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Homework Help: Balls - Projectile

  1. Feb 7, 2005 #1
    The balls are released from rest at a height of 5.0m at time t=0s. How long [tex]t_g[/tex] does it take for the balls to reach the ground? Use 10 m/s^2 for the magnitude of the acceleration due to gravity.

    i'll use the position formula for this problem:

    [tex]y = y_0 + V_{y_0}*t - 1/2*g*t^2[/tex]

    well i know that
    [tex]y_0[/tex] = 5.0 m
    g = 10 m/s^2

    i'm missing [tex]V_{y_0}[/tex]

    if i plug in t = 0, wouldnt it just cancel everything out? but it doesnt make sense.
  2. jcsd
  3. Feb 7, 2005 #2
    The balls are released from rest, which tells you that their initial velocty [tex]V_{y_0}[/tex] is zero. If you plug in t=0 you just get [tex]y=y_0[/tex], or 5.0 m, which just tells you that the balls do in fact start where they start (always a good check :smile: ).

    What you want to do to solve the problem is find out what value of t will give you [tex]y=0[/tex], since that's the 'height' of the ground.
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