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Help! Simple Problem I Think!

  1. Oct 8, 2004 #1
    hi i need help with a homework problem for physics: it goes like this:

    A roller coaster car of mass 310 kg (including passengers) travels around a horizontal curve of radius 40 m. Its speed is 18 m/s. What is the magnitude and direction of the total force exerted on the car by the track?

    _______N at _______° above the horizontal.


    The first answer I tried was calculating the normal force exerted by the track (which was simply mass * gravity, and 90 degrees above the horizontal). That was wrong. Then I tried giving the answer as the centripetal acceleration ((m * V^2)/ r), and the angle as 0 degrees above the horizontal. That was wrong too.

    So how do I solve this??????????? Does anyone know, please help?!!!
  2. jcsd
  3. Oct 8, 2004 #2


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    Staff Emeritus
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    Gold Member

    There are two forces acting on the car : (i) its weight acting downwards (ii) the normal reaction, acting at some angle (not vertically upwards) to be determined.

    As a result of these forces, the acceleration of the car is v^2/r inwards.

    Draw the free body diagram. Resolve all forces along the x, y directions. Apply Newton's second law to the forces and accelerations along these directions.

    PS : Next time, post such questions in "Homework Help"
  4. Oct 8, 2004 #3
    Hi, thanks for the probleb advice! and next time i'll post it in homework help.

    But one thing im not understanding: Wont the normal force be directly upwards and the weight be directly down, since the curve is not banked? Therefore the sum of forces in the y-direction will be ZERO right?

    Now for the x-direction: The only thing acting on the car is centripetal acceleration since there's no friction. Therefore that acceleration will be in the x-direction. But since centri. accel. is not really a 'force' it can't be counted. so im still confused.

    P.S. If someone experienced can calculate the answer to this problem and let me know how he got it, i would like that!!!
  5. Oct 8, 2004 #4

    Doc Al

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    Staff: Mentor

    As Gokul43201 stated, there are two forces acting on the car: its weight, acting downwards, and the force that the track exerts on the car, which acts in a direction that you must find. I would not call that latter force a "normal" force, since that assumes it acts in a particular direction: it's safe to assume that the car is somewhat "attached" to the track, not merely riding on top of it.

    Yes, vertical forces must add to zero; horizontal forces must provide the needed centripetal force.

    (I will move this to the homework help section.)
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