Quote:
Originally Posted by EricVanWyk
0 = The friction force from sliding over the carpet. I'm not so sure on this one - I think it is zero because it is right at the tip point?
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Correct. The distance is 0, so r * F will always be 0 for the torque due to friction around that point. This is the reason to choose the contact point of the leg.
Quote:
Originally Posted by EricVanWyk
This simple formula is pretty grokable, but is it true? Or more specifically, is it true enough?
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Assuming that the device is in a static configuration, i.e. it's not accelerating noticeably, all your math looks good. If you want to deal with acceleration, choose the center of mass of the device, and do torque calculations around that. Assuming that I'm not forgetting something, if you push below the center of mass, your equation is conservative when accelerating and should always return an answer that will work.