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Homework Help: Projectile Motion in Combination with Energy Problem

  1. Jun 13, 2012 #1
    1. The problem statement, all variables and given/known data
    A toy rifle shoots a spring of mass 0.008kg and with a spring constant of 350 N/m. You wish to hit a target horizontally a distance of 15m away by pointing the rifle 45o above the horizontal. How far should you extend the spring in order to reach the target?

    2. Relevant equations
    E = 1/2(k)(x)2
    F = kx
    KE = 1/2(m)(v)2
    Any relevant kinematics formulae.

    3. The attempt at a solution
    Normally for questions like this I would equate potential spring energy to the kinetic energy, but here I am missing a velocity as well as the spring compression length so I am a little lost.
  2. jcsd
  3. Jun 13, 2012 #2

    Andrew Mason

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    Science Advisor
    Homework Helper

    Use the formula for horizontal range for projectile motion letting sin2θ = 1 and R = 15 m. to determine the initial velocity v that is needed. From that you can work out the kinetic energy and, therefore, the potential energy of the spring.

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