# Simple fixed axis rotational problem

1. Apr 2, 2007

### emtilt

The problem statement, all variables and given/known data
A uniform stick of mass M = 2.1 kg and length L = 1.3 m is pivoted at one end. It is held horizontally and released. Assume the pivot is frictionless. Find the magnitude in newtons of the force Fo exerted on the stick by the pivot immediately after it is released.

I calculated the angular acceleration immediately after the stick's release to be 11.3 rad/s^2 for another step of the problem (and that was the correct solution for that part of the problem), but other than that I don't really know where to begin this problem. Could someone just provide a hint as to the direction I should be going in?

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2. Apr 2, 2007

### robb_

Running this through my head..., keeping in mind you have alpha, I would next think about what is causing that alpha.

3. Apr 2, 2007

### emtilt

Not entirely sure what you mean.

Gravity acts on the mass producing a torque of 13.368 Nm (from $$\alpha \frac{1}{3}ML^2 = \tau$$)...but I don't know where to go with that....

4. Apr 2, 2007

### robb_

You specify a moment of inertia, this depends upon where the axis of rotation is taken. This can be used to calculate the net torque, again with a specific axis of rotation in mind.

5. Apr 2, 2007

### emtilt

Ah, thanks. I got 5.14, which was correct, by taking the net torque with an axis at the center of the stick. This allowed me to get the torque provided by the pivot by dividing the 13.368 Nm from my previous post by two. The result could then be divided by L=1.3 to get the force.