Questioning Forces: A Physics Teacher's Ambiguity

[Physicscct]

My physics teacher did not give a clear definition. Is it a force or not?

 

[PeroK]

My physics teacher did not give a clear definition. Is it a force or not?

Technically, torque is a force times a distance. In SI units it has units of Newton-metres.

 

[lomidrevo]

My physics teacher did not give a clear definition. Is it a force or not?

When force is applied to a body, its momentum changes. When torque is applied to a body, its angular momentum changes (with respect to some rotational axis).
You can think of torque as an analogous quantity to force when you are studying rotational movement of bodies. Although force plays a big role in the definition of torque, it is not a force.

 

[haruspex]

Technically, torque is a force times a distance. In SI units it has units of Newton-metres.

To expand on that...
Force times distance arises in two ways that must not be confused.

If a constant force F is applied to a body, and the point of application moves distance s in the direction of that force then the force does work Fs on the body. In vector terms, F and s are vectors, but the work done is the scalar product: $W=\vec s.\vec F$.

If a force F is applied to a body and point P is at perpendicular distance s from the line of action of the force then the force exerts a torque Fs about point P.
In vectors, the torque is the cross product: $\vec\tau=\vec s\times\vec F$.

So note the differences...

• For torque, it is a distance at right angles to the force
• The torque of a force depends on the reference point P.

There is an interesting special case. If there are two equal and opposite forces but acting along different (parallel) lines then their net effect is a pure torque, and its value is the same for all reference points.

 

[sojsail]

Force is simply described as a push or a pull. An equally simple description for torque would be a twisting force. Like if you were using a wrench.

You may have noticed that you get better results with a wrench if you apply your force to the end of the handle instead of closer to the nut you are trying to tighten. That is why torque is F*r. The r is the distance from the point of application of your force to the turning axis of whatever you are trying to rotate. And there is another complication: if the angle theta between the direction of the force and r is not 90 degrees, then
torque = F*r*sin(theta)

The friction in the bearing of a wheel is a torque that will eventually stop a spinning wheel -- unless there is another torque to keep it spinning.

 

[sophiecentaur]

My physics teacher did not give a clear definition. Is it a force or not?

What was the definition that you were given by your teacher? Have you actually tried looking up the definition and seeing where it differs from what you were told?

 

[Herman Trivilino]

My physics teacher did not give a clear definition. Is it a force or not?

Not. To get a torque you have multiply a force by a distance. That distance is usually called the lever arm.

 

[zanick]

My physics teacher did not give a clear definition. Is it a force or not?

it is a rotational force... a force in a rotational plane. (plus all that was said above)

 

[Herman Trivilino]

it is a rotational force... a force in a rotational plane. (plus all that was said above)

Torque is not a force. Note that torque and force don't even have the same dimensions.

 

[zanick]

Torque is a force causing an object to rotate. Force is in Newtons and torque is in Newton-meters.

 

[lomidrevo]

That is not correct, torque is not a force! You might find some similarities between the two quantities (as mentioned in the posts above), but they are not the same.

 

[zanick]

i didnt say it was a force, i said it was a "rotational force". Is this not an accurate description?

 

[jbriggs444]

i didnt say it was a force, i said it was a "rotational force". Is this not an accurate description?

You said:

Torque is a force causing an object to rotate.

When asked whether torque is a force, an answer that it is a force is not accurate.

 

[lomidrevo]

From linguistics point of view, rotational force would still be a force, which is not correct. Moment of force is a correct terminology, if you wish to emphasize the relationship to linear force.

 

[A.T.]

i said it was a "rotational force". Is this not an accurate description?

No. Torque is the change of angular momentum per time, which doesn't require rotation. Linearly moving particles can have angular momentum around some point.