# Speed of CDs

1. Aug 27, 2005

### Donna

My son has asked me a question I cannot answer. Can someone help?

Does any given point on the outer edge of a CD (or any spinning object, like a record album), turn at a faster speed than a given point on the inside edge, because it is going a farther distance in the same amount of time?

What is the math involved?

Thanks.

Donna

2. Aug 27, 2005

### whozum

All points on the CD make the same number of revolutions per unit time, but the linear distance traveled varies with the distance from the center. The outermost part of the CD travels faster than the innermost part.

Linear velocity (speed) = Radius (Distance from center) * Angular velocity (revolutions per unit time)

3. Aug 27, 2005

### Fermat

Yes, the points further from the centre (of rotation) will move faster.

All the points have the same speed of rotation, though. e.g. 33 rpm.

But, the speed of rotation (called the angular velocity) is related to the circumferential speed of the point like this,

v = wr

where v = peripheral velociity
w = angular velocity
r = radius, i.e distance from the centre of rotation.

4. Aug 28, 2005