Equilibrium when applied to muscles and joints! Help, please!

1. Nov 6, 2005

confusedbyphysics

This seems like it should be easy but Im not sure what to do. Heres the problem:
The picture to the problem is here: http://www.webassign.net/giancoli/9-65alt.gif
"(a) Calculate the magnitude of the force FM required of the "deltoid" muscle to hold up the outstretched arm shown in Fig. 9-65. The total mass of the arm is 2.8 kg and = 12°.
(b) Calculate the magnitude of the force FJ exerted by the shoulder joint on the upper arm.
I found a) by doing .12 m (sin 12) F - .25 m (2.8 kg) (9.8 m/s^2) = 0. F = 264 N.
But how do I find the force FJ? Confused (we haven't had the lesson on this yet, but trying to get my homework done early because I have a busy week). THANKS for any help!!!

2. Nov 6, 2005

lightgrav

Now that you know all the Forces, you could treat it like a chapter 3 problem
(Sum of the force components (x,y) = 0 . you DID enjoy that, right?)

OR, you can sum torques around the tendon attachment point, to = 0
(pretending that it might rotate around this attachment point)
This gives the vertical component of the joint Force (Fjx applies no torque).

3. Nov 6, 2005

confusedbyphysics

thanks lightgrav, adding up the components worked. answer turned out to be 260 N. Thank you for the help!