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I am watching this example from Wikipedia:

I am wondering what factors would indicate that the collision of 2 objects will be inelastic (I know macroscopic scale impacts are never perfectly elastic). If these are 2 wagons of the same mass and they have both hard surface, there is no friction and collision takes place (mostly) in 1 dimension shouldn't it be more close to elastic.

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    $\begingroup$ Related: physics.stackexchange.com/q/121663/2451 $\endgroup$
    – Qmechanic
    Commented Jun 30, 2014 at 19:18
  • $\begingroup$ In the case shown, there is something causing the blocks to stay together... adhesive or a latch of some kind. Otherwise they would not stick, and the collision would have some elastic character. $\endgroup$
    – garyp
    Commented Jun 30, 2014 at 19:50

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In general, the elasticity of a collision is dependent on the properties of the colliding objects. In a perfectly elastic collision, no kinetic energy is dissipated, which means the collision creates no heat, no sound, etc. In a perfectly inelastic collision, the maximum possible amount of kinetic energy is dissipated as heat, sound, etc. This corresponds to the two particles sticking together after the collision.

In real life, most collisions are neither perfectly elastic nor perfectly inelastic, but rather somewhere in the middle. (One major exception to this is gas molecule collisions, which are perfectly elastic.) Some objects collide nearly perfectly elastically, such as billiard balls or steel ball bearings, while others collide nearly perfectly inelastically, such as balls of putty or mud. Without knowing the specific properties of the colliding objects (such as their elasticity, etc.), it is impossible to predict how elastic a collision will be.

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  • $\begingroup$ "In a perfectly inelastic collision, the maximum possible amount of kinetic energy is dissipated as heat, sound, etc. This corresponds to the two particles sticking together after the collision" Does it mean in a perfect inelastic collision case even if low mass object hit big mass object(standing) no matter how massive that object is the small object will move in the same direction as before the impact and stick to big object? $\endgroup$
    – user82115
    Commented Jun 30, 2014 at 20:57
  • $\begingroup$ Yes. In a perfectly inelastic collision between a small, moving object and a large, stationary object, the two will stick together after the collision and continue to move in the same direction, albeit with drastically decreased speed. $\endgroup$ Commented Jun 30, 2014 at 21:02
  • $\begingroup$ I see. Just for the "In a perfectly inelastic collision, the maximum possible amount of kinetic energy is dissipated as heat, sound, etc" Is it possible that all of kinetic energy will be dissipated as other form of energy heat, sound, etc ? $\endgroup$
    – user82115
    Commented Jun 30, 2014 at 21:27
  • $\begingroup$ There are lots of ways that the kinetic energy could be dissipated. Heat and sound are probably the most common, but that energy could also be dissipated, for example, in the form of light. In many inelastic collisions, the objects permanently deform during the collision. (Imagine two balls of putty sticking together and becoming more elliptical.) This change in shape requires some energy input, and this is another way that the kinetic energy can be dissipated away. Overall though, heat and sound are definitely the most common ways for energy to be dissipated in an inelastic collision. $\endgroup$ Commented Jul 1, 2014 at 2:33

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