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Homework Help: How far above the top of a fence?

  1. Jan 10, 2009 #1
    A ball of mass 0.4 kg, initially at rest, is
    kicked directly toward a fence from a point
    20 m away, as shown below.
    The velocity of the ball as it leaves the
    kicker’s foot is 19 m/s at angle of 61 ◦ above
    the horizontal. The top of the fence is 12 m
    high. The kicker’s foot is in contact with the
    ball for 0.04 s. The ball hits nothing while in
    flight and air resistance is negligible.
    The acceleration due to gravity is 9.8 m/s2

    1. Determine the magnitude of the average
    net force exerted on the ball during the kick.
    Answer in units of N.

    2. Determine the time it takes for the ball to reach the plane of the fence. Answer in units of s.

    3. Hint: Consider the diameter of the ball to be
    negligible. How far above the top of fence will the ball
    pass? Answer in units of m.

    4. What is the vertical component of the velocity
    when the ball reaches the plane of the fence?
    Answer in units of m/s.

    I have gotten all of the parts except 3. I have tried submitting .969, 1.148, and .8377 nut they are all wrong

    1 is 190
    2 is 2.1739
    4 is -4.68649

    Last edited: Jan 10, 2009
  2. jcsd
  3. Jan 10, 2009 #2

    Doc Al

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    Staff: Mentor

    What's the height of the ball as a function of time?
  4. Jan 10, 2009 #3


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    I am not sure how you got those values but I get something different

    Find the time to reach the fence (using the motion along X). Then plug that time in the Y equation.
  5. Jan 10, 2009 #4


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    I agree with your time of 2.17 s. I got 12.9 - 12 for the height, which agrees with your first answer to my 1 digit accuracy. I suspect the difficulty is in accuracy. That 12.9 - 12 kills two digits of accuracy so if you need 3 digit accuracy in the final answer you need to do everything to 5 or 6 digits. Even g = 9.81 doesn't cut it.
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