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Andrew Mason

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This may be easier to see by looking at a collision between two bodies in the frame of reference of the centre of mass of the colliding bodies. Since the only forces are between the two bodies, there are no external forces acting on the two-body system so the motion of the centre of mass cannot change (by the first law).

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But when the two bodies stick together after colliding, all motion with respect to the centre of mass ends. So kinetic energy in the centre of mass frame disappears.

BTW, it is not that the energy disappears. It can't. It is just that it is no longer macroscopic kinetic energy. The energy is converted to other kinds of energy.

AM

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A.T.

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There is no loss of energy. There is just loss of macroscopic kinetic energy, which is converted into microscopic kinetic energy (heat). There is only one form of momentum, while energy can be converted into other forms, so the conservation of energy is less obvious.Since there is a loss of energy

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Thanks. Now i understand ;)

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Khashishi

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I can think of one case where linear momentum is conserved while ‘linear ke’ isnt..

If two rough balls collide on a smooth surface.....they may gain angular momentum which adds up to be equal to that before the collision....however if they both spin faster, the would have more rotational kinetic energy.....but since total energy is conserved, the total linear ke would be reduced....

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A.T.

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YesLinear momentum is conserved as a result of newtons 3rd law?

That is usually the case.I can think of one case where linear momentum is conserved while ‘linear ke’ isnt..

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Andrew Mason

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Conservation of momentum is actually a bit more fundamental than Newton's third law. Newton's third law assumes absolute time and space, so it breaks down in Special Relativity and electromagnetism. But the law of conservation of momentum appears to have no exceptions.Linear momentum is conserved as a result of newtons 3rd law?

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I knew that the law of conservation of momentum applied at small scales and high speeds, but I thought Newton's 3rd law did as well! How does Newton's 3rd law not apply in some situations? Could you give me some links to sites/lectures/books that say this? Thanks :)Conservation of momentum is actually a bit more fundamental than Newton's third law. Newton's third law assumes absolute time and space, so it breaks down in Special Relativity and electromagnetism. But the law of conservation of momentum appears to have no exceptions.

AM

EDIT (clarification): I find this fact confusing because I always thought Newton's 3rd and momentum conservation were completely interchangeable concepts. Maybe you're thinking of Newton's 2nd?

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Andrew Mason

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See: http://en.wikipedia.org/wiki/Newton%27s_third_law#Importance_and_range_of_validityI knew that the law of conservation of momentum applied at small scales and high speeds, but I thought Newton's 3rd law did as well! How does Newton's 3rd law not apply in some situations? Could you give me some links to sites/lectures/books that say this? Thanks :)

EDIT (clarification): I find this fact confusing because I always thought Newton's 3rd and momentum conservation were completely interchangeable concepts. Maybe you're thinking of Newton's 2nd?

See this attempt to get around the problem: http://en.wikipedia.org/wiki/Weber_electrodynamics#Newton.27s_third_law_in_Maxwell_and_Weber_electrodynamics

Also, have a look at this link: http://physics.stackexchange.com/questions/43269/violation-of-newtons-3rd-law-and-momentum-conservation

AM

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