# Uniform circular motion lab question

1. Mar 18, 2013

### sunriserocks

Could any here smart people help me with the Lab question: 3. Why do we measure the period by timing 25 revolutions? Why not one revolution or 100
revolutions?
This is from http://www2.sunysuffolk.edu/denicog/labs/lab11_132.pdf
Lab from Uniform Circular motion.

Thank you so much!!!

2. Mar 18, 2013

### tiny-tim

welcome to pf!

hi sunriserocks! welcome to pf!
it's a compromise

too few revolutions, and the experimental error will be too high

but too many revolutions, and the speed will change

so you choose a number that you guess will minimise the total error

the book is advising you that, in this case, 25 is about right

3. Mar 18, 2013

### Emilyjoint

You need to realise that any measurement involves an uncertainty (error).
When you are dealing with oscillations there are 2 obvious causes of uncertainty.
1) you may mis-count the number of oscillations
2) you may mis-time the oscillations
Lets say that you can count the oscillations to within 1, ie you may miss one ! !. So if you decide to count 10 oscillations you could be out by 10%.....so count more than 10 oscillations if possible.
Lets say you can time with a stop watch to within 1 second.....by the same logic make certain that the time period you measure is more than 10 seconds.
What you decide to do depends on the circumstances. The principal behind it is uncertainty.
If 25 oscillations occur in 2 seconds this is not good enough because of the uncertainty in time.