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Homework Help: Dynamics/Kinematics Question

  1. Oct 22, 2006 #1
    Hey,
    An object is fired up a frictionless ramp 65 degrees to the horizontal with an initial velocity of 35 m/s, how long does the object take to return to the starting point.

    What I did was:

    cos 23 * 35m/s = Vyo
    Vyo = 31.72 m/s

    Vf = Vo + at
    0 m/s = 31/72 m/s + (9.8m/s^2)(t)
    t = 3.24s

    Then doubled time for down to equal: 6.5s

    The answer in the key is 7.9s.
    What did I do wrong here?

    Thanks,
     
  2. jcsd
  3. Oct 22, 2006 #2
    First thing I would do is draw a diagram. I would define a coordinate system in which the x-axis is parallel to the surface of the plane. That way all my calculations will deal with breaking components into the x-direction. Next I would use F=ma to calculate the acceleration of the object. Then apply projectile motion equations to calculate the time.
     
  4. Oct 22, 2006 #3
    I'm still getting 6.5s as an answer :/
     
  5. Oct 22, 2006 #4
    Starting with F=ma. The only force that is going to cause an acceleration is gravity (if we use the coordinate system I described).

    F = ma
    mgsin65 = ma
    a = gsin65

    Vf = Vi + at
    0 = 35 m/s + (gsin65)t
    t = 35/gsin65
    multiply it by 2 to get whole time
    t = 70/gsin65
    t = 7.88 seconds.
     
  6. Oct 22, 2006 #5
    ah,
    that makes sense now
    thanks very much
     
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