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Help with a mechanical energy exercise

  1. Apr 8, 2013 #1
    1. The problem statement, all variables and given/known data

    A spring cannon is used to shot horizontally a marble mall, whose mass is 75g, from a platform located 1.2 m from the ground. If the spring compression is 25 mm, the ball hits the ground 4,2m from the base of the platform. Not taking friction into account, determine:
    1-the mechanical energy of the ball during the motion
    2-the "k" constant factor of the spring
    3-how far the ball would hit the ground if the compression were 37mm.


    3. The attempt at a solution

    With no friction, i can use the principle of conservation of energy

    I want to compute the mechanical energy for the top of the platform.
    ##E_i=E_f## ##\Rightarrow## ##U_i+K_i=U_f+K_f##
    But ##K_i = 0## because the ball is not moving yet, and ##U_f=0## So,
    ##U_i-K_f=E## ##\Rightarrow## ##E=\frac{1}{2} k x^2 - \frac{1}{2} m v^2##

    However, I keep having two variables: velocity and k, and I don't know how to find the value of any of the two, even using other formulas. Can anyone give me a hint?
     
  2. jcsd
  3. Apr 8, 2013 #2
    When the ball leaves the platform, you know the distance it moves in the x- and y-directions.
    So you can work out its speed at this point.
    That will help you with the rest of the problem.
     
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