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fishes
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Homework Statement
Consider a person holding their forearm horizontally in such a way that the bicep muscle, attached to the forearm at a distance of 4cm from the pivot in the elbow, is vertical. The mass of the forearm is 1 kg and the distance of the centre of mass of the forearm from the pivot is 12 cm. What is the force exerted by the bicep muscle?
(a) 117.6 N. (b) 29.4 N. (c) 9.8 N. (d) 3.27 N.
10. The person now shifts their arm so that the bicep muscle is no longer acting vertically, but the forearm is still horizontal. What happens to the force in the elbow joint?
(a) Nothing.
(b) It changes direction but not magnitude.
(c) It changes direction and decreases in magnitude.
(d) It changes direction and increases in magnitude.
Homework Equations
Net sum of torques? Clockwise torques (negative) = Counterclockwise torques (positive)
The Attempt at a Solution
So I'm pretty sure I got the first question (I chose b). I worked it out by first calculating the force acting on the center of gravity (9.8 x 1) and then using the net sum of torques to figure out the force acting on the bicep.
I'm more concerned with the second question, I know that muscle force is independent of angle but I'm a bit confused as I know that muscle angle does affect muscle fiber length which in turn affects magnitude - are magnitude and force not the same thing in this instance? I know the force of the elbow joint (fulcrum) is acting downwards when the bicep is vertical and I would have assumed that it would stay this way but this is apparently wrong as no change in direction is not an answer. Is muscle angle only independent of muscle force when it is vertical? I'm pretty lost on how to solve this question, at the moment I would assume that a change in magnitude occurs which according to this question means a change of direction occurs as well. I would probably choose (d) just because I feel like the elbow joint would have to exert extra force to make up for the missing bicep force in order to keep the forearm up?
Cheers