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Homework Help: A simple projectile question

  1. Jul 9, 2010 #1
    1. The problem statement, all variables and given/known data

    in a soccer practice session the football is kept at the centre of the field 40 yards from the 10 feet high goalpost. a goal is attempted by kicking the football at a speed of 64 feet/second at an angle of 450(45 degree) to the horizontal. will the ball reach the goalpost?

    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jul 10, 2010 #2
    Write down the initial velocity, acceleration & distance for both x & y directions.
    Then you'll have to use the equations of motion.
     
  4. Jul 23, 2010 #3
    ok i have proceeded in this way. just check where am i wrong:-
    u (in both x and y directions) = 64/root 2 feet/sec
    distance of the ball from goalpost = 40 yards = 120 feet
    a (in x direciton)=0
    therefore, 120 = 64/root 2 x t + 1/2 x 0 x t^2 => t = 15 x root 2 / 8 second
    therefore, s (in y direction) in that time = 64/root 2 x 15 x root2 /8 + 1/2 x (-10) x 225/32
    = 85 feet (approx.)
    but, 85 feet is more than 10 feet which is the height of goalpost. therefore it will pass over the goalpost and it won't be a goal.
    therefore the answer is no.
    but in the textbook it is given that the answer is yes.
     
  5. Jul 23, 2010 #4
    I don't watch football. So I have no idea what the terms mean. But I think question is asking whether ball will be able to travel 120 ft. or not. which according to your calculation it would. (May be i am wrong i don't know)
     
  6. Jul 23, 2010 #5

    DaveC426913

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    Gold Member

    I'm still trying to figure out what a football is doing at a soccer practice....
     
  7. Jul 24, 2010 #6
    yah, if the question asks for that then i am done with the problem. it's quite easy. thanks a lot!
     
  8. Jul 24, 2010 #7
    i cannot believe what a silly mistake i was doing. i was taking everything in feet and second but was taking acceleration in y direction in metre/second square. now i have solved the problem and got the correct answer. the acceleration should be 33 ft/s^2 (approx.).
     
    Last edited: Jul 24, 2010
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