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B Inelastic Collisions in space

  1. Jan 7, 2017 #1
    Let's say that 2 objects inelastically collided in outer space.
    One has a Mass of 2kg, and velocity 5m/s.
    The other has a Mass of 4kg, and velocity is 0m/s. ( at rest)

    1) Energy cannot be created or destroyed. (Energy is conserved)
    2) Momentum is conserved.

    Total Momentum before was (2x5) + (4x0) = 10kg m/s
    Total Momentum after, therefore, also = 10kg m/s
    Hence, velocity of the 2 objects moving is 'v' in the equation :
    10 = (v x [4+2])
    so v = 10/6 m/s

    Now from Kinetic Energy perspective:
    Before -- k.e = 1/2 x 2 x 25 = 25J
    After -- k.e = 1/2 x [4+2] x 25/9 = 25/3 J

    So what happened to the K.E after the Collision for it to go from 25 to 25/3??
     
  2. jcsd
  3. Jan 7, 2017 #2

    PeroK

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    Where does energy ever go?
     
  4. Jan 7, 2017 #3
    I would usually say, "Lost as Heat or Sound..." etc if this happened on earth... But I can't imagine heat or sound being produced in outer space...
     
  5. Jan 7, 2017 #4

    PeroK

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    The Sun's pretty hot I would say!
     
  6. Jan 7, 2017 #5

    anorlunda

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    Both momentum and kinetic energy are frame dependent, so changing the frame of reference, post collision can mess up your calculations.

    Choose a point such as the initial position of the stationary object. Then calculate the velocities of both objects relative to this point both pre and post collision.
     
  7. Jan 7, 2017 #6
    So if you were to use the data in my example, how would you calculate it?
     
  8. Jan 7, 2017 #7
    I meant that the objects, (metal magnets most probably), wouldn't emit any heat/ light energy like the Sun, since they aren't luminous.
     
  9. Jan 7, 2017 #8

    PeroK

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    There's nothing to stop an object heating up in space, either due to absorbing radiation or through a collision with another object.
     
  10. Jan 7, 2017 #9
    Let's assume that the energy is lost via radiation.
    Then, from the above example, approx. 17 J of energy is lost in a very short time span. From calculations, frequency of that radiation would be 2.6 x 10^34, which is really close to the Maximum possible value achievable. This radiation can only occur theoretically and requires a dramatic event to happen. Just 2 Magnetic objects, with such a low mass and velocity, wouldn't be able to that.
     
  11. Jan 7, 2017 #10
    Here, I assumed that the 2 objects are magnetic and hence, clumped together. But it can be anything like an object with a glue or something.
     
  12. Jan 7, 2017 #11

    PeroK

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    Perhaps the objects simply heat up, the way they would in a terrestrial collision!
     
  13. Jan 7, 2017 #12
    Yes, perhaps. But I would like to know if that's what actually happens :)
     
  14. Jan 7, 2017 #13

    jbriggs444

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    How do you imagine that objects in terrestrial collisions heat up?
     
  15. Feb 4, 2017 #14
    If it is heat or radiation, then what if the same collision is happening on earth where sound is produced? By the conservation of momentum and a bit of calculation the energy loss should be the same. However since sound is produced, does it mean that the energy loss in heat or radiation that we have guessed will become smaller? Isn't it weird that the source of energy loss is different even though it is the same collision? I mean why will it specifically emit less radiation on earth?
     
  16. Feb 4, 2017 #15
    Since it is inelastic collision , because of the energy needed to deform the bodies the KE reduced
     
  17. Feb 4, 2017 #16

    Nugatory

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    It will be less weird if you consider that the two collisions are not exactly the same. In space we have a collision between two moving objects. On earth we have a collision between the same two moving objects plus the air molecules that happen to get in the way - that's a slightly different situation and it shouldn't be surprising that some of the energy ends up with the air molecules.
     
  18. Feb 5, 2017 #17
    The objects itself heated up , whether they are in space or in earth and the heat radiation itself not taking place in infinity small time.
     
  19. Feb 5, 2017 #18
    You have stated that this is an inelastic collision, therefore energy is not conserved and must be lost from the system by definition. Assuming your bodies are finite objects, the most likely route for energy loss would be in the deformation of the objects (depending on the nature of the objects there may be other energy loss mechanisms) during the collision, which would reduce the total kinetic energy of the bodies after the collision. This more than likely would result in the objects being heated, in which case the energy converted to heat during the collision could be subsequently lost through radiation.
     
  20. Feb 6, 2017 #19
  21. Feb 6, 2017 #20
    Part of the Kinetic Energy in the first object converted to strain energy in both the objects, the stored strain energy will be lost as radiation, sound.
     
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