# Homework Help: Static Equilibrium of deltoid muscle

1. Dec 4, 2008

### Bones

1. The problem statement, all variables and given/known data
Calculate the magnitude of the force, FM, required of the "deltoid" muscle to hold up the outstretched arm shown in the figure. The total mass of the arm is 3.3 kg. Then, calculate the magnitude of the force FJ exerted by the shoulder joint on the upper arm.
http://www.webassign.net/gianpse4/12-56.gif

2. Relevant equations

3. The attempt at a solution
We did a problem similar to this in class, but there was a 8.5kg object in the hand.
X direction:
Fjcos(theta)-Fmcos15=0

Y direction: Force of weight=32.2N
-Fjsin(theta)+Fmsin15 Force of object=83.3N
-32.3N-83.3N=0

tnet=Fm(0.12m)(sin15)-32.3N(0.24m)(sin90)-83.3N(0.52m)(sin90)=0
Fm=1644N

How would you this problem change is there is no weight in the hand like the above problem suggests?

2. Dec 4, 2008

### G01

If there was no weight, you would not include the force of gravity from that weight or the torque caused by that force in your calculations.

3. Dec 4, 2008

### Bones

Does this mean I just leave it out or do I alter the equation in some way?

4. Dec 4, 2008

### Bones

I got part a:
tnet=mg(24m)-Fmsin15(0.12m)=0
Fmsin15(0.12m)=(3.3kg)(9.8m/s^2)(0.24m)
Fm=(3.3kg)(9.8m/s^2)(0.24m)/0.03106m
Fm=250N

How do I figure out Fj for part b?