What is the coefficient of static friction for a penny on a rotating wheel?

[d.tran103]

Can someone look at this and tell me what I'm doing wrong? Thanks!

Homework Statement

You place a penny on a level rotating wheel at a point 5.00 cm from the center of rotation. You increase the speed of rotation gradually, and once you reach a rotational speed of 0.825 revolutions each second, the penny slides off the wheel. What is the coefficient of static friction

a) 0.718

b) 0.409

c) 0.253

d) 0.137

Homework Equations

Fc = mv^2/r = mac
v=wr

The Attempt at a Solution

Fc = mv^2/r = mac

mv^2/r = mac turns to v^2/r = ac. I converted 0.825 rev/s to rad/s by multiplying by 2pi/1rev to get 5.18 rad/s. I plugged w into v=wr, v=(5.18 rad/s)(.05m) to get v=.259 m/s. I plugged v into v^2/r = ac, .259^2/.05 and got 1.34. However, 1.34 isn't an answer. Can someone tell me what I'm doing wrong?

 

[Dick]

You found the acceleration. The question isn't asking for the acceleration. It's asking for a coefficient of static friction. That's the ratio between the normal force and the centripetal force. There should be a g=9.8m/sec^2 in there somewhere.

 

[PhanthomJay]

You solved for the centripetal acceleration. Now you need to find and identify the centripetal force.

 

[d.tran103]

Thanks, I was missing 1/9.8!