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Homework Help: Football proble, PLEASE HELP!

  1. Oct 8, 2008 #1
    1. The problem statement, all variables and given/known data

    A placekicker must kick a football from a point 36.0 m (about 40 yards) from the goal, and half the crowd hopes the ball will clear the crossbar, which is 3.05 m high. When kicked, the ball leaves the ground with a speed of 24.0 m/s at an angle of 45.0° to the horizontal.
    (a) By how much does the ball clear or fall short of clearing or fall short of clearing the crossbar? (Enter a negative answer if it falls short.)
    ______ m

    2. Relevant equations

    3. The attempt at a solution

    I am completely lost. Would someone please teach me how to get started with this problem?
  2. jcsd
  3. Oct 8, 2008 #2
    First split the 24m/s at 45 degrees into two componenets, vertical and horizontal. You can do this with trigonometry.

    Use constant acceleration formulae in the y plane to determine the amount of time that passes till the ball reaches 3.05m on the way down

    Hint: [tex]displacement = v_{initial}t + \frac{1}{2}at^2[/tex]

    Then solve in the x dimension to see how far the ball goes in this time.

    acceleration is constant, therefore distance = velocity * time
  4. Oct 9, 2008 #3
    There are lots of projectile problems out there, and this one is one of the more complicated ones. If you are "completely lost" that suggests you didn't understand how to do the easier ones and so you missed some really important insights which make this harder problem a lot easier.

    For example, if a ball is dropped from a height of 10 meters, and a second ball is launched horizontally from the same height at the same instant, which will hit the ground first? Why? What does this imply about the acceleration due to gravity? Does it have any effect on horizontal motion? If I kick a soccer ball down a hallway, will gravity speed it up or slow it down?

    The key here is to treat this as two problems - one dealing with horizontal (X) quantities only, and the other with Y quantities only. Like Rake-MC said, since the initial speed is at an angle, you'll need to separate it into its X piece and Y piece using trig.
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