Question

Since your forearm is not accelerating, the net force on it must be zero. The forces on it considered so far in this problem (weight of arm, contact force from hunk of metal, tension from tendon attached to biceps muscle) do not add up to zero! What other force acts on your forearm, and what is its magnitude and direction?

Answer 1

We generally consider the forces such that we balance the torque around the elbow. As elbow becomes the reference point, we do not consider the forces acting there as torque due to those would be zero (as distance from point of rotation is zero). But when we have to balance forces for translational equilibrium, we have to consider all the forces and hence the force acting on the elbow joint has to be considered. It is better to consider two components of this force, one is along the arm and other is perpendicular. An example of a hand holding a ball and some calculations are shown for reference below.