# Torque, force and rotational acceleration

1. Feb 21, 2010

### The_Journey

This isn't a homework question.

I'm really confused at this, say you're holding a massless ruler / stick and there is a mass (m) on it at a distance d from your hand. If you move the mass to 2d from your hand, would you have to apply twice the force as before, or quadruple to keep it stable?

I know torque = r cross F. F is constant (mg), so the force you would have to apply should be twice if you move it to 2d. But my teacher said it is quadruple because the rotational inertia of the stick would be m(2d)^2 which is 4 times the rotational inertia as before.

Can anybody explain to me if the force would have to be twice or 4 times as before?

Again NOT a homework problem, I just thought of this.

Some equations and math would be nice too.

2. Feb 21, 2010

### The_Journey

Nvm I'll just put this in the homework section.

3. Feb 21, 2010

### Naty1

NVM...you have two different situations, one static, one dynamic, hence two different answers....if you know the formula for rotational inertia you can work out that result using
v = wr

4. Feb 21, 2010

### The_Journey

Can you explain what you mean by static and dynamic? The stick is always stable.