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Urgent help needed : finding the intersection of two projectiles

  1. Oct 12, 2011 #1
    (I will try to correctly translate this, as this is a class in french)

    A person on the board of a swimming pool throws a ball (ball1) at a speed of 8 m / s at an angle of 40 degrees above the horizontal. At the time of launch, the ball is 3 m above the water, and 10 m from the opposite side of the pool. From the opposite side of the pool, someone else throws another ball(ball2) at an angle of 50 degrees above the horizontal.

    If the two are thrown at the same time: a) what should be speed of ball2 so that it touches ball1 in flight?

    I just don't know where to start... it seems as though there is information missing! I have tried decomposing the speed of ball1 : Vxo = 8cos40=6.13m/s and Vyo = 8sin40 = 5.14m/s but I don't know where to go from there... I know I need to show the speed where x1=x2 and y1=y2, but how do I do that??

    For projectiles we are given these equations :
    x = (Vxo)(t)
    Vy=(Vyo) - (g)(t)
    y = (yo) + (Vyo)(t) - 0.5(g)(t)^2
    Vy^2 = (Vyo)^2 - 2(g)(y-yo)

    I have no idea how to solve this... I have been thinking about this one for at least 2 hours. Please help! Thank you so much in advance :)
  2. jcsd
  3. Oct 13, 2011 #2
    Just to clear things up, the second ball is thrown at the same level a the water.

    Please help, this is due today!
  4. Oct 13, 2011 #3


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    Staff: Mentor

    You need to pick a frame of reference (coordinate system) and write the trajectory equations for both projectiles in that frame. Start by drawing a picture of the setup.

    The problem text doesn't state the initial height of the second ball. Is this an omission, or are you to assume that it's the same as the first ball's launch height?
  5. Oct 13, 2011 #4
    The second ball is at the same level as the water. On my homework there is a picture of the situation.
  6. Oct 13, 2011 #5


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    Staff: Mentor

    Okay, then you have enough information to write x and y component trajectory equations for both balls. For the first ball you have complete information (all the variables have values). For the second ball only the launch speed is missing, so you'll need to carry it through the math as an unknown. Suppose you call it v2. What would be the magnitudes of the x and y components of that velocity?
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