1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Simple Projectile Question
Loading...