How can I calculate the force needed to hold a book on a forearm?

[rice1am]

Homework Statement

Figure shows a forearm holding a book.
a.) Calculate the force the biceps muscle must exerts to hold the forearm and the book.
**all answers should be in three sigdigs.

Homework Equations

\tau = Frsin\theta

F=ma

The Attempt at a Solution

Well, I assumed that the torque of the book = the torque of the forearm; so, I just assumed that the F of the book = F of the forearm.

But I know that's completely wrong, and the answer has been provided by my prof, but she did not provide an explanation as to how she got that.

Please if anyone can answer my question! I have an exam on Wednesday and physics was never my best subject :| Thanks!

**Image of the problem is provided**

 

[loobloke]

Hi.

What you want to do is set the origin at variable positions.

For a system to be in equilibrium you need the sum of torques to be zero.
If you set an origin centered at the line of action, you will effectively eliminate any torque the force produces.
That leaves you with 3 torques to analyze, 2 of which are known.

It is important to note the convention of positive and negative torque in your calculations too.