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Homework Help: Baseball projectile motion

  1. Apr 8, 2012 #1
    1. The problem statement, all variables and given/known data
    A ball player hits a home run, and the baseball just clears a wall 21.0 m high located 130.0 m from home plate. The ball is hit at an angle of 35 degrees to the horizontal, and air resistance is negligible. Assume the ball is hit at a height of 1.0 m above the ground.

    a. What is the initial speed of the ball?
    b. How much time does it take for the ball to reach the wall?
    c. Find the velocity components and the speed of the ball when it reaches the wall.

    I don't know how to use any trig in this case since there isn't a velocity given.
  2. jcsd
  3. Apr 8, 2012 #2


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    Hello jonwasnothere. Welcome to PF !

    What have you tried? Where are you stuck.

    According to the rules of this Forum, you need to show some effort before we can help you.

    Use some variable, such as v0, for the magnitude of the initial velocity.

    If it takes the ball t seconds to reach the wall, what equations describe the following during those t seconds?
    The x component of the ball's velocity.

    The y component of the ball's velocity.

    The x coordinate of the ball.

    The y coordinate of the ball.​
  4. Apr 8, 2012 #3
    I don't really know what I'm doing, though. All I know is that:

    A: 35 degrees
    Delta x: 130m
    a: 9.8 m/s^2
    Delta y: 20m

    I don't know where to go from there, or anything. I drew out a diagram, but with no velocity I don't know how to apply trig in the situation so I'm thinking you can't. I was thinking you solve for Vo using Delta y=Vot+1/2at^2 but I don't know t either.
  5. Apr 8, 2012 #4
    As you see from here, when the ball is hit at the angle of 35°, it does not only go forwards but also upwards

    Thus you have to find the horizontal and vertical velocity or assign a value/variable and apply to standard formula for displacement.

    [itex]S=S_o+V_ot+\frac{1}{2}at^2 [/itex]

    Note for horizontal displacement the acceleration a=0.
  6. Apr 9, 2012 #5
    Try to figure out the vertical velocity before the horizontal velocity. If you can figure out the time it takes for the ball reach a height of 21m (at said vertical velocity) you can figure out the horizontal velocity.

    Make sure you draw a diagram which includes all your information, this will help you realise what equations to use and when.
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