Energy, Work, and Power Flashcards

A weightlifter holds a 100 kg barbell stationary above his head for 10 seconds. How much work is done on the barbell by the weightlifter during this time?

Answer: 0 Joules

In physics, work is defined as force multiplied by the distance over which the force is applied (Work = Force × Distance). Since the barbell is held stationary, its distance moved is zero. Therefore, no mechanical work is done on the barbell, even though the weightlifter is exerting an upward force to counteract gravity and is expending biological energy.

Two movers, Alex and Ben, lift identical pianos to the same second-story window. Alex lifts the piano in 2 minutes, while Ben takes 4 minutes. Which of the following statements is true?

Answer: Alex and Ben do the same amount of work, but Alex uses more power.

Work is calculated as force times distance. Since both movers lift identical pianos (same force) to the same height (same distance), they perform the same amount of work. Power is the rate at which work is done (Power = Work / Time). Because Alex completes the work in less time, he generates more power than Ben.

A child at the top of a playground slide is momentarily still before starting to slide down. At this highest point, the child's energy is primarily in what form?

Answer: Potential Energy

Potential energy is stored energy due to an object's position or state. At the highest point, before movement begins, the child's energy is stored as gravitational potential energy. As the child slides down, this potential energy is converted into kinetic energy, which is the energy of motion.

An electric motor's ability to do work over a period of time is rated in a specific unit. Which of the following is the standard unit of power?

Answer: Watt

The standard SI unit of power is the Watt (W). A Watt is defined as one Joule of work done per second (J/s). Joules are units of energy or work, Newtons are units of force, and Pascals are units of pressure.

Which of the following actions results in the greatest increase in an object's gravitational potential energy?

Answer: Lifting a 5 kg box onto a 2-meter high shelf.

Gravitational potential energy depends on mass, gravity, and height (PE = mgh). Lifting the 5 kg box 2 meters high increases its height against gravity, thus increasing its potential energy. Sliding the box horizontally does not change its height. Compressing a spring increases elastic potential energy, not gravitational. Lowering a box decreases its gravitational potential energy.

A pendulum swings back and forth. Ignoring air resistance, at which point in its swing does the pendulum bob have the maximum kinetic energy?

Answer: At the lowest point of its swing.

Kinetic energy is the energy of motion (KE = 1/2mv²), which is greatest when the object's speed is greatest. As the pendulum swings down from its highest point, its potential energy is converted into kinetic energy. The pendulum moves fastest at the very bottom (lowest point) of its arc, which is where its kinetic energy is at a maximum and its potential energy is at a minimum.