The Open Door Web Site

Mechanics

The Moment of a Force (also called torque)

The moment of a force is a measure of its turning effect.

The moment can be calculate using the following equation.

Moment = Force ×

 Perpendicular distance of force from "pivot"

Consider a person trying to open a door, by applying a force, of magnitude, F, as shown below.

The perpendicular distance, d, between the line of action of the force and the pivot (the hinge of the door) is rsina

Therefore, the moment of the force is given by

Moment = Frsina

The two obvious changes the person could make in order to open the door more easily are

i) he/she could increase the distance "r"
ii) he/she could push at 90° to the door.

space

If the angle between the line of action of the force and the door is 90°, we have

Moment = Fr

and it is clear that this is the maximum value of the turning effect for a given force.

This we know to be true from experience.

 

© David Hoult 2008