TBR inelastic collisions- slightly incorrect?

[Meredith92]

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From TBR page 184 in chapter 4 of physics

"Elastic collisions by definition have conservation of energy. as such, the objects do not deform as a result of the collision. the objects bounce hard off of one another.

Inelastic collisions by definition do not exhibit conservation of energy. as such, the objecs undergo deformation as a result of the collision."

Do they means do not exhibit conservation of KINETIC energy?? I think energy is ALWAYS conserved. Its just that the energy is being converted into internal energy and other forms of energy ( referenced http://hyperphysics.phy-astr.gsu.edu/‌hbase/elacol.html)

 

[brood910]

In this case, KE is the only energy source they have.
KE = energy. It is either conserved or lost.

And yes. If you lose energy, other system will gain that energy.
However, the book is talking about the object that is going thru the momentum.

Think of work.
If work is done on a system,

Ef - Ei = amount of energy lost.
We say energy was not conserved for the system on which the work was done.
However, that lost energy still goes to the other system that did work.

 

[Meredith92]

I thought we are defining our system as both of the objects involved with the collision (at least thats what we do for conservation of momentum- one loses momentum the other can gain it)
Therefore, isnt the change in energy the same as the change in momentum? If one loses kinetic energy to heat energy or to deformation (potential energy) of one of the objects, then energy should still be conserved.



after rethinking your comment... are you saying we shouldnt consider heat/deformation as energy... we should consider them as work??

i sort of get it... its like with thermodynamics internal kinetic energy can be lost as work to the surroundings.... ehhh but at the same time we derived those equations with the understanding of conservation of energy... so i still am not 100% sure ...
(sorry for that bit of rambling! and thanks for your help!)

 

[brood910]

No.
Change in energy equals to work, not to momentum.
Momentum is conserved ALL THE TIME regardless of whether it is elastic or inelastic (at least, in the CLOSED (no external forces) system, which is all we care about when we do momentum questions unless stated otherwise).

You do not conserve energy when there's work done on the closed system, WITH REGARDS TO THE SYSTEM YOU ARE TALKING ABOUT. If you are talking about the whole situation, of course, energy is ALWAYS conserved since the lost energy is gained by the other system that did the work. In this case, the lost KE is converted to heat, sound, etc, which we dont bother to care about.

So, yes. To be more specific, the book should have said KE is conserved. But I guess the book was referring to the collision itself, not other things like heat, etc..

 

[Meredith92]

Oo okay!!! That makes sense thanks so much for your help!!