Must answer all three parts for positive rating. Part C is asking for a number.

Question

Must answer all three parts for positive rating. Part C is asking for a number.

A man holds a 172-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 21.2 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 0.0890 m 0.330 m 0.0510 m

Explanation / Answer

Since the man holds the ball motionless, the ball and the arm are in equilibrium. Therefore, the net force, as well as the net torque about any axis, must be zero.

part a )  the net torque about an axis through the elbow joint is

sum of torque = M*0.0510 - (22 * 0.14 ) - (172 * 0.33) = 0

M = 1173 .33 N

part b )

net torque about an axis through the center of gravity is

t = -M*0.0890 + F * 0.140 m - 172*0.190 = 0

F = 979.33 N

direction must be downward ,because the forces must add to give a net force of zero

270 degree

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