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Collisions in outer space

  1. Dec 10, 2009 #1
    The question is:
    In outer space, 2 objects collide.
    1. Explain if the momentum of one of the objects is conserved.
    2. Is the total momentum of both objects conserved? Why?
    3. Before the collision, the two masses have non-zero total kinetic energy. After the collision, explain if their total kinetic energy can or can not be zero.

    Wouldn't momentum be conserved for all (of one object and for both)? It's either elastic or inelastic collisions and in both momentum is conserved? I thought the objects transfer their momentum to each other. Isnt kinetic energy zero if the objects are at rest? Please help.

    Thank you so much.
  2. jcsd
  3. Dec 10, 2009 #2
    Momentum is conserved for the system of objects. So in this case, the momentum of both summed is conserved. In response to the second question, you need to imagine a situation where the net momentum of the two spaeships is zero before and after the collision. Hint: remember momentum depends on velocity which depends on direction.
  4. Dec 10, 2009 #3
    So momentum is conserved for the system of objects but not for each individual object? For number two you said the momentum of both the objects is conserved, maybe because when there is an equal but opposite momentum for both objects, the net momentum equals zero? And for number 3, is the kinetic energy zero if the objects collide and stay still? Because kinetic energy is energy of motion, so if it was zero wouldn't the objects be not moving? Sorry if this is worded weird. Thanks for your help!
  5. Dec 10, 2009 #4
    Very good. I understand and you are right, so any combination of head on momenta which equals zero would result in zero kinetic energy if they stuck together. Now that may work for chewing gum, but not billiard balls. Space ships unlikely, but one might imagine a situation where there "tractor whip force fields get inextricably intertwined... More likely an expanding debris cloud with lots of KE.
  6. Dec 10, 2009 #5
    Okay, thanks a lot! Wow that would be a crazy thing to see! Just one more thing, can you explain again why momentum isn't conserved for each individual object? That's the only thing I'm still a little confused on. I understand that the system of objects conserves momentum, but is the individual object's momentum not conserved because of space?
  7. Dec 10, 2009 #6
    Well imagine taking a hammer to peanut brittle. Each small piece of peanut brittle is at rest and has zero momentum. You'd be hard pressed to predict where every little bit of peanut britttle goes, amazingly though if the hammer strike was perfect, the sum of all of the small pieces momenta would still be zero. Imagine the process in reverse.
  8. Dec 11, 2009 #7
    Ohh okay! Thanks a lot!
  9. Dec 11, 2009 #8
    You bet. BTW it's friday night, don't you have something better to do vs hanging here?:wink:
  10. Dec 11, 2009 #9
    No. Rowie does physics questions allllll night every night.
  11. Dec 11, 2009 #10
    Then remind your friend that all work and no play makes for a dull boy. I am here only because I have nothing better to do, enjoy helping, and find interacting with fri nite students less challenging and stressful than sunday nite students. Science can be a brutal mistress. But without curiosity, that joy of understanding and questioning, we'd all be on the savannah.
  12. Dec 11, 2009 #11
    ahhahah! I have finals in a couple days!
  13. Dec 12, 2009 #12
    Ok so I'm a little confused here

    a) Momentum is not conserved for the single object.

    b) The momentum of both are conserved because their sum equals zero.

    c) Since it is an elastic collision, the kinetic energy can not be zero because the space ships dont stick together?

    Do we have to factor in that the objects are in space? Meaning that there is not gravity?
  14. Dec 12, 2009 #13
    Are the answers I made earlier today correct? I can't seem to find the logic behind them. Would anyone please be willing to elaborate?
  15. Dec 12, 2009 #14


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    Homework Helper

    a) and b) are correct. c) is correct if the collision is elastic, but the question asks what happens in collisions in general. It does not specify elastic collisions.
  16. Dec 12, 2009 #15
    Oh, I see that makes sense. How come we couldn't assume that the collision was elastic?
    And since it is in space where there is no gravity, does it affect the collision?
  17. Dec 12, 2009 #16


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    Homework Helper

    Except for elementary particles, real-life collisions are never elastic. There's always some amount of heat, sound, electric sparks, or deformation, and these processes lose kinetic energy.

    As for your second question, the conservation of momentum always applies, gravity or not.
  18. Dec 12, 2009 #17
    Ohhh ok thankyou very much! So what you're basically saying is that the kinetic energy is nonzero?
  19. Dec 13, 2009 #18
    Unless they have equal and opposite momentum and stick together. Otherwise something wil be moving, and hence kinetic energy is present.
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