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Question about moments, fast answers please

  • Thread starter pyroknife
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moment=rXF. I was wondering, in order to find r, you have to find a point on where the force is being applied right and subtract whatever you're revolving around from that point. My question is, can that point be any point only the direction of the force? Someone told me it has to be the beginning point of where the force is first applied, but if that's the case, what if the profile doesn't specifythat "beginning point?"
 

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PhanthomJay
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moment=rXF. I was wondering, in order to find r, you have to find a point on where the force is being applied right and subtract whatever you're revolving around from that point. My question is, can that point be any point only the direction of the force? Someone told me it has to be the beginning point of where the force is first applied, but if that's the case, what if the profile doesn't specifythat "beginning point?"
The position vector r is from the point about which you are taking the moments to the point of application of the force. But if you don't know that point of application, you can choose any point along the line of action of the force, and choose that as your point of application.. That changes the vector r and the angle between the position and force vectors, but not the calculated moment. A sometimes and often easier way is to calculate the moment as force times perpendicular distance from the line of action of the force to the point about which you are summing moments. Watch plus/minus signs, and basic geometry/trig.
 

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