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Projectile motion off a ramp

  1. Sep 21, 2012 #1
    1. The problem statement, all variables and given/known data
    A skateboarder starts up a 1.0--high, 30 ramp at a speed of 7.9 . The skateboard wheels roll without friction. How far from the end of the ramp does the skateboarder touch down?


    2. Relevant equations

    x=x0+v0t+.5(a)(t2)
    v=v0+at

    3. The attempt at a solution
    This question seems pretty straight forward and I've found an answer but the website we use says it's wrong. First I found vx=7.9cos(30)=6.84 and vy=7.9sin(30)=3.95. I tried finding maximum y by solving vy=0=-9.8*t1 then t1=.405 and plugging that into y=1.8 then finding t2 when y=0 and y0=1.8 to get t2=.605 so ttotal=1.01 and plugging that into x=6.913
     
  2. jcsd
  3. Sep 21, 2012 #2
    A skateboarder starts up a 1.0--high, 30 ramp at a speed of 7.9 . The skateboard wheels roll without friction. How far from the end of the ramp does the skateboarder touch down?
    -------------------
    My understanding that it starts off a flat ramp top.
    I do not understand whether the friction has any factor in the calculation.
    With zero initial vertical velocity, in my calculation the distance from the base is 1.8m
     
  4. Sep 21, 2012 #3
    sorry 30 ramp is supposed to be 30o ramp
     
  5. Sep 21, 2012 #4

    Simon Bridge

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    What immediately occurs to me is that the ground (where the skateboarder touches down) is at a different height to the end of the ramp (where ballistic motion starts).
    The 7.9m/s is at the bottom of the ramp, not the top.
     
  6. Sep 22, 2012 #5
    Yeah that was it the 7.9 was at the bottom of the ramp not the end of the ramp thanks
     
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