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Homework Help: Projectile motion in physics

  1. Mar 10, 2012 #1
    1. The problem statement, all variables and given/known data

    a man throws a stone vertically upward with initial speed of 20 m/s from a building with height 54 m a)how long does the stone take to reach 40 m/s
    b)what is its velocity before it reaches the ground
    c) what is the magnitude of its acceleration at its maximum height?


    2. Relevant equations
    vf=vi+at
    xf=xi+vit+0.5at
    vf^2=vi^2+2a(xf-xi)

    3. The attempt at a solution
    I just want someone to tell me whether what I have done is right or wrong
    so for a)
    I took vi=20m/s
    vf=0m/s (I was going to calculate how long it takes to reach the maximum peak)
    then I used vf=vi+at (a as-9.8)
    I got 2.04 s
    Then I took vi=0
    vf=40
    then used the same thing and got t as ~4 and then I added them together
    b) At first I calculated the distance it takes to go from the building to the peak
    using xf=xi+vit+0.5at
    xi=0
    vi=20
    t=2.04
    a=-9.8
    I got ~22m
    Then I added to the 54
    then I used vf^2=vi^2+2a(xf-xi)
    vi=0
    a=-9.8
    xf=76
    xi=0
    and got vfinal
    c) I said it was 9.8 m/s^2
    So did I do anything wrong and if I did tell me what it is
     
  2. jcsd
  3. Mar 10, 2012 #2
    You're correct, but it can be done simpler
    For (a), v = u + at, just put v=-40, u=20, a=-9.81
    For (b), v2 = u2 + 2as, just put u=20, a=-9.81, s=-54
     
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