Question 4: A machine requires 1,000 watts of input power but delivers only 800 watts of useful output. What is its efficiency?
- 80% (Correct answer)
- 125%
- 20%
- 60%
Correct answer: 80%
Efficiency = (Useful Output / Input) × 100 = (800 / 1,000) × 100 = 80%.
Question 5: Which of the following is NOT a form of mechanical energy?
- Kinetic energy of a moving car
- Potential energy of a raised weight
- Chemical energy in a battery (Correct answer)
- Elastic energy in a stretched rubber band
Correct answer: Chemical energy in a battery
Chemical energy in a battery is stored chemical energy, not mechanical energy; mechanical energy includes kinetic and potential forms.
Question 6: A lever allows a worker to lift a 400 lb rock by applying only 100 lbs of force. If the effort arm moves 4 feet, how far does the load move?
- 16 feet
- 1 foot (Correct answer)
- 4 feet
- 8 feet
Correct answer: 1 foot
Work in = Work out: 100 × 4 = 400 × d, so d = 400/400 = 1 foot.
Question 7: A car engine produces 200 horsepower. If 1 hp = 550 ft-lb/s, what is the engine's power in ft-lb/s?
- 2,750 ft-lb/s
- 110,000 ft-lb/s (Correct answer)
- 550 ft-lb/s
- 1,100 ft-lb/s
Correct answer: 110,000 ft-lb/s
200 hp × 550 ft-lb/s per hp = 110,000 ft-lb/s.
WTMA Energy, Work, and Power 3
A ball rolling along a flat surface gradually slows down due to friction.
Where does the kinetic energy go?