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Homework Help: Physics Kinematics Problem.

  1. Sep 17, 2010 #1
    1. The problem statement, all variables and given/known data

    A ball is thrown straight upward and rises to a maximum height of y above its launch point. Show that the velocity of the ball has decreased to a factor α of its initial value, when the height is y2 above its launch point is given by y2=(1-α^2)y.
    Also SHOW that y2=(1-α^2)y.

    2. Relevant equations
    y=vt+1/2at^2, where a=-9.8m/s^2
    v1=v0+at (maybe)

    3. The attempt at a solution
    I've already attempted a solution that involves plugging in y=vt +1/2at^2 and did a bunch of algebra; I was writing so fast I probably did something illegal; one thing i was worried about was plugging in (v1-v0)/t for the acceleration, and canceled out the t^2. I also attempted a solution that involved looking at derivatives of both sides. That is, y2' = ((1-α^2)y)' with respect to t, after plugging in the aforementioned equation of kinematics.

    After all the algebra was said and done, I ended up with .5v(final)t+.5v(initial)t=y2, which definitely sounds incorrect to me.
    Is there a way to modify the kinematics equation y=vt+1/2at^2 to include y2?
  2. jcsd
  3. Sep 17, 2010 #2


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    Homework Helper

    Be careful with the notations. If you use y for the height in terms of t denote the maximum height by something else, say ym. Also, vt in the equation for y(t) should be v0t. So the correct form of the first equation is y=v0t+1/2*at^2.
    The equation for the velocity as function of time is correct in the form v=v0t + at.

    Plug in a=-g for the acceleration and work further with the new equations.

    You have to prove that when the velocity is v=αv0 the height is y=ym(1-α^2).

    That is almost all right! But you cancel out t if you plug in t=(v-v0)/a for t in the first equation. Then you will get the height y in terms of v instead of t.

    Can you proceed?

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