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Homework Help: Circular motion - Please help, I think the book's wrong?

  1. Apr 12, 2008 #1
    Circular motion - Please help, I think the book's wrong!?

    1. The problem statement, all variables and given/known data
    A car of mass 400kg travels over a bump of radius 10m on a track in a fun park.

    a) What force is exerted by the track on the car when it travels over the top of the hump at a speed of 6 m/s.

    b) What is the minimum speed needed for the car to leave the track at the top of the hump?

    2. Relevant equations
    F = mv^2/t

    3. The attempt at a solution
    a) v= 6m/s , Fnet=?
    F= (400)(6^2)/ 10
    Fnet = 1440 N - WRONG!
    RIGHT - 2600N

    How did they get that answer? - Could someone please help with this issue.

    Kindest Regards,

    Attached Files:

  2. jcsd
  3. Apr 12, 2008 #2
    There should be a centripetal force on the car pointing into the hump. At the top of the hump, the answer you got is the centripetal force, but at the same time there is the normal force of the car pointing in the opposite direction. The net force exerted on the car by the hump should be that of the centripetal force subtracted from the normal force.
  4. Apr 12, 2008 #3
    Thank you very much for your answer, you really have got me thinking now.

    Does that mean:

    F= 400*9.8
    Fnet = 3920N
    Centripital Force = 1440

    =>Fnet - Fc
    => 3920 - 1440
    F= 2500N

    Am I correct?
  5. Apr 12, 2008 #4
    Seems right to me. The answer would be between 2500N and 2600N, so I'm supposing your book may have rounded or used 10 instead of 9.8 for the acceleration due to gravity.
  6. Apr 14, 2008 #5

    The matter is solved, and I am very happy with all the kind hearted assistance I received.

    Have a great afternoon.

    Kindest Regards,
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