# Homework Help: Center of mass problem

1. Nov 13, 2005

### Nm

To determine the location of his center of mass, a physics student lies on a lightweight plank supported by two scales L = 3.90 m apart.
If the left scale reads 250 N, and the right scale reads 106 N, find the student's mass.
Since the student is at rest, I know the scales read normal force = mg, and I found the center of mass to be 1.161 m. I found the masses from the two scales, but do I just add them together as the student's mass because subtracting the two masses and just using the left scale's mass aren't correct. Any hints are appreciated.

Last edited by a moderator: May 2, 2017
2. Nov 14, 2005

### Nm

Does anyone have any suggestions? Thanks.

3. Nov 14, 2005

### Astronuc

Staff Emeritus
The mass of the student and lightweight board provide a downward force as a result of gravity. The scales support that force in equilibrium, so the force (indicated by weight) upward must balance the force downward.

To the sum of the scales is the gives indicates the total weight, from which one calculates total mass.

How the weight is distributed between the scales is determined by where the CM lies between the two scales. The closer a scale is to the the CM, the more weight it would bear. This distribution of weight can be found by using the moments, which one has presumably done.