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Find the time the ball is in motion

  1. Sep 23, 2007 #1
    help please!!!

    1. A ball is thrown horizontally from the top of a building 22.8 m. high. The ball strikes the ground at a point 52.1 m. from the base of the building.
    a. Find the time the ball is in motion.
    b. Find the initial velocity of the ball.
    c. Find the x component of its velocity just before it strikes the ground.
    d. Find the y component of its velocity just before it strikes the ground.

    vx =

    voY = 0
    aY = -9.8 (gravity)
    dY= -22.8

    3. I found the time to be 2.16 s., but the rest is tricking me up. The answer to b can't be zero can it? I don't understand that part, and i'm just at a loss for how to find the x and y components. So please...can anybody give me some help here?
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 23, 2007 #2


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    Staff: Mentor

    It would be helpful if one showed the calculations.

    The time is found from the freefall in the vertical direction.

    Neglecting air resistance, the horizontal velocity is constant, so the velocity is found by dividing the horizontal displacement by the time it takes to travel that distance.

    There is a horizontal velocity (constant) and a vertical velocity (time dependent), which changes with the acceleration due to gravity.
  4. Sep 23, 2007 #3
    Yeah I used d=vo(t) + 1/2at^2 to find the time.

    so to find the horizontal velocity, I would use dx=vx(t), right?
    and then how would i go about finding the y?
  5. Sep 23, 2007 #4


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    Staff: Mentor

    Yes, in the horizontal, vx = dx/dt.

    When the ball is thrown, unless stated, one may assume is it thrown horizontally. If thrown up at and angle, it will have a vertical climb (upward velocity in y) to an apex and then freefall. If thrown downard at an angle, it will have a downard initial velocity.

    See this example - http://hyperphysics.phy-astr.gsu.edu/hbase/traj.html#tra11
  6. Sep 23, 2007 #5
    i understand the x now, but i still don't understand the y...so would you use the free fall formula to find it or what?
  7. Sep 23, 2007 #6
    or one of the four equations for constant acceleration?
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