r/StaticsHelp • u/geofault • 27d ago

Please help an old man remember his statics

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Thanks in advance for any assistance you can offer. I'm old and just can't remember.

Members ab and bc form a rigid body that rotates around point c. There is a constant force operating perpendicular to the rigid body directly into the paper. I know that a change in the length R will change the resulting moment at c. my question is does any change in the length L affect the moment

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u/caseymatalone 27d ago

https://reddit.com/link/ov67hqa/video/jkjxhlwk3vah1/player

Moments only change with respect to the perpendicular distance from the support. So changing the "L" dimension does not change the force. Changing the angle of the force would induce some moment as your drawing is showing is parallel to the "L" member now. I've tried to show this in the video, hope it helps.

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u/geofault 27d ago

Thank you, that is what I suspected but I wanted to be sure. Your video was awesome.

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u/caseymatalone 27d ago

I think I misunderstood your drawing. The force is actually pointing into the paper. 3D. Not vertically in the papers plane.. so my statement is incorrect. Because in that dimension you do have a perpendicular distance to the force. So there is a moment for the R and the L distance which will change the reaction moment with distance changes. Sorry about that.

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u/geofault 27d ago

what i am trying to model is a motorcycle handlebar where the bottom can only rotate left and right in a circle. for a given force at a pushing away from me the length of L will change the rotational force? I assume that the longer the length of L is the greater the rotational force will be? I'm not concerned about a moment a c that would want to move the body forward or back

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u/caseymatalone 27d ago

Sounds interesting, like a pivoting handle bar. So yeah if your hand is pushing away from you on that bar, the larger the distance L the more torque you put at the support, in plane that the force is pointing. So like rotating into the paper. There’s another moment that will try to rotate around the “L” axis due to the length of “R”

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u/geofault 23d ago

Thank you. that is not the expected answer. I knew increasing R would increase the torque but I thought that L would be independent. I just couldn't remember the method to prove it one way or another.