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SAT Subject Test in Physics

  • #1
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[SOLVED] SAT Subject Test in Physics

Homework Statement


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?


Homework Equations


F=ma, a=(Vf-Vo)/t


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.
 

Answers and Replies

  • #2
Doc Al
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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?
Careful: The time in the barrel is not the same for the two projectiles.
 
  • #3
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Oh, thats true, so how can I get around that?
 
  • #4
Doc Al
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44,882
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Oh, thats true, so how can I get around that?
How do the accelerations compare? How does final speed (at the end of the barrel) depend on distance and acceleration?
 
  • #5
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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?
 
  • #6
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Wait, v doesn't equal a/d, I don't know what I'm thinking...
 
  • #7
119
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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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