elastic and inelastic collisons

[doctorjoy]

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I want to make sure I have the concept correct. I was listening to the audio osmosis CD 2 lecture four on the discussion of elastic and inelastic collisons. The example given said that, On a frictionless air table 2 magnets are slid toward each other and repel without making contact and bounce off in opposite directions; they said the answer is elastic because magnetic force is conservative. Now here is my question if the same two magnets were slid toward each other and were attracted to one another and took off in the same direction wouldn't the collision be inelastic or would it still be elastic because the maganetic force is conservative? 😕

Anybody can answer...

 

[bjb305]

i would think that it would have to be perfectly inelastic cause by definition, a perfectly inelastic collision occurs when 2 objects collide and stick together, as would 2 magnets

 

[Shrike]

You need more information: what happens after the collision? The first "collision" is elastic for the reason stated -- no energy is lost. (In practice, that's wrong because there is molecular rearrangement occuring, but if the crystal structure of the magnets is stable this can safely be neglected.) If the magnets actually hit each other, then you have to consider the impact itself, so more forces are at work than just the magnetic. If the magnets bounce off of each other, and no energy is lost when they hit, the collision is elastic; if they stick to each other or they bounce but after losing energy, it is inelastic. (With magnets, the latter situation is, of course, more likely in practice.)

 

[Gentle]

You need more information: what happens after the collision? The first "collision" is elastic for the reason stated -- no energy is lost. (In practice, that's wrong because there is molecular rearrangement occuring, but if the crystal structure of the magnets is stable this can safely be neglected.) If the magnets actually hit each other, then you have to consider the impact itself, so more forces are at work than just the magnetic. If the magnets bounce off of each other, and no energy is lost when they hit, the collision is elastic; if they stick to each other or they bounce but after losing energy, it is inelastic. (With magnets, the latter situation is, of course, more likely in practice.)

Doctorjoy, it'll be safe for MCAT purpose to know that:

-Inelastic collision is when there's conservation of momentum ONLY
M1V1 initial + M2V2 initial = (M1+M2)Vfinal
The objects, magnets in your example, stick together and move with a common vel.

-In Elastic collision, there's conservation of momentum PLUS Kinetic energy
i.e M1V1 initial + M2V2 initial = (M1+M2)Vfinal and

1/2M1V1^2 initial + 1/2M2V2^2 initial = 1/2M1V1^2 final + 1/2M2V2^2 initial

The objects DON'T stick together, they bounce off and take seperate paths with diff. vels.

Then know how and when to apply them and solve ur probs.

 

[Shrike]

Ah, I missed that in your second example they actually stuck. In such cases (common on the MCAT), the collisions are perfectly inelastic.

Bounce = elastic

Bounce with no energy loss (first example) - perfectly elastic

No bounce = (perfectly) inelastic

 

[doctorjoy]

Thanks just making sure of myself. It has been along time since I took physics