# Homework Help: A solid cylinder has a mass of

1. Nov 16, 2007

### bww

A solid cylinder has a mass of 10kg and a radius of .2 meters. Starting from rest it achieves a rotational velocity of 2 revolutions per second after 4 seconds.
a. What torque is required to cause this?
I found the Inertia, (.5)(10(.2) =.2 and found alpha= 2*2pi/4 and got 3.142. Then I took
.2(3.142) to find the Torque of .628Nm.

The problem I encounter is on part b.
B. How long did it take for the cylinder to rotate through its first 2pi radians?
I think it sounds rather basic but the question confuses me. Am I suppose to use the radius of .2 and the 3.142. I really don't know what is wanting me to do.

And then for part c.
C. Assuming this is produced by a string wound around the cylinder, how hard in Newtons, would the string have to be pulled?
I use the Torque formula again right? F*L? so i take like .628 and divide it by .2 to get 3.14N?

Those are my thoughts, great appreciation to anyone who can help me out! Thanks!

2. Nov 16, 2007

### Math Jeans

For part B, what it is asking is how long it takes for the cylinder to rotate one full circle. That is what 2pi radians is.

3. Nov 16, 2007

### Shooting Star

B) Use theta=1/2(alpha*t^2), where theta = 2*3.142. Find t.
C) Torque = Force*radius. What you've written is correct.

Last edited: Nov 16, 2007