# Circular motion (Newton's 3 Laws)

1. Sep 25, 2008

### southernbelle

1. The problem statement, all variables and given/known data
A coin placed on a 12 in record will revolve with the record when it is brought up to a speed of 33 and 1/3 rev/min provided the coin is not more than 4 in from the axis.
A) What is the coefficient of static friction between coin and record
B) How far from the axis can the coin be placed without slipping if the turntable rotates at 45 rev/min?

2. Relevant equations
ac= v2/R
T= 2pie r/v
Fc = mac
Fc=mv2/r

3. The attempt at a solution
Fc=Ff
Ff=coefficient of friction x m x g

So...
v2/r = coefficient of friction

1= 2pie x 4/vcoin
Vcoin = 8pie

so..
8pie/4 = coefficient?
that gives me a really big number.

And.. can you give me a hint to start on the b part?

2. Sep 25, 2008

### LowlyPion

w = 2πf = v/r This means that v = 2π*f*r

From your force equation you know that F = m*a = mv2/r = u*m*g

That means that u = v2/g*r = (2π*f*r)2/g*r = r*(2π*f)2/g

For part b you can use the fact that u is constant to develop an equation of like products between f and r.

Last edited: Sep 25, 2008