2. Bill is holding a 12 kg barbell in his hand. His upper arm is perpendicular...
2. Bill is holding a 12 kg barbell in his hand. His upper arm is perpendicular to the ground, and his elbow is flexed to produce a 40º angle at the elbow joint. His forearm has a mass of 12 kg. The distance from the elbow joint to the weight in his hand is 35 cm. The center of mass (COM) of his forearm is located 14 cm from the elbow joint.
- Draw a free body diagram of this scenario. A free body diagram is a sketch of the objects of interest only, showing all of the known and unknown forces acting on the objects.
- Calculate the moment at Bill’s elbow joint due to the barbell and the weight of the forearm.
- What direction is this moment acting in?
- What muscle group is primarily activated to maintain this position?
- What is the direction of the moment that must be produced by these muscles to maintain static equilibrium?
Homework Answers
(a)
(b)
Moment at Bill’s elbow joint due to the barbell and the weight of the forearm,
M = Mb + Mf
M = mb*g*r1*sin40 + mf*g*r2*sin40
M = 12*9.8*0.35*sn40 + 12*9.8*0.14*sin40
M = 37.039 N.m
(c)
direction is this moment is into the page.
(d)
Elbow flexors are primarily activated to maintain this position.
(e)
Moment produced by the muscles should be directed out of the page to maintain static equilibrium.
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