Question

In the human arm, the forearm and hand pivot about the elbow joint. Consider a simplified model in which the biceps muscle is attached to the forearm 3.80 cm from the elbow joint. Assume that the person's hand and forearm together weigh 15.0 N and that their center of gravity is 15.0 cm from the elbow (not quite halfway to the hand). The forearm is held horizontally at a right angle to the upper arm, with the biceps muscle exerting its force perpendicular to the forearm.

Answer 1

Part A
Find the force exerted by the biceps when the hand is empty.
F =59.2
Correct


Part B
Now the person holds a 80.0- weight in his hand, with the forearm still horizontal. Assume that the center of gravity of this weight is 33.0 from the elbow. Find the force now exerted by the biceps.
F =754

Part D
Under the conditions of part B, find the magnitude of the force that the elbow joint exerts on the forearm.
F = ???????

Answer 2

May i know how you do part A? Cause i think something seems wrong to me somewhere. It you could tell me how you work out Part A, it would clear my doubts.

Answer 3

Just the steps will do.

Answer 4

I used torque with axis over elbow which makes F=(W*0.15)/0.038

Answer 5

And that is the correct answer?

Answer 6

yes, Part B is also correct. For part D, answer should be 659 N but I can't get it

Answer 7

80 is in kg or in N

Answer 8

oh, sorry, it deleted all units while I pasted. All weights are in N and all length are in cm

Answer 9

Part B answer is actually 75.4N right? Not 754

Answer 10

nope, it is 754 N.
3.8 cm = 0.038 m

Answer 11

The arm is in equilibrium. Because you have calculated the moment of each force and balanced it. But translational equilibrium is not balanced.

Total upwards force = total downwards force
Force by elbow joint + Force by biceps = weight of arm + weight of the 80N weight.
Force by elbow join + 754 = 15+80
Force by elbow joint = 754-15-80 = 659N

Answer 12

Sorry i took so long, was so confused by the lack of units. And then i misread the question, ignoring that they assumed the biceps is actually on the forearm. lol.

Answer 13

Oh wow it's pretty easy, I struggled with this of a while. I got confused because of F of biceps is perpendicular to forearm so I didn't count it... Thanks very much!!

Answer 14

@timoha :) Love that name