# Homework Help: Motion in a plane II, Coin on a turntable with Vmax

1. Oct 18, 2011

### SherBear

1. The problem statement, all variables and given/known data

A 3.90 g coin is placed 12.0cm from the center of a turntable. The coin has static and kinetic coefficients of friction with the turntable surface of Ws= 0.800 and Wk= 0.410.

What is the maximum angular velocity with which the turntable can spin without the coin sliding?

2. Relevant equations
?

3. The attempt at a solution
? no idea
can you please show all the steps and the solution, thank you!

2. Oct 18, 2011

### LawrenceC

Draw a free body diagram of the coin showing all forces. Then look at the forces and determine how they interact with one another.

3. Oct 18, 2011

### SherBear

Does a turntable turn clockwise? Normal force up, mg down, Ff opposite of the motion if clockwise then it's counterclock wise?

I have Vmax=SQRT Ws r g=SQRT .800*.12m*9.81 m/s^2= .938 m/s, but I don't know if that's a relevant equation.

Then I have an equation for angular velocity=Vt=wr

4. Oct 18, 2011

### LawrenceC

The turntable is turning at constant speed so you are not concerned with tangential force here. We want to determine what the maximum speed is.

At the point of impending coin movement outward, what forces are exactly balanced?

5. Oct 18, 2011

### LawrenceC

"I have Vmax=SQRT Ws r g=SQRT .800*.12m*9.81 m/s^2= .938 m/s"

Use parentheses.

Your calculator seems to cause problems.

I am signing off for the day. You are on the right track.

Angular velocity = V/r

Last edited: Oct 18, 2011
6. Oct 18, 2011

### SherBear

I think the normal force and gravitational forced are the balanced forces.

7. Oct 18, 2011

### SherBear

ok i did the calc with para, and the result was Vmax=.9704 m/s
then i plugged that into the m=v/r
.9704 m/s / .12 m = 8.09 rad/s ?

8. Oct 19, 2011

### LawrenceC

That is what I calculated. Good work.

9. Oct 19, 2011

### SherBear

Thank you Lawrence! and for all your help and hardwork also! :)

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