What is the Torque Problem in Muscle Mechanics?

[Noctix]

Homework Statement

A person with upper arm vertical and forearm horizontal holds a 4.5-kg iron cannon ball. Assume the mass of the forearm and hand is 1.5 kg, with a center of mass 15 cm from the elbow. The center of the cannonball is 32 cm from the elbow, and the force of the biceps is applied 5.0 cm from the elbow. (a) What force is exerted on the forearm by the biceps muscle?
correct answer=330N

Homework Equations

Tau₁+Tau₂+...=0

The Attempt at a Solution

Tau(biceps)+Tau(forearm)+Tau(cannonball)=0
r₁m¹gsin90° + r₂m²gsin90° + r₃m³gsin90°=0

plug in variables=wrong answer

thanks in advance, i really appreciate all help.

 

[PhanthomJay]

Homework Statement

A person with upper arm vertical and forearm horizontal holds a 4.5-kg iron cannon ball. Assume the mass of the forearm and hand is 1.5 kg, with a center of mass 15 cm from the elbow. The center of the cannonball is 32 cm from the elbow, and the force of the biceps is applied 5.0 cm from the elbow. (a) What force is exerted on the forearm by the biceps muscle?
correct answer=330N

Homework Equations

Tau₁+Tau₂+...=0

The Attempt at a Solution

Tau(biceps)+Tau(forearm)+Tau(cannonball)=0
r₁m¹gsin90° + r₂m²gsin90° + r₃m³gsin90°=0

plug in variables=wrong answer

thanks in advance, i really appreciate all help.

You have 3 masses listed, but there are only 2. Please show your math.

 

[Noctix]

Sorry, that was unclear. I assumed the mass for the biceps would be the mass of the cannonball plus the mass of the forearm because the biceps are exerting force on both.
is that fair to assume?

 

[Noctix]

I drew a picture similar to the illustration in my textbook for the other visual people like myself.
[PLAIN]http://img718.imageshack.us/img718/3923/physicsg.jpg


**My internet shuts down for the night in two minutes, so if anyone responds I will not be able to answer your questions till tomorrow. Any tips to point me in a general direction on how to solve this are greatly appreciated. Thanks

 

[PhanthomJay]

It is not correct to assume that the force in the biceps equals the mass of the arm, hand, and cannonball. Nor can you assume that the force in the biceps equals the weight of the arm, hand, and cannonball. There is also another force acting at the elbow. Sum torques = 0 about the elbow to solve for the bicep force. Show the numbers you are using.