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Homework Help: Finding Velocity When Only Given Height

  1. Nov 21, 2011 #1
    A football is kicked from a height of 2.0m to a maximum height of 8.0m. Calculate the football's initial velocity and the time taken to reach the maximum height.


    I have no idea how to even approach this question. How do I apply formulas to solve this question given only the height of the football??
  2. jcsd
  3. Nov 21, 2011 #2

    Doc Al

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    Staff: Mentor

    What formulas describe the velocity and position of a projectile? (I would assume the football moves vertically.)
  4. Nov 22, 2011 #3
    the dispalcement is 6 metres and the final velocity is zero and the acceleration due to gravity is -g(-9.81) so use suvat equation V(squared)=U(squared) + 2AS
  5. Nov 23, 2011 #4
    Either do I, with the information you're given.

    Since footballs are generally not kicked straight up, they follow a parabolic path. Without knowing the horizontal distance from the point where it was kicked to the point where it reached it's peak height, you cannot come up with a definitive answer.
  6. Nov 23, 2011 #5
    Wow! The kicker must be a giant!
  7. Nov 24, 2011 #6
    @zgozvrm , the question is valid and the answer can be found just by using the two equations of constant acceleration

    v^2 - u^2 = 2as

    remember to only use the part of the ball's motion from the start t=0s to the time at which it reaches it's max hieght....

    another thing that we will have to assume constant verticle acceleration for this question (which in practise is not)
  8. Nov 24, 2011 #7
    The problem can't be solved unless you consider the motion to be ONLY vertical, in that case [itex] V^2 - V{_0}^2 = 2as [/itex] looks pretty good :)
  9. Nov 24, 2011 #8

    assuming the ball is thrown vertically upwards and neglecting air resistance, it can be solved

  10. Nov 25, 2011 #9
    correct me if i am wrong, i think the problem can be solved if we have just on more quantity ... ie the angle at which the ball was kicked .... (the angle to the horizontal)

    thus the initial velocity would be having two components (one in the x direction the other in the y direction) and the y component will be
  11. Dec 3, 2011 #10
    That is correct.

    Like I said before, you need more information in order to solve the problem; like angle or distance kicked. Or, perhaps, the football's velocity as it hit the ground (which will differ from the initial velocity since it was kicked from a height of 2 meters!).
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