The bones of the forearm (radius and ulna) are hinged to the humerus at the elbo

Question

The bones of the forearm (radius and ulna) are hinged to the humerus at the elbow. The biceps muscle connects to the bones of the forearm about 2.15 cm beyond the joint. Assume the forearm has a mass of 2.35 kg and a length of 0.445 m. When the humerus and the biceps are nearly vertical and the forearm is horizontal, if a person wishes to hold an object of mass 6.55 kg so that her forearm remains motionless, what is the force exerted by the biceps muscle? Number Tools Muscle × 102 Humerus Radius Elbow Ulna Hand

Explanation / Answer

F = force by biceps

r = distance of force at bicep from the elbow = 2.15 cm

m = mass of forearm = 2.35 kg

L = length of forearm = 0.445 m = 44.5 cm

M = mass to be held = 6.55 kg

using equilibrium of torque at the elbow joint

F r = mg(L/2) + MgL

F (2.15) = (2.35 x 9.8) (44.5/2) + (6.55 x 9.8) (44.5)

F = 15.67 x 102 N

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