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Homework Help: Perpendicular Forces and Change in Momentum

  1. Oct 8, 2012 #1
    Okay, I feel like I am just missing something that should be very easy to see, but I can't seem to wrap my head around this concept. Can anyone explain to me why a force perpendicular to the momentum only changes the direction of the momentum and not the magnitude?

    By my logic, if Fnet=Δp/Δt, and thus, Δt*Fnet = mΔv, then shouldn't the perpendicular force change the velocity of object in the direction perpendicular to the current direction of momentum? And because the velocity changes in the new direction, wouldn't the magnitude of the momentum change since the magnitude of the velocity changed?

    Hopefully you can follow my logic and see where it is flawed! Thanks in advance!
  2. jcsd
  3. Oct 8, 2012 #2


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    Remember that velocity is a vector, and as such, it has magnitude and direction. If its direction changes, its magnitude does not necessarily change. A change in its direction results in a change in its velocity, even if its magnitude does not change. An object with momentum moving in a curved path must be acted on by a force perpendicular to the momentum vector, which does not change its speed. Hint: motion in a circle.
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