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Net force

  1. Jul 31, 2009 #1
    1. The problem statement, all variables and given/known data
    A toy train moves along the track at 9.0m/s. When the power is turned off, the train comes to a stop in 6.0s. The net force on friction acting on the train to stop it is

    2. Relevant equations

    3. The attempt at a solution
    how do i find mass and acceleration so that i can find force??
  2. jcsd
  3. Jul 31, 2009 #2

    Doc Al

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    Staff: Mentor

    Enough information is given for you to figure out the acceleration using kinematics, but not enough to find the friction force. Are you sure you are presenting it word-for-word as it was given?
  4. Jul 31, 2009 #3
    yes im one hundred percent sure.
  5. Aug 2, 2009 #4


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    This bothers me. Consider a real life freight train moving at the same initial speed and stopping in the same time. Surely the average force acting on it must be greater in this case. Something is missing or the problem cannot be done the way it is phrased.
    Last edited: Aug 2, 2009
  6. Aug 2, 2009 #5
    Yes, the scaling factor is the mass of the freight train compared to that of the toy train. Their accelerations would be the same, but as their masses are different, the force required to bring about those accelerations would differ.

    As for my perspective on the problem, I could single out the coefficient of kinetic friction, but couldn't for the life of me find an expression for the mass in terms of the question's data, so the actual force eluded me.
  7. Aug 2, 2009 #6

    Doc Al

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    Right. As I said, there is not enough information given to solve for the net force.
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