Calculating Power
You can calculate power by dividing the amount of work done by the time needed to do the work.
Power
When using SI units in the power formula, work is in joules (J) and time is in seconds (s). The SI unit of power is the watt (W), which is equal to one joule per second. Thus, a 40-watt light bulb requires 40 joules each second that it is lit. This amount of power is also approximately equal to lifting your textbook a height of one meter in half a second.
Math Skills
Calculating Power
You exert a vertical force of 72 newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds. How much power is used to lift the box?
Read and Understand
What information are you given?
Plan and Solve
What formula contains the given quantities and the unknown?
Replace each variable with its known value and solve.
Look Back and Check
Is your answer reasonable?
36 watts is not a lot of power, which seems reasonable considering the box was lifted slowly, through a height of only 1 meter.
Math Practice
Your family is moving to a new apartment. While lifting a box 1.5 m straight up to put it on a truck, you exert an upward force of 200 N for 1.0 s. How much power is required to do this?
You lift a book from the floor to a bookshelf 1.0 m above the ground. How much power is used if the upward force is 15.0 N and you do the work in 2.0 s?
You apply a horizontal force of 10.0 N to pull a wheeled suitcase at a constant speed of 0.5 m/s across flat ground. How much power is used? (Hint: The suitcase moves 0.5 m/s. Consider how much work the force does each second and how work is related to power.)
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