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Homework Help: Finding initial projectile velocity with angle and a point in the path?

  1. Sep 23, 2010 #1
    1. The problem statement, all variables and given/known data
    A ball is hit from the ground with an angle of 36 degrees above the horizontal. The ball hits a target 30m ahead and 4m above the ground. Find the initial velocity of the ball.

    2. Relevant equations
    Vavg = Δd/Δt
    aavg = Δv/Δt
    Δd = voΔt + 1/2aΔt2

    3. The attempt at a solution

    Δx = 30m
    Δy = 4m
    Vox = Vo * cos 36
    Voy = Vo * sin 36
    Δx = voxΔt + 1/2ax(Δt)2
    Δx = (Vo * sin 36)Δt + 1/2ax(Δt)2
    Since ax = 0 then
    Δx = (Vo * sin 36)Δt

    This question appeared on a quiz today and caught me totally by suprise. My teacher had never assigned homework questions like this ever and my textbook did not have any questions like it either. I've asked my teacher for help, but he didn't give me a clear answer. I've tried this question again at home for another two hours but I still couldn't get anything out of it. I know I have to find Δt, but I can't seem to isolate it. I would put all my attempts on the post, but I have already destroyed them in my fustration. Please help me! I'm losing my mind!
  2. jcsd
  3. Sep 23, 2010 #2
    That is the correct equation for the x component of motion.

    However, the reason your getting stuck is because you need another equation so that you can eliminate the unknown variable time.

    This is where the y component of the motion will come in. Do this like you did to find the x component.

    Remember: When the question asks for the initial velocity, it wants the velocity in the direction of launch, not a component of the velocity.
  4. Sep 23, 2010 #3
    I'm sorry I didn't initially post it, but I did find Δy, which was

    Δy = VoyΔt - -4.9m/s^2

    I also tried substituting Δx/Δt in place of Vox and Δy/Δt for Voy, but I still could solve for Δt because Δt was always dividing the direction.
  5. Sep 23, 2010 #4
    So first of all, the equation is delta Y = (Vo_y)*t + (1/2)(-9.8)t^2.

    It should not be a minus negative 4.9. You also left off the t^2.

    Also, your equation for delta x should be cos(36) not sin(36).
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