# Homework Help: Rotational dynamics - pleaaaaase help

1. Oct 17, 2007

A uniform board of mass m and length L is pivoted on one end and is supported in the horizontal position by a rope attached to the other end. Another rope, attached to the board a distance L/3 from the pivot point, is being pulled straight down with a constant force of magnitude F.

1) How far is the axis of rotation from the center of mass of the board?
Express your answer in terms of quantities given in the problem introduction.

2) How far is the axis of rotation from the point of application of force F?
Express your answer in terms of quantities given in the problem introduction.
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ok... what?!?!? There are no diagrams for this Q. And I'm a visual person. Care to explain what's going on???

2. Oct 17, 2007

The board is horizontal right now. The length of the board is L so if you pretend there's a coordinate plane: the pivot at one end is at x=0 and the other end is at x=L. Can you fill in the rest?

3. Oct 17, 2007

i think so ... so if the axis of rotation is L/2 from the center of mass of the board ... then the axis of rotation is L/3 from the point of application of force F??? i'm not quite sure on the second part

4. Oct 17, 2007

Ah, I think I see it now... So: Suddenly, the rope attached to the end of the board breaks. How many forces are acting on the board after the rope attached to the end of the board breaks?

First there were three forces... one on each side and one from below??? and now there is only one??

5. Oct 18, 2007

The only difference is that the force at L is gone, so now there are two forces. Which one did you forget?

6. Oct 18, 2007

actually there are three... what are they?? the ones on the left and right and gravity?

7. Oct 18, 2007

### learningphysics

so the answers are 1)L/2 and 2)L/3

are there more parts to the problem?

8. Oct 18, 2007

another Q: Find the angular acceleration alpha of the board immediately after the rope breaks.

so i know that Torque = I(alpha) ..... mhm... how would i go about using the quantities introduced in #1 box to find the net torque ...

Last edited: Oct 18, 2007
9. Oct 18, 2007

Last edited: Oct 18, 2007
10. Oct 18, 2007

### learningphysics

11. Oct 18, 2007

thanks for the response :)