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Kinematics Problem (Find Speed)

  1. Sep 17, 2012 #1
    1. The problem statement, all variables and given/known data
    A daredevil motorcyclist attempts to leap a 48m wide gorge. At the side where the cyclist starts, the ground slopes upward at an angle of 15 degrees. Beyond the far rim, the ground is level and is 5.9m below the near rim. what is the minimum speed necessary for the cyclist to clear the gorge?


    2. Relevant equations

    Kinematics Equations

    s=d/t
    d= vf(t)-0.5(a)(t)2

    3. The attempt at a solution

    First I just used Pythagorean therm to find Δd. 482+5.92= c2

    c= 48.36

    Used equation to find time

    Δd = VfΔt-0.5(a)Δt2
    48.36= -0.5(-9.8)Δt2
    48.36 = 4.9Δt2
    48.36/4.9=Δt2
    9.86 =Δt2
    + - 3.14 = Δt

    s=d/t
    s=48.36/3.14
    s=15.4 m/s

    I think I was suppose to do something with 15 degrees.

    I got the answer as 15 m/s, but I believe its wrong.
     
    Last edited: Sep 18, 2012
  2. jcsd
  3. Sep 17, 2012 #2
    Maybe you show us your workings and if we can spot any errors you might have made.
     
  4. Sep 18, 2012 #3
    First I just used Pythagorean therm to find Δd. 482+5.92= c2

    c= 48.36

    Used equation to find time

    Δd = VfΔt-0.5(a)Δt2
    48.36= -0.5(-9.8)Δt2
    48.36 = 4.9Δt2
    48.36/4.9=Δt2
    9.86 =Δt2
    + - 3.14 = Δt

    s=d/t
    s=48.36/3.14
    s=15.4 m/s

    I think I was suppose to do something with 15 degrees.
     
  5. Sep 18, 2012 #4
    1. Why vf=0?
    2. The vertical height difference 5.9m below the launching point.
     
  6. Sep 18, 2012 #5
    How do I solve it without putting Vf as 0, and yes the vertical height difference is 5.9m below the launching point.
     
  7. Sep 18, 2012 #6
    Have you learned about vectors?
    [itex]\vec {A}=\vec {B}+\vec {C}[/itex]

    The motorcyle launches at 15° above the ground.
    Since it is vector in 15° direction, you can resolved it to 2 components.
    Y direction which affected by gravity and a constant X direction.
     
  8. Sep 18, 2012 #7
    Can you please show me, how you would solve this please ??
     
  9. Sep 18, 2012 #8
    If you do it once in lifetime, it is useful in giving you the answer.
    I'll only tell you what prequisite knowledge that you should acquire in order to solve this type of question.

    If you want to do physics problems, you should know about vectors and its operations.
    Secondly you should know/remember/derive(this makes you less equations to remember) SUVAT equations.
     
  10. Sep 18, 2012 #9

    vela

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    This is a projectile motion problem. What can you tell us about projectile motion? What equations do you have that apply to projectile motion?
     
  11. Sep 18, 2012 #10
    If you have a question why we need vector components, then you are on the way to solve the problem.
    http://imageshack.us/a/img805/2684/20946647.jpg [Broken]
     
    Last edited by a moderator: May 6, 2017
  12. Sep 18, 2012 #11
    I'm really really sorry to hijack, but just wondering, is the answer 21.75m/s? Attempted it as practice, and just want to check the answer if I made any errors. Sorry! And Thanks!
     
  13. Sep 18, 2012 #12

    vela

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    Nope. I get v0=25.4 m/s.
     
  14. Sep 18, 2012 #13
    Okay thanks lots! Time to redo then... :/
     
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