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Speed distance and not a lot of hair left

  1. Apr 25, 2012 #1
    1. The problem statement, all variables and given/known data
    a balls inital velocity is 72 km/ph and falls 100 meters with gravity @ 10 meters per second and i need to work out the time it takes to fall to ground. i was given s=ut+0.5*gt^2



    2. Relevant equations
    s=ut+0.5gt^2
    v^2=u^2+2as
    t=(v-u)/a


    3. The attempt at a solution

    v2=u2+2as
    v=squr u^2+2as
    v=squr 20^2+2*10*100
    v=48.98979486

    t=(v-u)/a
    t=(48.98979486-20)/10
    t=2.898979486 seconds

    100=20*2.898979486+0.5*10*2.898979486^2

    is this correct??
    and is there a way to do this with just the s=ut+0.5at^2 formula ?
     
    Last edited: Apr 25, 2012
  2. jcsd
  3. Apr 25, 2012 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    hi steveandy2002! :smile:

    (try using the X2 button just above the Reply box :wink:)
    s = 100, u = 72, a = 10 …

    what's the difficulty? :confused:
     
  4. Apr 25, 2012 #3
    not sure what you mean, i need to get time not s
     
  5. Apr 25, 2012 #4
    All you need to do is plug in your known values into s=ut+[itex]\frac{1}{2}[/itex]gt2. Then you move everything to one side and solve the quadratic equation. What are you having issues with?
     
  6. Apr 25, 2012 #5
    Don't forget to change the units on u from km/hr to m/s.
     
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