If I use the total before as the expected value I get 200% percent error. So Total before = -0.0031 kg m/s and total after = -0.0091 kg m/s. If it is substantially more than 2 (like 4 or 5), then your measurement contradicts conservation of momentum (so it is probably flawed.Okay, so I did an elastic collision with Vernier carts and magnets.Ĭart one started with -0.1205 kg*m/s ended with +0.1027 kg*m/sĬart two started with +0.1174 kg*m/s ended with -0.1118 kg*m/s If the absolute value of that ratio is less than 2, your measurement is fine (in terms of statistics, your measurement is not significantly different from what you expected). Third, divide the difference between your measured and theoretical value by the error of the difference- i.e., (actual value-theoretical value)/(sqrt(2) * E). If the two measurements have the same error E, the error of the difference/sum is sqrt(2) * E. In the case of addition and subtraction, combined error (error of the sum or difference) is just the quadrature sum of the individual errors (error of each term being summed).

Second, determine the appropriate combined-error rule for the calculations you are doing. You can do this by repeatedly measuring the same (known) quantity and calculating the standard deviation of measurement results, by checking the documentation on your measurement device to find it's uncertainty, or by asking your instructor for assistance regarding the measurement uncertainty of the device (other ways I can think of depend heavily on what the experimental setup is.) You could easily calculate a percent difference but the answer would be utterly meaningless- you can't just say because the percentage is small the values are the same, because if you were working with very high precision equipment that percent difference could be way more than your measurement uncertainty! The appropriate way to address this question as is as follows:įirst, estimate the error in each of the two measurements (in your setup, the error in each is probably the same). Pretend the difference you were expecting was actually 1 N s, and and you measured 1.0172 N s. You need to stop and think about what you're doing.

How to find percent error