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Homework Help: Physics Problem: Center of Mass

  1. Oct 19, 2004 #1
    With the engines off, a spaceship is coasting at a velocity of +250 m/s through outer space. The ship carries rockets that are mounted in firing tubes, the back ends of which are closed. It fires a rocket straight ahead at an enemy vessel. The mass of the rocket is 1200 kg, and the mass of the spaceship (not including the rocket) is 2.0 106 kg. The firing of the rocket brings the spaceship to a halt. What is the velocity of the rocket?

    I used the equation vcm=(m1*v1+m2*v2)/(m1+m2) but the answer I got is wrong. Please help ASAP!!
  2. jcsd
  3. Oct 19, 2004 #2
    The idea is that whole of the momentum of the ship is transferred to the rocket.

    Just use the conservation of momentum. The momentum which was shared by both the rocket and ship before is now with the rocket alone.

    This will tell you the rocket's velocity.

  4. Oct 19, 2004 #3

    Doc Al

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    Staff: Mentor

    This formula calculates the velocity of the center of mass (CM) of the system. That velocity does not change. If you wanted to apply this to the problem, you'd calculate the velocity of the CM before and after, and set them equal. Do it and you'll see that is equivalent to applying conservation of momentum.

    Please do not double post!
    Last edited: Oct 19, 2004
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