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CalTran
12-08-2011, 07:48 PM
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?

Ninja_Bait
12-08-2011, 08:05 PM
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?

The way I understand it is a force applied at a distance from the axis of rotation in such a way that the object rotates. That's the nice and easy Fxr version. (I think about the cross product in terms of finding the component of force that is perpendicular to the moment arm, which requires the sine function hidden in the cross product. The direction of torque is something like the side from which the rotation would look counterclockwise, but that's a little less important than the magnitude) The easiest example to understand is a door. You apply a given force at any point on the door, and your ability to move the door - i.e. the torque - is dependent on the angle of the force (parallel to the door would obviously not move it) and the position of the force (near the hinge is harder than near the knob).

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.

Andrew Lawrence
12-08-2011, 08:21 PM
Try watching these:

DonRotolo
12-09-2011, 10:53 PM
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?

Tristan Lall
12-10-2011, 12:28 AM
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...)It's mostly English speakers (in colloquial use) and Americans (all the time) that tend to confuse them. Americans are among the few still using the 'same' units for both weight and mass (pounds force vs. pounds mass, which aren't the same at all). If you're just learning this for the first time, you should probably use SI (which uses newtons for force, and kilograms for mass). Otherwise, use slugs (http://en.wikipedia.org/wiki/Slug_(mass)) and pounds (force), if SI isn't an option.

Ninja_Bait
12-10-2011, 08:58 AM
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?

I believe he means moment of inertia, which is like inertia/mass for translational motion but instead for rotational. Unlike inertia, MoI changes based on the axis of rotation.

DonRotolo
12-10-2011, 10:19 AM
I believe he means moment of inertia, which is like inertia/mass for translational motion but instead for rotational. Unlike inertia, MoI changes based on the axis of rotation.
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.

FWIW, I've been called a Slug many times.

Ether
12-10-2011, 10:51 AM
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?

Chris is me
12-10-2011, 10:57 AM
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?

2.73 kilograms, or just over 6 pounds.

Ian Curtis
12-10-2011, 11:48 AM
The way I understand it is a force applied at a distance from the axis of rotation in such a way that the object rotates. That's the nice and easy Fxr version.

Torque is defined as rxF. Fxr is the negative of rxF, so it is important for your sign conventions. :)

Ninja_Bait
12-10-2011, 11:55 AM
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.

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.

Torque is defined as rxF. Fxr is the negative of rxF, so it is important for your sign conventions.

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.

SenorZ
12-11-2011, 09:54 PM
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.

FrankJ
12-12-2011, 10:04 AM
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?

PAR_WIG1350
12-12-2011, 06:40 PM
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?

I can deal with Nm and foot-pounds, but kilogram meters make me sick. Unfortunately, that seems to be here to stay.

Ninja_Bait
12-12-2011, 08:26 PM
I can deal with Nm and foot-pounds, but kilogram meters make me sick. Unfortunately, that seems to be here to stay.

I like to use ounce-miles.

PAR_WIG1350
12-12-2011, 11:44 PM
I like to use ounce-miles.

1ounce*1mile/1 * 5280 ft/1 mile * 1lbf/16oz = 5280/16 ft*lbf = 330 foot-pounds. Actually, that isn't an unreasonable unit for some applications, impact wrenches come to mind (especially the big pneumatic ones).

Chris Hibner
12-13-2011, 08:05 AM
I like to use ounce-miles.

I had a first year college class where we were reviewing unit conversion. In one of the problems the professor had us calculate the speed of an object in furlongs per fortnight. Every time I think of unit conversions, I laugh about that.

DonRotolo
12-13-2011, 08:52 PM