A particle moves on a given straight line with a constant speed v. At a certain time it is at a point P on its straight line path. O is a fixed point. Show that →OP×→v is independent of the position P.
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Solution
The particle moves on the striaght line PP' at speed v.
From the figure,
→OP×v=(OP)vsinθ^n=v(OP)sinθ^n=v(OQ)^n
It can be seen from the figure, OQ=OPsinθ=OP′sinθ′
So, whatever may be the position of the particle, the magnitude and direction of →OP×→v remain constant.