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Torque?
So, in Physics, we just started learning about Torque, and I was wondering if there was a simpler explanation than the one given by my teachers about inertia being equal to weight and what not. Would anyone be able to explain it in simpler terms?
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Now, force equals mass times acceleration, and acceleration equals angular acceleration times radius. So substitute and now you have Amr^2 (just pretend that A is alpha), but that really only applies to a point mass. What you actually have is the sum of all the little bits of torque. That's equal to the sum of all the little bits of mr^2 (or the integral of r^2 dm if you have an understanding of calculus), which is I, multiplied by A which is the same for all the particles. So torque is moment of inertia times angular acceleration. That doesn't really explain the F=ma and tau=IA analogy, but I hate that, too. |
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Torque and Inertia are two different things.
Torque is how hard you twist something. One typical unit of measure is foot-pounds. Three pounds of force turning something from Two feet away exerts six (2*3=6) foot-pounds of torque. Often Torque is shown in Pound-feet to emphasize the units. Inertia is the property of matter to resist acceleration due to its Mass. Things not moving tend to stay that way unless something pushes it. Weight and Mass are NOT the same, but we humans tend to confuse them, because they are measured in the same units (pounds, or kilograms, or...) These values are approximately equal on the earth's surface (due to Earth's gravity) Does that help, or did I misunderstand the question? |
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FWIW, I've been called a Slug many times. |
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An astronaut on the surface of the Moon is holding a string, at the end of which is tied a Moon rock. The tension in the string is 1 lbf. What is the mass of the rock? |
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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. Quote:
Can someone with immense Chief Delphi editing power fix that for me? kthxbye. |
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I teach my conceptual physics students that torque is a twisting force, one that may cause rotation.
Pushing with a force inward on the end of a door produces no rotation, but the same force exerted on the face of the door may make the door move. In AP Physics we discuss the mass distribution of the object being rotated, how the moment of inertia will affect the angular acceleration, etc. Little more tricky, but way more fun. |
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What causes some confusion it in SI units, torque is expressed as Newton meters, which is the same unit as a Joule (work). But torque is not the same as work although they have the same unit. Until you multiple torque by radians traveled. Through the magic of unitless numbers, you get work.
As for what units to use... If you live in the states, get used to working in both because it is going to be a missmash for the foreseeable future. :( Thinking about Lb mass verses lb force gives me a head ache so I never use lb mass. I have not used a slug since school. I do have mechanics use a slugging wrench to tighten bolts. Does that count? |
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