# Forces21.pdf - 2.2 Moments The moment of a force is a...

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2.2 Moments The moment of a force is a measure of its tendency to rotate an object about some point. The physical significance of a moment will be discussed later. We begin by stating the mathematical definition of the moment of a force about a point. 2.2.1 Definition of the moment of a force . To calculate the moment of a force about some point, we need to know three things: 1. The force vector, expressed as components in a basis ( , , ) x y z F F F , or better as x y z F F F = + + F i j k 2. The position vector (relative to some convenient origin) of the point where the force is acting ( , , ) x y z or better x y z = + + r i j k 3. The position vector of the point (say point A ) we wish to take moments about (you must use the same origin as for 2) ( ) ( ) ( ) ( , , ) A A A x y z or ( ) ( ) ( ) A A A A x y z = + + r i j k The moment of F about point A is then defined as
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