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# Mechanics watch

1. I'm trying to work my way round a nice mechanics exercise and i've got stuck on one of the questions.

I'm giving the vector equation of a force which incorporates a position vector, x and the cross product of the velocity and another vector, say a.

Given that x and x' are perp to b initially, I need to show that they continue to be perpendicular - any hints?
2. Can you post the actual problem?
3. You mean a force of the form ?
4. It's an assesed question so I didn't want to post the full problem as i'm only looking for hints.

F=ax+b(cross)x'
x is position vector, x' the velocity

a is a constant and b is a vector.

I'm just wanting someone to nudge me in the right direction. I know what properties must hold for x and x' to be initally perp to b but don't know how to show that they remain so.

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Updated: November 10, 2008
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