Physics - Coefficient of a Friction

You will have to use the Standard Error of the Mean.
Why? SEM shows the deviation of the mean of a sample (your estimation about the coefficient) around the population's mean (the exact value of the coefficient). When you calculate the mean of a sample , it is expected that the value is different from the original mean of the population. To find how different is the value you estimated from the original value, you use the Standard Error of the Mean.
The range should be x ± 2sx , or x ± 1,96sx to be more precise, with a possibility 5% that the coefficient of friction does not belong to this range (possibility of error)
sx is the Standard Error of the Mean. Note that the higher the number of observations, the smaller is the range (more accurate estimation).

Standard Deviation is different. SD shows how much does the observations of your experiment varies from the mean. Practically, by using SD you can estimate the range were new observations are most likely appear.

Many thanks!