# Need help with Tangential Acceleration

1. Sep 29, 2008

### eyvhgi557r

1. A coin placed on a 12" record will revolve with the record when it is brought up to a speed of 33.33 rpm. Provided the coin is not more than 4" from the axis...

2. a) What is the coefficient of static friction between the coin and the record?

3. I have calculated the velocity of the coin to be 27.92 in/s and the radial acceleration to be 194.92 in/s^2. Don't I need to calculate the tangential acceleration now to calculate the total acceleration? How do I calculate the tangential acceleration? I know the formula for ta is at = dv/dt.

2. Sep 29, 2008

### tiny-tim

Hi eyvhgi557r!

Yes, aT = dv/dt.

But in exam questions like this, you can assume that the record is "brought up" to the final speed so gradually that the tangential acceleration can be taken to be zero.

So just forget it!

3. Sep 29, 2008

### eyvhgi557r

Great! Thanks. So how do I calculate the coefficient of static friction from the radial acceleration/force? Is the force acting on the coin in the opposite direction of the radial acceleration considered "n", the normal force?

4. Sep 29, 2008

### tiny-tim

No … the normal force is always normal to the surface … in this case, the surface is horizontal, so it's the vertical force, mg.