The figure below a weightlifter holds a dumbbell in place. The biceps muscle exe

Question

The figure below a weightlifter holds a dumbbell in place. The biceps muscle exerts a force on the forearm. The upper arm exerts a force on the elbow joint. Gravity exerts a force at the center of mass of the forearm, and the barbell exerts a force at the hand. Draw the free body diagram and torque diagram for the situation below. 7. Biceps muscle Elbow In the above problem, the dumbbell weights 60 Ibs. The forearm weighs 15 lbs. The biceps is attached to the forearm 2.0 in. from the elbow joint; the center of mass of the forearm is 7.30 in. from the elbow joint. (Note: these units are English rather than metric. Pounds are a unit of force in this system, and the unit of torque for this problem is inch-pounds.) Find the force exerted by the biceps to lift the dumbbell. What is the magnitude of the force exerted by the elbow joint. Show your work and explain your reasoning. 8.

Explanation / Answer

Since the weightlifter is holding the dumbbell, we can consider it is in static equilibrium.
In static equilibrium, the net torque about any point is zero
Consider the net torque about the elbow joint.
FM x 2 - WA x 7.3 - WO x 14.5 = 0
FM = [WA x 7.3 + WO x 14.5] / 2
= [15 x 7.3 + 14.5 x 60] / 2
= 489.75 lbs    ( = 2178.52 N)

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