# Homework Help: Velocity of rotating disc

1. Mar 1, 2016

### JJK1503

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

Find the velocity v of a general point on a plate rotating at 800 rpm which is r meters from the center.

This is literally a copy and paste job.

2. Relevant equations

period = T = 1 / f ; f = frequency

3. The attempt at a solution

This is what I did please tell me if it is correct

800 rpm = 40 / 3 rps = f

T = 1 / f = 3 / 40

v = distance / time = (2 pi r) / (3 / 40) = (80 pi r) / 3

2. Mar 1, 2016

### RUber

That is correct.
You could also have seen that 40/3 rps can be directly translated into a speed by replacing the r in rps with the distance covered in 1 revolution (2 pi r).
That would give you the same result $\frac{80 \pi r}{ 3}$m/sec.

3. Mar 1, 2016

### Staff: Mentor

Yes, that works. You need to carry the "seconds" unit through to the end though. The period T has seconds as its unit.

Otherwise you'll end up with a "velocity" with units of distance rather than distance/time.

4. Mar 1, 2016

### JJK1503

Awesome!!! Thank you.

5. Mar 1, 2016

### JJK1503

Ohh... I see. Thank you