Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Conservation of Linear Momentum of spaceship

  1. Oct 1, 2006 #1
    With the engines off, a spaceship is coasting at a velocity of +230 m/s through outer space. The ship carries rockets taht 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 1300 kg, and the mass of the spaceship (not including the rocket) is 4.0 x 10^6 kg. The firing of the rocket brings the spaceship to a halt. What is the velocity of the rocket?

    I'm pretty sure the equation I have to use is: mAvA + mBvB = (mA + mB)V'

    I'm just unsure on which side to set equal to 0, because it's difficult for me to picture this in my mind. I have a hunch that the left side is equal to 0, because the rocket was in motion initially?

    So if I did that, it would be 0 = (1300)(v'A) + (4.0 x 10^6)(230)

    And then solve for V'A.

    Also, I'm stuck on this problem:

    Batman (91 kg) jumps straight down froma bridge into a boat (mass = 510 kg) in which a criminal is fleeing. The velocity of the boat is initially +11 m/s. What is the velocity of the boat after Batman jumps into it?

    I tried to use linear momentum equations but none of them get me the right answer of 9.3 m/s ~_~' Any ideas on what I am supposed to do?
    Last edited: Oct 1, 2006
  2. jcsd
  3. Oct 1, 2006 #2
    Is the total momentum of the system (spaceship + rocket) equal to zero before the rocket is fired? How about after the rocket is fired?
  4. Oct 3, 2006 #3
    the formula you have to use is:
    mAvA + mBvB = mA'vA' + mB'vB'

    batman has no vA so vA is 0.
  5. Oct 3, 2006 #4


    User Avatar
    Homework Helper

    (mass of the spaceship + mass of the rocket)*velocity of the spaceship = mass of the rocket*velocity of the rocket.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook