Energy, Work, and Power Flashcards

A ball rolling along a flat surface gradually slows down due to friction. Where does the kinetic energy go?

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.

Which unit is commonly used to measure electrical power?

Answer: Watt

The watt (W) is the SI unit of power, including electrical power, equal to one joule per second.

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)?

Answer: 200 ft-lb

The potential energy at the top (10 × 20 = 200 ft-lb) converts entirely to kinetic energy just before impact.

A machine requires 1,000 watts of input power but delivers only 800 watts of useful output. What is its efficiency?

Answer: 80%

Efficiency = (Useful Output / Input) × 100 = (800 / 1,000) × 100 = 80%.

Which of the following is NOT a form of mechanical energy?

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.

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?

Answer: 1 foot

Work in = Work out: 100 × 4 = 400 × d, so d = 400/400 = 1 foot.

A car engine produces 200 horsepower. If 1 hp = 550 ft-lb/s, what is the engine's power in ft-lb/s?

Answer: 110,000 ft-lb/s

200 hp × 550 ft-lb/s per hp = 110,000 ft-lb/s.