14. Line Vectors





When a force acts on a body it can cause it to move in a particular direction and it can also cause it to twist. (Think of a spanner or wrench turning a bolt.) In order to measure the twist we introduce the concept of the moment of a line vector about a point.
It doesn’t matter which point P on we choose because we get the same value of the moment for all points of . To see this, suppose that Q is another point on . Then is parallel to v and so × v = 0. Therefore,
When calculating the moment of a line vector (,v), the particular choice of the point P on does not affect the answer but if you change the point O you will get a different result. This is why we always refer to the moment about a point O.


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