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Newtons third law and collision of bodies

  1. Nov 27, 2011 #1
    Hi friendz
    I m new to this forum
    I have a problem in a question which is as follows-
    If a body of mass m collides with a body of mass 2m it changes its direction of motion buy when it collides with a body of same mass m it stops and when it collides with a body of mass m/2 it does not change its direction of motion but there is only change in its speed.But when we do this by newtons third law the third case does not prove true.If you got the answer then reply me fast.
    Thanks a lot.
     
  2. jcsd
  3. Nov 27, 2011 #2
    Welcome to the Physics Forums!

    One can give better help if you give some more detail how the Newton's 3rd law seems troublesome.
     
  4. Nov 27, 2011 #3
    If a body of mass m collides with a wall it changes its direction because when it collides with wall it transfer its energy to the wall but due to wall's elastic nature it transfer its energy back to a body due to which it get reaction and when it collides with a body of same mass then on collision it tranfer its tranfer its all energy and the rest body start moving and first body stop due tranfer of all its energy.Then please tell me third case with same with the same energy concept.
     
  5. Nov 27, 2011 #4

    Doc Al

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    Newton's 3rd law is true regardless of the masses involved. Why do you think it doesn't work in the third case?
     
  6. Nov 27, 2011 #5
    sir please tell me how it works in third case
     
  7. Nov 27, 2011 #6

    Doc Al

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    How what works? Newton's third law works exactly the same in all cases.

    In terms of energy, when the mass collides elastically with a stationary mass of lesser value, some (but not all) of its kinetic energy will transfer to the other mass. It continues to move in its original direction, just at a lower speed.
     
  8. Nov 27, 2011 #7
    sir please read this two examples- when you and your friend sit on two.chairs (both of same mass)and you push your friend's forward then you got equal and opposite force in backward direction.but when a body of mass m strikes with a body of same mass m it stops why.another w
    example when you kick a football with a force F it gives reaction on your foot with equal and opposite force but when a body of mass M collides with a body of mass M/2 it get a small reaction due to which there is small change in its speed.why? sir please tell me what i am missing.and reply soon.
     
  9. Nov 27, 2011 #8

    Doc Al

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    Right.
    They exert equal and opposite forces on each other, just like always. Note that the change in momentum of each mass is equal and opposite. While one comes to rest, the other starts moving. In this particular case, things work out just right (due to conservation of momentum and energy) so that the first mass comes to rest.
    In both case, Newton's 3rd law applies. When mass M collides with mass M/2, they exert equal and opposite forces on each other. In this particular case (due to conservation of momentum and energy) the first mass just slows down while the second mass speeds up.

    The effect of the collision on the resulting velocities will depend on the relative masses of the bodies colliding. But Newton's 3rd law always applies.
     
  10. Nov 27, 2011 #9
    Whether m stops or not depends on your frame of reference. The third law is really about the conservation of momentum. If you think about it in those terms it will make more sense.
     
  11. Nov 27, 2011 #10
    But as we know that when we kick a football with a force f (such that mass of foot is M then in opposite it gives us reaction but when a box of same mass M collide with a body of mass M/2 and apply a force F then it does not get equal reaction because if it gets equal reaction then it must stops running.
     
  12. Nov 27, 2011 #11

    Doc Al

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    Why do you think this? The forces that two colliding objects exert on each other are always equal and opposite.
     
  13. Nov 27, 2011 #12
    O.K sir I got the point.And thanks for reply on my questions.Thank you very...... much
     
  14. Nov 27, 2011 #13
    On what factor the transference of kinetic energy from one body to another body depends on colliding bodies.
     
  15. Nov 27, 2011 #14

    Doc Al

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    It depends on the relative sizes of the two masses. By applying conservation of momentum and energy, you can figure out the final velocity of each mass in terms of their initial velocities. (At least for simple, straight-line collisions.)

    (Note: It also depends on the nature of the collision, whether energy is conserved or not. The examples in this thread all assume a perfectly elastic collision.)
     
  16. Nov 27, 2011 #15
    Thanks a lot.......
     
  17. Nov 27, 2011 #16

    Doc Al

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    You might find this helpful. It describes the special case of head-on elastic collisions where the target object is initially at rest: Elastic collisions, target at rest
     
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