# A student does 100 J of work on the handle of a bicycle pump. The pump does 40 J of work pushing the air into the tire. What is the efficiency of the pump?

Dec 21, 2015

I calculate 40% efficiency.

#### Explanation:

The efficiency of a device can be calculated using these steps:

1) Measure the amount of work put in (input);
2) Measure the work that the device puts out (output);
3) Divide the output by the input;
4) Multiply by 100 to get a percentage.

As an equation, this becomes:

"Efficiency" = ("output")/("input")*100

The input is the $100 J$ of work done by the student. The pump's output is $40 J$. We can substitute these numbers into the equation and get:

$\text{Efficiency} = \frac{40}{100} \cdot 100$
"Efficiency" = 40%

Efficiency values are always expressed as percentages. This pump is not particularly efficient - less than half the work put in by the student is actually used to pump air! The rest is converted to heat and sound, then lost to the surroundings.