# Rotational Inertial and Torque question

1. Nov 14, 2005

### skinnyabbey

Can anyone help me out?

The top has a moment of inertia
of 0:0002 kg.m2 and is initially at rest. It is
free to rotate about a stationary axis AA'. A
string, wrapped around a peg along the axis
of the top, is pulled in such a manner as to
maintain a constant tension of 5.22 N in the string.
If the string does not slip while wound
around the peg, what is the angular speed
of the top after 82.1 cm of string has been

I have a problem with this question. I don't know how to calculate the linear distance given to an angular distance. I found the acceleration first using Torque=(I*acc). But then I don't know what to do next after this. My acceleration turned out to be 26100 rad/s^2

2. Nov 14, 2005

### Chi Meson

in order to check your torque, we knweed to know the radius of the peg.
edit:

aha! by reverse engineering I have discovered that you have used the force as the torque (unless the radius of the peg is 1 meter, which I doubt).

If the radius of the peg (that the string is wrapped around) is not given, then there is not enough information to solve this problem.
$$\tau = I \alpha$$
$$\tau = Fr$$
$$Fr = I \alpha$$

Last edited: Nov 14, 2005
3. Nov 14, 2005

### hoseA

Try square root of [(2*Tension*String Length)/(Moment of Inertia)] .

4. Nov 14, 2005

### hoseA

also watch your units... (i.e. make sure length is in m NOT cm)

5. Nov 14, 2005

### skinnyabbey

thanks.it worked.

6. Nov 15, 2005

D'oh!