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Homework Help: Finding speed of the car, kinematics

  1. Sep 24, 2011 #1
    This is an even numbered problem from Cutnell and Johnson 8 edition....Can somebody tell me if the answer is correct? I spent a good 30 minutes dissecting the problem, so here's to hoping I am right :)

    A child operating a radio-controlled model car on a dock accidentally steers it off the edge. The car's displacement 1.1 s after leaving the dock has a magnitude of 7.0 m. What is the car's speed at the instant it drives off the edge of the dock?

    t = 1.1 s
    Voy = 0 m/s
    The displacement after 1.1 seconds is 7 meters. This means, from the point where it fell off the edge of the dock, to its position after 1.1 seconds, the distance between the two points is 7 meters.

    We are trying to find Vox which is the same thing as Vx because there's no acceleration in the x-direction.

    My Work
    Voy = 0 m/s

    Find Vy when t = 1.1 seconds passes

    Vy = Voy + at = 0 - (-9.8*1.1)
    Vy = -10.78 m/s

    Find the distance it has fallen in the y-direction during the 1.1 seconds.
    y = Voyt + 0.5at2 = 0.5(-9.8)(1.12)
    y = -5.929 meters

    Using A2 + B2 = C2
    5.9292 + B2 = 72
    B = 3.72114 meters <----this is the distance in the x direction that it travels during 1.1 seconds

    So Vx = B/t = 3.72114/1.1
    Vx = 3.383 m/s

  2. jcsd
  3. Sep 24, 2011 #2


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    Homework Helper

    Hi, Jehan,

    your work is correct and nice, but why have you calculated vy? It was not needed.

  4. Sep 24, 2011 #3
    It seems like the reasoning and execution is flawless, good job!
    One point though, just for future reference: there's alot to be said for rounding off, on occasion.
    You have your v_y there, at 5.92.... It's not unreasonable to bring to ~6...
  5. Sep 24, 2011 #4
    thank you both! it feels so stressful not knowing whether it is right or wrong. having both your approval puts me at ease! i can finally get some sleep!!! thank you!!!!!
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