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Homework Help: Projectile Motion: Finding Time

  1. Nov 19, 2007 #1
    1. The problem statement, all variables and given/known data Ok, so i'm having alot of trouble finding time when it is not given in a projectile motion problem. Here's what I have set up:

    X
    A=O
    Vi=100.0
    VF=100.0
    D=?
    T=?

    Y
    A=-9.8
    Vi=O
    Vf=?
    D=8848m
    T=?

    2. Relevant equations

    Distance(d=1/2at2=vit)




    3. The attempt at a solution
    Ok, here's where my problem is, I can never get the time right. I have failed 2 tests and numerous homework assigments because something is wrong. Here's my attempt at it:

    8848=1/2(-9.8)t2

    I divide both sides by 8848 then square root it.


    I ALWAYS get the wrong time and in turn that gives me the wrong distance and in turn I fail the test.
     
    Last edited: Nov 19, 2007
  2. jcsd
  3. Nov 19, 2007 #2
    The general idea to the projectile motion problems is:

    1. Find time using the relationship of displacement in the y-direction.
    2. FInd displacement in the x-direction x= v*t keeping in mind that in the x direction the velocity will be constant.

    Now in this particular case i believe that you only have to worry about the first step.

    y = .5 gt^2 => t = sqrt(2y/g)
     
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