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Homework Help: Spring GUN

  1. Oct 21, 2007 #1
    1. The problem statement, all variables and given/known data

    A spring gun (k = 22 N/m) is used to shoot a 56 g ball horizontally. Initially the spring is compressed by 19 cm. The ball loses contact with the spring and leaves the gun when the spring is still compressed by 12 cm. What is the speed of the ball when it hits the ground, 1.5 m below the spring gun?
    ____ m/s

    2. Relevant equations

    kinetic energy, potential energy and spring constant equation

    3. The attempt at a solution

    I have attempted to do this problem with the kinetic energy, potential energy and spring constant equation. But I must be messing it up some how with the two sping compression lengths or the 1.5 m above ground. Can anyone shed some light on how to do this one?
  2. jcsd
  3. Oct 21, 2007 #2


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    Staff Emeritus
    Science Advisor

    Well there are two parts to this problem - horizontal velocity (speed) and vertical velocity (speed). To get the 'speed' of the ball 1.5 m below, one needs to find the resultant from both velocities.

    The spring provides the energy to achieve a horizontal motion (velocity and KE).

    The springs stored mechanical energy is transformed as the spring extends from 19 cm to 12 cm, so find the energy from that change. The spring energy becomes the kinetic energy of the ball.

    When the ball leaves the gun, it begins a free fall under gravity, so use equation of motion for free fall under constant acceleration of gravity.

    Here is a good reference - http://hyperphysics.phy-astr.gsu.edu/hbase/traj.html

    For elastic or spring potential energy - http://hyperphysics.phy-astr.gsu.edu/hbase/pespr.html
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