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?

1 Answer
Dec 21, 2015

I calculate 40% efficiency.


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

#"Efficiency" = 40/100*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.