# Is Torque the Same as Moment of a Force? A Confusing Semantic Issue

• Cameron12345
In summary, Torque and Moment are two terms that are used to mean the same thing, but Torque is used primarily for moments about an axis of rotation, while Moment is more general.
Cameron12345
I'm confused when it comes to the rotational version of Newton's Second Law:

τ = Iα
Is the torque τ the same thing as the moment of a force? I'm not sure if there is a discrepancy between British and American schools but I was always taught that a torque was a special case of moments and is calculated when two equal forces act in opposite directions on a new object. All the rotational dynamics question questions seem to suggest that torque is actually what I know as the "moment of a force". Am I missing something?

This can be a fuzzy area of physics. The MOMENT of a force is F x perpendicular distance from the pivot ===Fxr. In my experience the term 'MOMENT' is used in balancing lever type applications!
TORQUE is force F x distance from pivot (spot any difference?) tends to be used in rotational dynamics applications
COUPLE is when 2 forces produce a turning effect (2 torques?) couple = 2F x r or F x 2r or...(just to add to confusion) F x D (D =diameter)
This is more about usage of terms than physics principles. You have to get used to it !

lychette said:
This can be a fuzzy area of physics. The MOMENT of a force is F x perpendicular distance from the pivot ===Fxr. In my experience the term 'MOMENT' is used in balancing lever type applications!
TORQUE is force F x distance from pivot (spot any difference?) tends to be used in rotational dynamics applications
COUPLE is when 2 forces produce a turning effect (2 torques?) couple = 2F x r or F x 2r or...(just to add to confusion) F x D (D =diameter)
This is more about usage of terms than physics principles. You have to get used to it !
In my experience, "torque" is never about force times non-perpendicular distance. Only the perpendicular distance is ever relevant. If I push my torque wrench end-on, its reading remains stubbornly at zero.

A.T.
Agree broadly with lychette. There's no real distinction, but I'd say that torque is used for moment about an obvious or intended axis of rotation, like the centre of a nut that's being loosened on a bolt, or about a shaft in an engine or motor. We tend to use the word 'moment' when we're considering force x perp distance from some arbitrary point in a structure, say, in order, perhaps, to determine conditions for equilibrium.

+1 Torque and Moment are frequently used to mean the same thing although general use seems to be..

Torque: May not have an associated force and distance or both force and distance are unknowns.

Moment: You can usually "factorise" a Moment into a force and a distance because one or both are known. A moment may or may not cause a resulting linear force as well.

Couple: Combination of forces and distances that result in a pure torque without an additional resulting linear force. eg special case of a Moment where there are two equal but opposite forces.

Edit:
I was always taught that a torque was a special case of moments and is calculated when two equal forces act in opposite directions on a new object.

That would be a couple.

jbriggs444 said:
In my experience, "torque" is never about force times non-perpendicular distance. Only the perpendicular distance is ever relevant. If I push my torque wrench end-on, its reading remains stubbornly at zero.
Here we go...Like moment, TORQUE is force x perpendicular distance from the pivot. If the line of action of the force is not perpendicular to the pivot then the perpendicular distance from the line of action of the force from the pivot must be used.
Sigh !...'what other distance would you use' ...

lychette said:
Here we go...Like moment, TORQUE is force x perpendicular distance from the pivot. If the line of action of the force is not perpendicular to the pivot then the perpendicular distance from the line of action of the force from the pivot must be used.
Sigh !...'what other distance would you use' ...
lychette said:
[...]MOMENT of a force is F x perpendicular distance from the pivot
[...]TORQUE is force F x distance from pivot (spot any difference?)
The difference that I spotted and commented on is the lack of the word "perpendicular". If you meant something else, perhaps you could be less coy and spit it out.

jbriggs444 said:

The difference that I spotted and commented on is the lack of the word "perpendicular". If you meant something else, perhaps you could be less coy and spit it out.
I see what has happened...you did not see the word 'perpendicular' in the previous statement or you thought it applied to only that statement.
we have a semantics problem...sorry
I hope that we have not confused anybody

lychette said:
I see what has happened...you did not see the word 'perpendicular' in the previous statement or you thought it applied to only that statement.
we have a semantics problem...sorry
I hope that we have not confused anybody
Yes indeed, I had parsed it as if "perpendicular" applied to only the one and not the other.

## 1. What is the difference between torque and moment of a force?

Torque and moment of a force are often used interchangeably, but they are technically different. Torque is the measure of the turning force of an object, while moment of a force is the measure of the tendency of a force to rotate an object around a specific point.

## 2. How are torque and moment of a force calculated?

Torque is calculated by multiplying the force applied by the distance from the pivot point, while moment of a force is calculated by multiplying the force applied by the perpendicular distance from the pivot point to the line of action of the force.

## 3. What units are used to measure torque and moment of a force?

Torque is typically measured in units of Newton-meters (Nm) or foot-pounds (ft-lb), while moment of a force is measured in units of Newton-meters (Nm) or pound-feet (lb-ft).

## 4. How do torque and moment of a force affect rotational motion?

Torque and moment of a force are both important factors in rotational motion. Torque is responsible for causing objects to rotate, while moment of a force determines the direction and speed of the rotation.

## 5. What are some real-life examples of torque and moment of a force?

Torque can be seen in everyday activities such as opening a door or using a wrench. Moment of a force is evident in situations such as a lever or a seesaw, where a force applied at a distance from the pivot point causes rotation.

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