The principle of moment, or Varignon's Theorem, as defined by S-cool – The Revision Website, is “a force multiplied by the perpendicular distance from the line of action of the force to the pivot.”
A good example to show this principle is the swinging of a door. When a door is closing, it does not travel in a straight line, instead it swings from a hinge. This is torque, or moment. Attempting to close a door with the force close to the hinges takes more effort than closing a door from the handle. Attempting to close a door at the hinge, or pushing into the hinge, provides no movement at all, no matter the amount of force used.
In order for force to make an object turn, a pivot has to be available. In the above case, the pivot is the hinge. Another example is a see-saw. In this case, the pivot is in the center of the object. When it is balanced, it is level. However, if someone applies force to one end of the see-saw, the other end will rise. The force around the pivot is called moment. If force is applied to the other end of the see-saw, the turning forces become balance. At this time the moments are equal and opposite.
The equation for working out a moment is moment = force x distance. Moments are measured in newton metre or Nm.
To use an example from Physics Net, if a force of ten N is placed on a see-saw two meter from the pivot. The equation is 10 x 2 = 20 Nm. To achieve balanced moments ten N would also need to be applied at the other end, two meters, from the pivot. Another way to achieve balance would be to apply 20 N at one meter from the pivot. As long as the product from multiplying Force and distance equal the moment, it is balanced.
This same principle can apply when a water faucet is turned on, or when a when a bolt is tightened. As long as the distance from the pivot point, the center of the faucet or the bolt, and the amount of force are multiplied together, the moment can be figured. When loosening a tight bolt, this principle shows that it would take less force if a longer wrench is used, than when using a shorter one.
This principle can come in handy in many different circumstances that a person may encounter in a day. As long as the formula moment = force x distance is remembered, the solution is right at hand.