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Q & A: What is Torque ?

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Most recent answer: 10/22/2007
What is torque?
- Claire (age 13)

"Torque" is a way of describing how hard something is turned or twisted, in the same way we talk about "forces" as something that pushes or pulls. If we want to move a heavy box across the floor, we need a large force to push it. In the same way, if we want to turn a rusty bolt we will need a large torque.

Suppose you do have a rusty bolt that is stuck and you need to turn it. You know you will need a large torque to do this, but how can you make this? It's easy! It turns out that torque is the product of force and distance. In other words, to make a large torque you need either a large force or a large distance. To turn a rusty bolt you would put a wrench on it and push or pull on the end of the wrench. The "force" is how hard you push or pull, and the "distance" is how far from the bolt you are pushing or pulling. If you push or pull far away from the bolt, which requires a long wrench, the torque is bigger than if you push close to the bolt. If you have a really long wrench you dont need much force to turn your bolt, whereas if you only have a small wrench you will need to push or pull much harder to produce the same torque. Another important ingredient is that you push on the wrench in a direction perpendicular to the wrench.

You may also have heard the word torque describing the performance a car engine, for example you might read that a certain engine produces "high torque". This means that the engine is good at turning the wheels of the car.

I hope this helps.

If you want to learn more about forces and torques, have a look in any physics textbook.

(published on 10/22/2007)

Follow-Up #1: torque

torque is the product of the force and distance.. but what distance .... which distance ???
- laila (age 18)
Cairo, Egypt
Good question. It's only meaningful to give the torque around some particular axis, not torque in general. In the case of the stuck bolt, we'd be interested in torque around the axis down the middle of the bolt. So the distance is the distance out from the axis to the point where the force is acting.  Also, the only part of the force that contributes to the torque is the part at right-angles to the line of the axis. 

Mike W.
Lee H

(published on 10/22/2007)

Follow-up on this answer.