# Homework Help: Uniform circular motion on a table

1. Oct 5, 2008

### cj3

1. The problem statement, all variables and given/known data
A small block with mass m rests on a frictionless horizontal tabletop a distance r from a hole in the center of the table. A string tied to the small block passes down through the hole, and a larger block with mass M is suspended from the free end of the string. The small block is set into uniform circular motion with radius r and speed v.

2. Relevant equations
a=v2/R
F=ma

3. The attempt at a solution
I really have know idea...but here goes nothing
Mg=Ma
Mg=Mv2/r
g=v2/r
v=sqrt(g/r)

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2. Oct 5, 2008

### Rake-MC

What is the question..? That is just a statement.
I assume they are asking for the velocity at which larger block is neither rising nor falling?

3. Oct 6, 2008

### cj3

yes, sorry, thats what theyre looking for

4. Oct 6, 2008

### Rake-MC

You have got the right idea with the equation, by letting $$ma = \frac{mv^2}{r}$$

however this is where the error is. You cancelled the masses. You can't do this because the question specifically states that the mass hanging on the bottom of the string is larger than that on the table.

So your equation is now: $$m_1 a = \frac{m_2 v^2}{r}$$ where $$m_1 > m_2$$

Solve for v.