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pratu043 Group Title

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?

  • 2 years ago
  • 2 years ago

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  1. Xishem Group Title
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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.

    • 2 years ago
  2. Yahoo! Group Title
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    it is just same as recoil velocity of Gun..

    • 2 years ago
  3. pratu043 Group Title
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    but even in recoiling of gun, how can the final momentum be 0?

    • 2 years ago
  4. Yahoo! Group Title
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    v = - Mv/m = -50*3/300 = -0.5m/s

    • 2 years ago
  5. pratu043 Group Title
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    Yes, I know the answer but I can't understand how total momentum is 0.

    • 2 years ago
  6. Yahoo! Group Title
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    Law of conservation of momentum..says that momentum is conserved

    • 2 years ago
  7. Yahoo! Group Title
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    Do..u knw abt Rocket propulsion

    • 2 years ago
  8. Xishem Group Title
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    Momenta are vectors.

    • 2 years ago
  9. Xishem Group Title
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    If you have two vectors in opposite directions, they can cancel eachother out.

    • 2 years ago
  10. Yahoo! Group Title
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    even though the mass gets reduced..the momentum is conserved

    • 2 years ago
  11. pratu043 Group Title
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    even if they are different?

    • 2 years ago
  12. Xishem Group Title
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    The girl's momentum is in one direction and the boat's in another direction.

    • 2 years ago
  13. Yahoo! Group Title
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    yup..@Xishem has a point there

    • 2 years ago
  14. pratu043 Group Title
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    I mean do they cancel each other even if the magnitudes are different?

    • 2 years ago
  15. Xishem Group Title
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    The magnitudes aren't different.

    • 2 years ago
  16. Xishem Group Title
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    Remember, momentum is a function of both mass and velocity.

    • 2 years ago
  17. Xishem Group Title
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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.

    • 2 years ago
  18. pratu043 Group Title
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    So mass*velocity of the bullet or girl will always be equal to mass*velocity of gun or boat?

    • 2 years ago
  19. Xishem Group Title
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    Yes, assuming you are in a closed system. Which you are.

    • 2 years ago
  20. pratu043 Group Title
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    What is a closed system?

    • 2 years ago
  21. pratu043 Group Title
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    Please help I have a test coming up soon.

    • 2 years ago
  22. Xishem Group Title
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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.

    • 2 years ago
  23. Xishem Group Title
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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).

    • 2 years ago
  24. pratu043 Group Title
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    Thanks I got it now.

    • 2 years ago
  25. pratu043 Group Title
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    This doesn't happen in the case of a collision right?

    • 2 years ago
  26. Xishem Group Title
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    Momentum is not conserved in an inelastic collision. Momentum is conserved in an elastic collision.

    • 2 years ago
  27. Xishem Group Title
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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.

    • 2 years ago
  28. pratu043 Group Title
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    Thanks.

    • 2 years ago
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