Uniform Circular Motion Question about Experiment (1 Viewer)

[petemod]

Hi,

Our class did an experiment last week on 'uniform circualr motion' and now I have to an experimental report.

I am trying to calculate the mass of a rubber stopper that was swinging around in 'uniform circular motion' with a radius of 1m and that it took 0.503 secconds to complete one rotation.

How do I find the mass of the rubber stopper?

He wrote a formula on the board: (shown below as if you typed in calculator)
Fc = (mv^2) / (r)

But if m = mass and I know r and the velocity I still have another unknown?

Thanks

 

[falcon07]

There should be a second mass on the end of the string to provide the Fc. As a result Fc=m2 x g. Putting that back into the other equation, you get:
m2 x g = (m1 x v^2) / (r)

 

[petemod]

So if the mass hanging to string was 250g then:
250 x g = (m x 12.49^2) / (1)
But what is g, gravity? Thats probably a stupid question.
Thanks

 

[falcon07]

Yeah, so g = 9.8

Making the final answer (be careful to put mass in kg not grams)
0.250 x 9.8 / 12.49^2 = m
m = 0.016kg

 

[petemod]

Thanks,
That seems to be a very small amount of mass though? Cant really remember how heavy it was but I thought it was a bit more than 16grams

I calculated the velocity by:
v = (2nr) / T
v = (2 x n x 1) / 0.503
= 12.49

Does that look right?

 

[airie]

Yes g is gravity, 9.8ms^-2. Remember to convert your measurements to SI units though: in this case you should put 0.25kg instead of 250g, and solving the equation will get you the mass of the rubber in kg.

EDIT: And that velocity does look right to me

 

[petemod]

Thanks falcon and airie, hopefully it is right

 

[alcalder]

But always remember in the report to account for errors. There could have been errors due to the fact that the string is not massless and would have contributed in some way on both ends.

What about air resistance. There could be lots of factors. Just be creative.

 

[Mtirnova]

Can sum1 pls send me this prac.. relyy need help with the theory.. email..Mtirnova@hotmail.com