Energy, Work, and Power Flashcards

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?

Answer: 5,000 watts

Power = Energy / Time = 500 J / 0.1 s = 5,000 watts.

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?

Answer: 60,000 ft-lb

Work = Force × Distance = 2,000 lbs × 30 ft = 60,000 ft-lb.

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.

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.

Which of the following machines can have an efficiency greater than 100%?

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.

A worker applies 50 lbs of force at an angle of 90° to the direction of motion. How much work is done?

Answer: 0 ft-lb regardless of distance

Work = Force × Distance × cos(90°) = F × d × 0 = 0; a perpendicular force does no work.

A bicycle rider coasts down a hill and gains speed. Which energy conversion is occurring?

Answer: Gravitational potential to kinetic

As the rider descends, gravitational potential energy converts into kinetic energy, increasing speed.

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)?

Answer: 83,400 ft-lb/min

Power = (200 gal/min × 8.34 lb/gal × 50 ft) = 83,400 ft-lb/min.