units of torque

[dstebbins]

As I read the Wikipedia article on torque, found here, I notice that the SI unit of torque is the newton-meter, not the Joule. Why is that? The base units for torque come out to the same as work or energy (kgm^2/s^2), and it's even calulated with the same math as work (force times length), so why is it not refered to as a Joule?

[swansont]

Because a Joule is specifically defined as a unit of energy. Torque and energy are not the same thing.

[dstebbins]

Then why is work still defined as a Joule?

[Klaynos]

Then why is work still defined as a Joule?

 

Because work is an energy transfer.

[Severian]

Torque is more analagous to a force than an energy. But since it is basically force on a lever arm, you need to state the lenght of the lever, so Newton-metres.

[dstebbins]

Because work is an energy transfer.

 

Is work the transfer of energy, or is force the transfer of energy? Think about it: Work is defined as force times distance. If you don't move the object, there's no work done. I could push on my house with my bare hands all day long, nothing's going to happen. I'm not doing any work, just exerting meaningless force, but I am still depleting energy, am I not?

 

But if energy were really the ability to do work as the definition says, then I should be able to push and push forever and not deplete a single Joule of energy because no distance = no work, and by defnition, no work = no energy, therefore, no distance SHOULD = no energy, yet it does.

 

So is energy really the ability to do work, or is it the ability to exert force? By that logic, the unit of energy should be a Newton.

[swansont]

Energy is the capacity to do work.

 

You, as an engine, may dissipate heat in trying to move your house, but you do no work for exactly the reason you note: no displacement. The internal workings are a question of thermodynamics, while the macro result is mechanics.

[dstebbins]

Okay, you're going to have to explain your vocabulary to me. Dissipate? Nore? Internal vs. Macro?

[swansont]

http://dictionary.reference.com/

[dstebbins]

Actually, I figured it out, but not because of anything you told me. When I push on my house, I am performing work, just not on the house. I'm pushing myself back away from the house due to Newton's 3rd law; therefore, work is being done, and energy is being burned. I can also force myself forward at the cost of the same amount of energy that I exerted pushing backward, to stay in a stationary position, but just because the works cancel out doesn't mean their non-existent.

 

So nevermind. I figured it out, although on my own.

[swansont]

You are misapplying Newton's third law: you push on the house so it pushes on you; it is not the case that the reaction force is you pushing yourself back from the house. It's still you pushing on the house — the original force.

 

The conceptual issue with this is that it ceases to be a true mechanics problem if you want to account for all of the energy, and becomes a thermodynamics problem. Using physics definitions you are not doing any work.