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Homework Help: Simple Projectile Question

  1. May 19, 2005 #1
    Well its been a while since i've done any physics problems and im having a problem with a real simple problem. The problem is:

    You know that the height (in m) of a projectile is given by the formula

    h = - g * t^2 / 2 + vi * t
    where t is the time from launch, g is the acceleration due to gravity (9.8 m/sec2) and vi is the initial vertical velocity component (in m/sec).

    Calculate the time at which a projectile will reach a certain height.

    vi = 20
    desired height = 10

    So far i have this done, but i always get a negative number and the answers should be 0.58 sec and 3.50 sec.

    10 = -9.8t^2/2 + 20t

    10(2 + 20t) = -9.8t^2
    20 + 200t = -9.8t^2

    9.8t^2 + 200t + 20 = 0

    (-200 + sqrt(200^2 - 4(9.8*20)) / 2(9.8)
    -200 + sqrt(40000 - 784)) / 19.6
    (-200 + 198.0303)/19.6

    Im sure its just a stupid math mistake, but i cant find it. Also where we even given the correct equation?
     
  2. jcsd
  3. May 19, 2005 #2
    10 = -9.8t^2/2 + 20t
    <={this is the step where your mistake is done}=>
    10 - 20t = -9.8t^2/2
    <==>
    20 - 40t = -9.8t^2
    <==>
    9.82t^2 - 40t + 20 = 0
    and not 9.8t^2 + 200t + 20 = 0
     
  4. May 19, 2005 #3
    Thanks a lot, wow dont i feel stupid now :tongue:
     
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