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Force a function of velocity?

  1. Aug 30, 2005 #1
    hi, i'm stuck on my very first physics homework problem for my junior level mechanics course... i am not sure if the force in this problem is dependent on velocity.

    a block of mass m slides down a frictionless incline. it is released at height h above the bottom of the loop. when it reaches the bottom, it begins going up a semi-circle of angle 45 degrees and radius R. the point where it begins going up the semi circle is labeled point A. basically, the block has acquired a certain velocity, and then starts going up a ramp. the point where the ramp begins is point A.

    the question is, what is the force by the inclined track on the block at point A?

    i realize the force by the track on the block is the normal force, however it appears this force would also be a function of the velocity, and not just of the mass of the block, but i am not sure. i am stuck because i am not sure if the force would simply be the normal force, or if it would be a function of the velocity (i know how to find the velocity).

    am i totally wrong, and the force is simply the normal force, and not a function of the velocity? if there is acceleration upward at point A, which is what i suspect, then is it dependent on velocity (as i can only imagine it is)?

    any help would be greatly appreciated.

    this is problem 2-25 in thornton/marion's classical dynamics, part (a)
  2. jcsd
  3. Aug 30, 2005 #2
    heh, i swear i've thought about this problem for a couple of hours total (i looked at it first yesterday), and i just had a breakthrough immediately after i posted the question.

    clearly the ramp part of the system can represented as an angular system which has a certain angular velocity at point A. this will allow me to find the acceleration at that point, and thus the force! correct?
    Last edited: Aug 30, 2005
  4. Aug 30, 2005 #3


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    I don't think you have to go so far as to change to an angular representation of the objects position, if that's what you're saying. You know the block is undergoing centripetal acceleration, so you can use mv^2/r to find the net force on it. This is equal to the normal force plus gravity, so subtract gravity and you have your answer.
    Last edited: Aug 30, 2005
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