Chapter 09, Problem 22 A man holds a 164-N ball in his hand, with the forearm ho

Question

Chapter 09, Problem 22 A man holds a 164-N ball in his hand, with the forearm horizontal (see the figure). He can support the ball in this position because of the flexor muscle force M, which is applied perpendicular to the forearm. The forearm weighs 24.7 N and has a center of gravity as indicated. Find (a) the magnitude of M and the (b) magnitude and (c) direction (as a positive angle counterclockwise from horizontal) of the force applied by the upper arm bone to the forearm at the elbow joint. Upper arm bone Flexor muscle Elbow joint 0.0510 mi 0.0890 m 0.330 m (a) Number (b) Number (c) Number Units Units Units

Explanation / Answer

Here, as the man holds the ball motionless so the arm and ball are in equilibrium hence the net force and net torque is zero.

(a) Now the net moment (torque) about the axis of elbow joint

M*0.0510 = 24.7 * (0.0510+0.0890) + 164*0.330

M = 57.5/0.0510

M = 1129 N

(b) here as we know that sum of forces is zero

F(bone) = M - 24.7 -164 = 940.2 N

And the direction is downward

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