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Homework Help: A spring gun problem

  1. Dec 19, 2016 #1
    1. The problem statement, all variables and given/known data
    A mechanical gun of mass M(g) uses a spring to shoot a bullet of mass M(b). The energy of the compressed spring before firing is E. Assuming the gun is at rest before firing, what is the recoil speed of the gun immediately after it shoots the bullet?

    2. Relevant equations

    Conservation of momentum (M(g)+M(b)vi= M(g)vbf+M(g)vgf
    Conservation of Energy
    3. The attempt at a solution
    I first tried to isolate vgf(speed of gun after the bullet is fired), which gave me

    -Vgf= M(b)vbf/M(g), I dont what to do after this...
  2. jcsd
  3. Dec 19, 2016 #2
    On the relevant equations section, you wrote conservation of energy, yet you did not use it.
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