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pratu043

I have a doubt in the answer key to the problem on law of conservation of momentum. The question: A girl of mass 50 kg jumps out of a rowing boat of mass 300 kg on to the bank with a horizontal velocity of 3 m/s. With what velocity does the boat move backwards? Answer key says: m1(mass of girl) = 50 kg m2(mass of boat) = 300kg v1(velocity of girl) = 3 m/s v2 = ? m1v1 + m2v2 = 0 How can the momentum of the whole system after the girl jumps out of the boat be 0? The boat will have some momentum right?

  • one year ago
  • one year ago

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  1. Xishem
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    Right. The boat must have some change in velocity, since the momentum must be conserved and the momentum of the girl changed.

    • one year ago
  2. Yahoo!
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    it is just same as recoil velocity of Gun..

    • one year ago
  3. pratu043
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    but even in recoiling of gun, how can the final momentum be 0?

    • one year ago
  4. Yahoo!
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    v = - Mv/m = -50*3/300 = -0.5m/s

    • one year ago
  5. pratu043
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    Yes, I know the answer but I can't understand how total momentum is 0.

    • one year ago
  6. Yahoo!
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    Law of conservation of momentum..says that momentum is conserved

    • one year ago
  7. Yahoo!
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    Do..u knw abt Rocket propulsion

    • one year ago
  8. Xishem
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    Momenta are vectors.

    • one year ago
  9. Xishem
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    If you have two vectors in opposite directions, they can cancel eachother out.

    • one year ago
  10. Yahoo!
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    even though the mass gets reduced..the momentum is conserved

    • one year ago
  11. pratu043
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    even if they are different?

    • one year ago
  12. Xishem
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    The girl's momentum is in one direction and the boat's in another direction.

    • one year ago
  13. Yahoo!
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    yup..@Xishem has a point there

    • one year ago
  14. pratu043
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    I mean do they cancel each other even if the magnitudes are different?

    • one year ago
  15. Xishem
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    The magnitudes aren't different.

    • one year ago
  16. Xishem
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    Remember, momentum is a function of both mass and velocity.

    • one year ago
  17. Xishem
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    If one object with 1kg mass and 10m/s velocity is moving right and another of 10kg mass and 1m/s velocity is moving left, the total momentum is zero.

    • one year ago
  18. pratu043
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    So mass*velocity of the bullet or girl will always be equal to mass*velocity of gun or boat?

    • one year ago
  19. Xishem
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    Yes, assuming you are in a closed system. Which you are.

    • one year ago
  20. pratu043
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    What is a closed system?

    • one year ago
  21. pratu043
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    Please help I have a test coming up soon.

    • one year ago
  22. Xishem
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    A closed system is a physical system which doesn't exchange any matter in or out of itself and which isn't affected by outside forces.

    • one year ago
  23. Xishem
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    It basically just means that you are only considering the two objects and not nonconservative forces like friction which would change the overall momentum. (This case is frictionless, but that's one case where momentum won't be conserved).

    • one year ago
  24. pratu043
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    Thanks I got it now.

    • one year ago
  25. pratu043
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    This doesn't happen in the case of a collision right?

    • one year ago
  26. Xishem
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    Momentum is not conserved in an inelastic collision. Momentum is conserved in an elastic collision.

    • one year ago
  27. Xishem
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    In the real world, momentum is never conserved in collisions. When two pool balls hit eachother for instance, they release some sound upon impact -- this is lost energy. They are also always under the effect of friction from the table.

    • one year ago
  28. pratu043
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    Thanks.

    • one year ago
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