A machine does 75 J of work in 5 s. What is its power?

A machine does 75 J of work in 5 s. What is its power?

• A. 75 W
• B. 0.15 W
• C. 15 W
• D. 6 W

Answer: (C)

Power is the rate at which work is done, measured in watts (joules per second). To find power, divide the work by the time: 75 joules ÷ 5 seconds = 15 joules per second, i.e., 15 watts. This also checks out because 15 watts sustained for 5 seconds does 15 × 5 = 75 joules of work. The other values would not produce 75 J in 5 seconds (for example, 75 W would yield 375 J, 0.15 W would yield 0.75 J, and 6 W would yield 30 J). So the correct power is 15 watts.

Power is the rate at which work is done, measured in watts (joules per second). To find power, divide the work by the time: 75 joules ÷ 5 seconds = 15 joules per second, i.e., 15 watts. This also checks out because 15 watts sustained for 5 seconds does 15 × 5 = 75 joules of work. The other values would not produce 75 J in 5 seconds (for example, 75 W would yield 375 J, 0.15 W would yield 0.75 J, and 6 W would yield 30 J). So the correct power is 15 watts.