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Homework Help: Simple Kinematics Question

  1. Jun 23, 2010 #1
    1. The problem statement, all variables and given/known data
    A child on a trampoline remains in the air for 1.5s after having jumped straight up. What height did the child reach?


    2. Relevant equations
    All kinematics equations
    I was thinking of using Net Force but we don't have the mass and if you use it how could you find displacement.

    3. The attempt at a solution

    d = vit + 1/2at2
    vi= 0
    So:
    d= 1/2at2
    d = 1/2(9.81m/s2)(1.5s)2
    d= 11m

    The answer is suppose to be 2.8m
     
    Last edited: Jun 23, 2010
  2. jcsd
  3. Jun 23, 2010 #2

    Doc Al

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    Staff: Mentor

    The time given is the total time in the air. So what's the time it took to reach the highest point?
     
  4. Jun 23, 2010 #3
    Half of that which is 0.75s, and wow. There it is, simplest question. Thanks man!

    Omg I keep getting stuck on these questions, kinematics is so easy but I cant remember anything. Check back I will have a new question posted in 1min.

    1. The problem statement, all variables and given/known data
    A truck uniformly decelerates by 5.00m/s2 to a velocity of 60.0m/s [W] in 30.0s. What was the truck's initial velocity before the driver stepped on the brakes?

    2. Relevant equations
    All kinematics equations
    I'm looking for an equation but I don't have distance so I can't figure one out that will work. The closest forumla I could maybe see working is
    vf2 = vi2 + 2ad

    3. The attempt at a solution
    I'm not sure.

    (EDIT)

    Found it:

    Solution

    a = Δv / Δt
    a = vf - vi / Δt

    Rearrange it and you get this
    vi = 60m/s -at

    Put in the variables
    vi = 60m/s - (-5.00m/s)(30.0s)
    vi = 60m/s + (5.00m/s)(30.0s)

    Answer:
    vi = 210 m/s [W]
     
    Last edited: Jun 23, 2010
  5. Jun 23, 2010 #4

    Doc Al

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    Staff: Mentor

    Looks good!
     
  6. Jun 24, 2010 #5

    Filip Larsen

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    Gold Member

    Based on the numbers you quote the result is correct, but then it sure is one fast truck. If I were you I would double check that the given numbers are correctly taken from the problem text.
     
  7. Jun 24, 2010 #6

    Doc Al

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    Staff: Mentor

    Yes, the numbers are comically unrealistic. Worth checking that you copied the problem correctly, but it's also not unusual for problems to use unrealistic data. (Unfortunately.)
     
  8. Jun 24, 2010 #7
    i guess im just lucky, my textbook uses realistic numbers, it even has a textblock on the side that says they're all realistic =]..

    and yes, that's an insanely fast truck xD
     
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