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henry3369

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## Homework Statement

A uniform plank of length L = 6.0 m and mass M = 90 kg rests on sawhorses separated by D = 1.5m and equidistant from the center of the plank. Cousin Throckmorton wants to stand on the right-hand end of the plank. If the plank is to remain at rest, how massive can Throckmorton be?

http://imgur.com/AVrdxIJ

## Homework Equations

net torque = 0

## The Attempt at a Solution

In my book it says that for problems involving torque, the reference point is arbitrary. I don't understand how this is possible.

For example: when I calculate the mass of the person in this problem using the right sawhorse as the center of rotation, I find the mass to be 30 kg. But if I take the net torque about the center of mass (which is at the center of the plank), then the torque on the left will be zero with only one torque on the right (the person).

So mg x 3 = 0, which means the mass of the person has to be zero, and the normal force from the saw horse has to be zero because the mass is concentrated to the left of the it.

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