# Moment of force ? some clarifications needed.

## Homework Statement

Regarding the moment of force, this is the definition given in the book
The tendency of rotation is called turning effect of a force or moment of the force about the given axis. The magnitude of the moment of force F about a point is defined as the product of the magnitude of force and the perpendicular distance of the point from the line of action of the force

## Homework Equations

Please refer my attachment ( fig 1) which has been given in the book to illustrate this example.
Now my doubts are 1) is PO line of action of force?
2) what that arrow pointing in the anticlockwise direction represent?

## The Attempt at a Solution

#### Attachments

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Doc Al
Mentor
Now my doubts are 1) is PO line of action of force?
No. The 'line of action' of the force is a line parallel with the force and passing through the point of application.
2) what that arrow pointing in the anticlockwise direction represent?
The direction in which that force would cause the object to rotate. Torque, at least in these simple situations, can be clockwise or counterclockwise. In this case it's counterclockwise.

is it PF then? pls help because i could not understand the concept of line of action of force

Doc Al
Mentor
is it PF then? pls help because i could not understand the concept of line of action of force
"F" stands for the force vector, not a point. The line AP extended is the line of action of the force.

Thanks a lot sir. But its been mentioned " perpendicular distance of the point from the line of the action of force"
Here the point is P and the line AP is line of action of force. Then How OA qualifies as perpendicular distance, because OA is perpendicular distance to A from the axis of rotation.

Doc Al
Mentor
But its been mentioned " perpendicular distance of the point from the line of the action of force"
"The point" is the point about which you are finding the moment of some force.
Here the point is P and the line AP is line of action of force.
No. P is the point of application of the force; the point about which you are computing the moment is point O, not P.

NascentOxygen
Staff Emeritus
The arrow for F (extended both ways to infinity) is the line of action of force F. I can't see any right angles marked in, but it looks like OP is not the perpendicular distance to O. Maybe it's OA that is the perpendicular distance from 0 to the line of action of F? You need to mark it clearly. So the moment of F about O is OA * F and the turning tendency is approximately anticlockwise. (Providing I'm correct that F is almost parallel to the y-axis marked "AXIS", is it?)

I wouldn't refer to an axis of rotation, since nothing is rotating (that I know of). I'd rather see a circular arrow drawn about 0, rather than stuck in mid-air near the end of F, to indicate the direction of the turning tendency of the force F about point O.

If F were to cause rotation about 0, then the axis of rotation would be out of the page directly towards you, i.e., perpendicular to the page. *

* Your hand-sketched diagram is not clear; I may be interpreting it differently to how it should be. I'm saying F is almost parallel to AXIS, but it could be viewed as being almost perpendicular to it, too. Only you know what it should be.

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Thanks a lot Nascent Oxygen and Doc Al for your detailed explanations