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Homework Help: Physics Help- Projectile Motion?

  1. Jun 5, 2013 #1
    A baseball leaves the bat at a height of 1.23 m above the ground, traveling at an angle of 45.0 degrees with respect to the horizontal. The initial velocity of the ball is such that, for an unobstructed trajectory, it would land 120 m from the home plate. The ballpark fence is at a distance of 110 m from the plate. What is the maximum possible height of the fence, if the play is certain to be a home run?
    I have tried this problem and am lost. I know I need the equation s=s0 + v0(t)+.5at^2 and the v(t)=v0 + at. I have made a column for horizontal and vertical. Can someone please explain how to do this problem? Thanks!
  2. jcsd
  3. Jun 5, 2013 #2
    It would be easier for us if you would have shown us the complete attempt. We would help from where you went wrong.
  4. Jun 5, 2013 #3
    I tried to find the components first by using cos45 and sin45, but I don't know if I can use the 120 m or that's the wrong thing? Then I tried to use tan45 = max height/60 and got 60. I just don't know where to go from here.
  5. Jun 5, 2013 #4
    For solving projectile problems you must first separate the x-axis component and the y-axis components. That is the velocity, displacement, acceleration. Now the only thing common between these two components is time (t).

    First find the speed of the ball. The range is given. What is the relation between speed and range?

    Now once you get the speed you can easily the problem.
  6. Jun 5, 2013 #5
    Speed = Distance/Time. As for acceleration, I know a=0 for horizontal and a=-g for vertical. I am still confused how to find the speed of the ball though. Which equation can I use for that or is there another method?
  7. Jun 5, 2013 #6
    Sorry i missed the part 'hit 1.23m above the ground'. Now

    1) The x-axis and the y-axis displacement is given. What are these?

    2) Now as the displacements are known use the second kinematic equation for each x and y axes. Form the two equations. Use these two equations to get the value of speed.(What are are these two equations?)

    This is the first part of the problem.

    In the second part of the problem the X-axis displacement is known. Use the value of v from part one. Again form the two equations(the second equation of kinematics will be used). Solve for the unknown.
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