Percent Error
When we do an experiment in physics or science, there is always some possibility of
error. This error can come from several sources: the limitations of the tools we are using
to measure, extraneous (outside) forces . Of course some errors are the result of less
than careful work.. Whatever the cause, errors occur. The important part is that we can
measure the extent of the error. Often the amount of error suggests a new way of
looking at what is going on with the experiment.
The general formula for % error is
accepted !measured
accepted "100
The vertical bars mean “absolute value.” Generally, %error is a positive number.
The accepted value is the value you should get. It might be listed in a book. For
example, when we measured the rpm’s of the drill using the strobe light, we compared
them to the manufacturer’s values printed inside the drill housing. The manufacturer’s
numbers were the accepted values and our measurements were the measured values.
Example: When using number three pulleys on the both the motor and the drive of the
drill, we measured the drill rate to be 1700 rpm. The accepted value (from the
manufacturer) was 1600 rpm. What was our % error?
Accepted = 1600 rpm
measured = 1700 rpm
Put it in the calculator like this ((1600-1700)/1600) X 100
You should get -6.25%
Don’t keep the negative sign
Instead of an accepted value, we might get a theoretical value. The theoretical value
would be the result of a calculation we do knowing the laws of physics. For example, if
we were to let a hot wheels car roll down a ramp, we might calculate that its velocity at
the bottom of the ramp (using Newton’s laws and the measured acceleration due to
gravity) should be 2.54 meters/second. This would be the theoretical value. It didn’t
come straight from a book. We might measure the car’s velocity to be 2.48 m/s.
Theoretical value: 2.54 m/s
Measured value: 2.48 m/s
theoretical !measured
theoretical "100
((2.54-2.48)/2.54) X 100 = 2.36% error
