What is moment of force?

1 Answer
Jul 17, 2014

It is the rotational effect of a force, it is equal to the force multiplied by the perpendicular distance between a pivot and the force.


A moment is the name for the turning effect that forces exert on objects. For example imagine pushing a door open. You push on the door handle and the door rotates around its hinges (the hinges are a pivot). You exerted a force that caused the door to rotate – the rotation was the result of the moment of your pushing force.

Pushing a door open is a very helpful application of moments to think about. Think about the location of the door handle – it is on the opposite side of the door to the hinges. The reason for that is that the moment of a force is related to the size of the force and the size of the perpendicular distance between the force and pivot. The larger the perpendicular distance the larger the turning effect (moment).

If you try to push a door open close to the hinges you will need a considerably larger force!
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More about moments
In the diagram below there are two forces: F1 and F2. We can find the moments of both forces by taking moments about the points where the other force acts - i.e. we treat one force as a "pivot" in order to find out the moment of the other.

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The moment due to force F1 .
Take moments about the point where F2 acts. Moment # = F1*d#.

The moment due to force F2 .
Take moments about the point where F1 acts. Note F2 is not perpendicular to the distance, d. In this case we either need to determine the component of the force which is perpendicular to the distance, or we need to determine the component of the distance which is perpendicular to the line of action of the force. In this case we will use the former method, and use the vertical component of F2 (#F2_V#). Moment # = F2_V*d#.