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Originally Posted by DonRotolo
No, I meant what Tristan said, the point being that Lbf and Lb are not the same, despite the same name. I got it almost right.
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I'm sorry, I'll be a little clearer. You're absolutely right that inertia is equivalent to mass and not weight, and that imperial units are stupid. But that's not really at question here.
When you talk about rotational motion, including torque, you talk about analogous quantities between rotation and translation. Torque is to force as angular acceleration is to acceleration as moment of inertia is to mass. That's what the OP's teacher means (or should mean) by "inertia is equal to weight". Translated into good physics that means that
torque equals angular acceleration times moment of inertia just as
force equals acceleration times mass. The comparison should make rotational motion less intimidating, because the equations are just like the ones in translational motion, with a different set of variables.
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Originally Posted by Ian Curtis
Torque is defined as rxF. Fxr is the negative of rxF, so it is important for your sign conventions.
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Right, sorry about that. I'm always mixing up cross products. Fortunately the direction of torque is kind of silly and you usually just need magnitude, F times r times sin(theta). For direction, just remember that forces in the anticlockwise direction yield positive torques and you'll be fine.
Can someone with immense Chief Delphi editing power fix that for me? kthxbye.