Define power. What are the units?

• A. Power = work/time; units: watts (W)
• B. Power = mass × acceleration
• C. Power = force × distance
• D. Power = velocity × time

Answer: (A)

Power is the rate at which work is done or energy is transferred. When you push something and it moves, you’re doing work, and power tells you how quickly that work happens. The unit of power is the watt, defined as one joule per second. So average power equals work divided by the time over which the work is done. Since work itself is force times distance, power can be thought of as energy moved per second, which is why its units are joules per second. If you look at instantaneous power, it can also be written as P = F × v when the force and motion are in the same direction, which again yields joules per second (watts). The other expressions mix up different ideas: force × distance is work, not power; mass × acceleration is force; velocity × time gives distance, not a rate of energy transfer.