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SAT Subject Test in Physics

  1. May 24, 2008 #1
    [SOLVED] SAT Subject Test in Physics

    1. The problem statement, all variables and given/known data
    Assume that every projectile fired from a toy cannon experiences force F along the length of the barrel. If a projectile of mass M leaves the barrel with speed, V, at what speed will a projectile of mass 2M leave the barrel with?

    2. Relevant equations
    F=ma, a=(Vf-Vo)/t

    3. The attempt at a solution
    The force F, produces acceleration of V/t for mass m, so the same force should produce F=(2m)(v/t), so the velocity equals v/2?

    This is a multiple choice question with choices A) v/2, B)v/sqrt(2), C)v, D)2v, E)4v

    The book says the answer is B, but gives no explanation.
  2. jcsd
  3. May 24, 2008 #2

    Doc Al

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    Staff: Mentor

    Careful: The time in the barrel is not the same for the two projectiles.
  4. May 24, 2008 #3
    Oh, thats true, so how can I get around that?
  5. May 24, 2008 #4

    Doc Al

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    Staff: Mentor

    How do the accelerations compare? How does final speed (at the end of the barrel) depend on distance and acceleration?
  6. May 24, 2008 #5
    The final speed is Acceleration/Distance, and we know the distance must be the same for both objects. Can we rearrange the v=a/d and substitute it into F=ma?
  7. May 24, 2008 #6
    Wait, v doesn't equal a/d, I don't know what I'm thinking...
  8. May 24, 2008 #7
    Oh, I found out how to do it, just use v^2=2ad and work it out.

    EDIT: Sorry for the obnoxious triple post.
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