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ttk3
Jun25-08, 01:01 PM
1. The problem statement, all variables and given/known data

The moment of inertia for an arm or leg can be expressed as I = AmL^2, where A is a unitless number that depends on the axis of rotation and the geometry of the limb and L is the distance from the center of mass. Say that a person has arms that are 31.30 cm in length and legs that are 40.69 cm in length and that both sets of limbs swing with a period of 1.20 s. Assume that the mass is distributed uniformly in both the arms and legs.

Calculate the value of A for the person's arms.


L arm = .313 m
T = 1.20

2. Relevant equations

A = (g/L) (T/2pi)^2

3. The attempt at a solution


(9.8/.313) (1.20/2pi)^2 = 1.142

I'm not sure where I'm going wrong with this problem. After the derivation of the equation it's plug and chug. I looked up the equation I derived and it's correct. Can anyone lend me a hand?
alphysicist
Jun25-08, 03:18 PM
Hi ttk3,

What does L represent in the equation? You plugged in the length of the arm but I don't think that's correct.
ttk3
Jun26-08, 08:50 PM
It says that L is the distance from the center of mass. Wouldn't the center of mass be located at the top of the arm (the point from which the pendulum swings)?

The equation is derived from the attached equation, and L would be the arm length in that one I thought.
alphysicist
Jun26-08, 09:06 PM
It says that L is the distance from the center of mass. Wouldn't the center of mass be located at the top of the arm (the point from which the pendulum swings)?

The equation is derived from the attached equation, and L would be the arm length in that one I thought.

I can't view the attachment yet, but the center of mass would not be at the top of the arm. The problem indicates that the mass is uniformly distributed in the arm.

Pretend the arm is a uniform rod. Where is the rod's center of mass? It's not at either end of the rod.

Also, when you say that L is the distance from the center of mass, is that all it said? That does not sound like it is complete. Shouldn't it be something like, L is the distance from the center of mass to the shoulder (arm's pivot point)? Because distances are between two points.
ttk3
Jun29-08, 11:07 AM
The question is a direct copy and paste from the program. So if the center of mass is the center of the arm, would I the lenght of the arm divided by two for my L value?
alphysicist
Jun29-08, 12:48 PM
If L is the distance from the center of mass to the shoulder, then that sounds right to me. What do you get?