An arm is held out horizontal. Take the shoulder as the axis. Assume that the ce
ID: 2040189 • Letter: A
Question
An arm is held out horizontal. Take the shoulder as the axis. Assume that the center of mass of the arm is 0.23 m from the shoulder joint, and the arm's mass is 3.2 kg.
The tendon of the deltoid muscle is attached to the bone of the upper arm some distance from the shoulder and acts at an angle of about 12° to the arm. As a result, the "lever arm" of this force by the deltoid muscle (i.e. perpendicular distance from the line of action of the force to the axis) is 0.025 m.
What force must be exerted by this muscle on the arm, in units of N? Give your answer to 3 significant figures. Use g = 9.8 N/kg.
Explanation / Answer
let m = 3.2 kg, d = 0.23 m
x = 0.025 m
F_muscle = ?
As the arm is in equilinrium net force and net torque acting on it must be zero.
Apply net torque about axis of rotation = 0
F_muscle*x - m*g*d = 0
F_muscle = m*g*d/x
= 3.2*9.8*0.23/0.025
= 289 N <<<<<<----------------Answer
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