Forces, Equilibrium and Moments

There are times that multiple forces act on a single object. An object is said to be in an equilibrium when the sum of all the forces equals zero . The model that interests us most is an object which has a single pivotal point. The turning effect of an applied force about a pivotal point is defined as moment ( torque).

The moment is the product of a force and the distance from the point where the force is applied to the pivotal point . Moments are expressed in lb.oft or kg.ometer in the metric system. The idea is best demonstrated in above figure(a). If the force applied is F and the arm is A then the moment that is affecting the bolt is: .

In (b), a board placed on a pivot with two weights placed one on each end. For the board to remain in an eqilibrium, the sum of moments must equal sero . In other words, W1xL1 = W2o L2 or W1xL1 - (-W2xL2) = 0.

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Last update May 17, 2005

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