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Homework Help: Two part 1d kinematics problem. Need help, not solution!

  1. Sep 19, 2010 #1
    1. A parachustist jumps from an airplane and freely falls y=49.4 m before opening his parachute. Thereafter, he decelerates at a=2.02 m/s2. As he reaches the ground, his speed is 3.11 m/s. How long was the parachutist in the air?

    For this one I managed to get the solution already using a method someone else showed me. However, I fumbled a long time with this problem and got the wrong answers despite having a strategy in mind.

    What I did was the following. I split the problem up into two parts: before the parachute opens and after. These are the variables as I defined them at the beginning of the problem:

    v0=0, s0 = 0, for when parachutist jumps out plane.

    v1 = ?, s1 =49.4m, a1 = 9.8 m/s2, t1 = ?, upon parachute release

    v2 = 3.11 m/s, s2 = ?, a2 = -2.02 m/s2, t2 = ?, upon reaching ground

    1. First I tried to find the time t2 between parachute release and reaching ground.

    0.5at2 + v2t + s1

    0.5(-2.02 m/s2)t2+(3.11 m/s)t +49.4m

    using the Q-formula I got t2=8.71


    2. Then I tried finding v1

    v2=v1+at2

    3.11 m/s = v1 + (-2.02 m/s2)(8.71s)
    v1 = 20.7 m/s

    3. Using v1, I could then find t1

    t1 = (v1-v0)/a

    t = 2.11s

    The red part I think is where the mistake was. For that formula, I had to use v1, which I didn't know, but even if I did, would this approach still work? Plugging in values for v1, which I found later using a different method, still gave me wrong figures for t.

    Rather than telling me how I could obtain a solution, please show me the mistake with this strategy and whether I could have tweaked it to make it work. I have struggled a long time with kinematics concepts and I admit my problem solving skills are subpar, so a physical description of what is occuring would be very helpful.

    Thanks!
     
  2. jcsd
  3. Sep 19, 2010 #2

    rl.bhat

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    Homework Helper

    For the first part you can use two equations.

    1) vf1^2 = vo^2 + 2*g*s. Here vo = 0.
    2) vf1 = gt. Find t1.

    For the second part follow the same procedure.
     
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