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I know there are a number of questions concerning torque on this site, but I still don't quite understand what it is. A video by veritasium on gyroscopic precession on you tube really got me puzzled about the nature of torque. Perhaps I have some terminology errors which account for my lack of understanding, but assuming I roughly know what I am talking about when I say torque, my questions are:

  1. What can torque be described as? (Is it a force, energy, etc.)
  2. How can its vector be perpendicular to the force applied? Is this the conclusion drawn from some mathematical work or is there some other logic behind it?
Sandejo
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Jaywalker
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3 Answers3

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Torque is a twisting force that tends to cause rotation (or elastic twisting) of the object to which it is applied. The vector is merely the axis of rotation and a direction along that axis (e.g. in or out).

Think of using a wrench to tighten a bolt. You pull on the wrench in one direction but the axis of rotation of the bolt is perpendicular to that. As you tighten the bolt, the direction you are applying force changes as the wrench moves (consider say a quarter turn of the bolt) but the axis of rotation remains the same.

If you tighten a conventional bolt clockwise, it rotates into the threaded hole - this establishes a choice of direction along the axis - i.e. a vector.

The magnitude of the twisting effect depends on how long the wrench is. This is a matter of leverage. So the units of torque are force x distance: N m.

RedGrittyBrick
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  • So the vector of torque really just indicates on which plane the rotation takes place because contrary to the actual direction of force, the axis remains the same? – Jaywalker Jan 12 '16 at 14:25
  • Jaywalker, no it also indicates a choice of direction normal to the plane of rotation - see updates answer. When you undo a bolt, the plane of rotation is the same but the torque vector points in the opposite direction. – RedGrittyBrick Jan 12 '16 at 14:27
  • But in essence the vector does not indicate the force being exerted in the direction of the vector – Jaywalker Jan 12 '16 at 14:28
  • @Jaywalker: You need to be careful when considering the exact circumstances where you are thinking of "force" separately from "torque". E.g. consider torque applied to a coupling between two in-line rotating shafts. In the case of a wrench and bolt, the force applied by the operator to the wrench is in a different direction than the direction of the torque experienced by the bolt. – RedGrittyBrick Jan 12 '16 at 14:32
  • This is one type of torque. There is also another type described as the equipollent moment, or the torque due to a force at a distance. The toque you describe is really a force couple, which is a special case of the force-moment. – John Alexiou Nov 16 '20 at 13:21
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First of all It is not a force. It is the turning effect of a force.Like force is responsible for linear acceleration of a particle, in the very same way torque is responsible angular acceleration of a particle about a given point. Linear accn is the rate of change of velocity in the same way angular accn is rate of change of angular. Now a question comes into the mind why does it depends on force and radius vector of point of application of force from the axis of rotation. Imagine a door. First apply some force perpendicular to the plane of door near the knob. Then apply the same amount of force near the hinge of the door. Now you can see the difference. In the first case the door closed in Less time as compared with the second situation. Why?? Because the torque which is force x radius vector is greater in the second situation, and so is the angular acceleration. Hope this helps.

Chaitanya Garg
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In classical mechanics,kinematics variables in translational motion are in analogy with rotational motion so that simple problem models could be obtained.So torque comes out as vector that is in analogy with force showing rotating effect. Sign convention used for angular velocity,angular acceleration are in analogy with that In Translational mechanics.

Mohit Saini
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