A person holds a 100-kg weight above their head at a height of 2.0 meters for a total of 10 seconds. Determine the efficiency of the person at doing work on the weights.

A) 0%

B) About 25%

C) You need to do a few minutes of calculations

D) Efficiency cannot be determined with the given information

Solution: A. The person does no work on the weights because it is not moving. The person presumably uses some chemical energy for the given time, so the efficiency (W/E_chemical in this context) is zero.

The textbook efficiency of 25% is only an estimate under fairly optimal conditions for extracting useful work from the human body.