22.   A man holds a 178-N ball in his hand, with the forearm horizontal (see the drawing). He can support the ball in this position because of the flexor muscle force , which is applied perpendicular to the forearm. The forearm weighs 22.0 N and has a center of gravity as indicated. Find (a) the magnitude of  and (b) the magnitude and direction of the force applied by the upper arm bone to the forearm at the elbow joint.

 

w0429

 

Take the torques about the elbow joint.  This way the force of the upper arm has no lever arm and will therefore have no torque about elbow joint.

 

 

 

 

 

 

 

 

 

 

 

The magnitude of the force of the upper arm bone we get from summing forces in the y direction

 

 

Minus sign means we had assumed upward force, so instead it is actually a downward force.

 

 

 

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This page last updated on January 11, 2020