# Homework Help: Circular motion experiment

1. Feb 25, 2012

### paperdoll

1. The problem statement, all variables and given/known data
I did this circular motion experiment where we had to change the mass of weights on the bottom of a fishing wire and we kept the radius the same. The questions in the post lab say to graph "centripetal force vs velocity". I worked out the velocity by using the 2pi*r/t formula but I'm not sure about the centripetal force. Would it be the mass on the bottom of the string providing the centripetal force? Very confused and would appreciate some help

Thanks

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Feb 25, 2012

### rollcast

The formula for clculating centripetal force is:

$f =\frac{mv^{2}}{r}$

r is the radius from the axis of rotation to the mass.

3. Feb 25, 2012

### paperdoll

I know this is the formula but apparently in this experiment, I'm supposed be only using the mass to find the centripetal force because the masses are hanging off the bottom of the string while the top is being whirled around.

4. Feb 25, 2012

### tiny-tim

hi paperdoll!
oh, you mean the string goes from a mass up to the centre, through a frictionless loop, then carries on to a weight that's being whirled around?

so the mass stays still?

in that case, the centripetal force is the tension T in the string

since the tension is the same throughout the string (because the loop is frictionless), it's also the only force that's holding up the mass …

so doing F = ma on the mass we have T - mg = 0,

so in this case, yes, the centripetal force (= T) is mg

5. Feb 25, 2012

### paperdoll

Thanks! That clears things up now :)