Final answer:
The force the biceps muscle must exert to hold the forearm in position is calculated using equilibrium conditions, considering the weight being held and the angle of the forearm. The force at the elbow joint is determined as the reaction to the muscle's force and the weight's torque.
Step-by-step explanation:
When analyzing the forces in the forearm of a weightlifter holding a weight, we consider several forces, including the unknown force at the elbow (FE), the tension in the muscle (FB), and the weight of the object being lifted (w). Given the forearm positioned at ß = 60° with respect to the upper arm, the first step in finding the tension the biceps must exert is establishing a free-body diagram and then applying the equilibrium conditions.
In the scenario where the weightlifter is holding a 50.0-lb weight, which is equivalent to 222.4 N, we must calculate the tension FB that the muscle must exert. This is done by using the second condition of equilibrium where the pivot point is the elbow, making the torque caused by FE zero. This simplifies the problem to finding only FB.
To calculate FB, we need to take into account the torque due to the weight of the object and its distance from the pivot point (elbow). Since the forearm is at an angle of 60 degrees, we use trigonometry to resolve the forces. The torque generated by the biceps must counteract the torque from the weight of the object.
The force on the elbow joint is the reaction force to these torques. If the angle of the forearm changes, for example to 45 degrees, the torque on the muscle and elbow joint will change due to the different lever arm length, which directly affects the force required by the muscle to maintain equilibrium.