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    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?
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    Can you post the actual problem?
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    You mean a force of the form F = (x -a) \times b ?
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    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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