Elastic Vs. Inelastic

[shaq786]

How do you know if a collision is Inelastic or Elastic when the passage does not state it??????

anyone have any strategies for this????

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[ComicBookHero20]

inelastic : objects "Stick together", kinetic energy is not conserved

I think u can usually assume elastic if they dont state anything like whats above...but the test has to tell you, and they usually do...

 

[VPDcurt]

if the two objects "stick" together after the collision, I believe it is inelastic.

 

[chopper]

Only 'perfectly inelastic' stick together.

There are three options:
1. Elastic: momentum and KE conserved - they will tell you this
2. Inelastic: momentum conserved, KE not conserved. still lots of variables.
3. 'perfectly' inelastic: momentum cons, KE not cons. They will say something like 'stick together'. They like using this one b/c the algebra is easier (the final mass is just m1+m2, only 1 final v).

Not sure if they try to 'trick you' into one or the other. I dont' remember ever being unsure of which type of collision

 

[BisMuOxo]

Only 'perfectly inelastic' stick together.

There are three options:
1. Elastic: momentum and KE conserved - they will tell you this
2. Inelastic: momentum conserved, KE not conserved. still lots of variables.
3. 'perfectly' inelastic: momentum cons, KE not cons. They will say something like 'stick together'. They like using this one b/c the algebra is easier (the final mass is just m1+m2, only 1 final v).

Not sure if they try to 'trick you' into one or the other. I dont' remember ever being unsure of which type of collision

👍 Good explanation

 

[VPDcurt]

so what would be an example of an inelastic collision (not perfectly inelastic)?

 

[smc927]

Perfectly elastic: all mechanical energy conserved, no energy dissipated to internal energy(heat). EX: atoms colliding, ball that bounces to the same height it was dropped from. AKA - things bounce back perfectly.

Perfectly inlastic: Drop an egg in the frying pan. It falls, plops, and sticks. Theoretically we'll say it doesn't bounce back at all.

Somewhere in between: head on car crash. Two cars collide. They both bounce back (elastic) but both are dented up (some of the energy was absorbed by the body/frame, bending it = inelastic).

How to figure it out on the MCAT? If you grasp these basics you "shouldn't" have trouble recognizing it on the MCAT.

 

[fonzy]

Simply put, in an ideal [or a perfect] elastic collision the velocity of approach should be equal to the velocity of separation. However, in practice this never is the case.
Somehow I remembered a parameter called 'coefficient of restitution [e]'. It is defined as:

e= velocity of separation / velocity of approach

During a perfectly elastic collision of two bodies, the velocity of approach is always equal to the velocity of separation [as stated above], so that e = 1 in elastic collisions.
In a perfectly inelastic collision the velocity of separation is zero, so that e = 0 in a totally inelastic collisions.


CLASSIFIATION of ONE-DIMENSIONAL COLLISION:

Type Kinetic Energy Restitution

Perfectly Elastic Conserved e = 1

Partially Elastic Not Conserved 0 < e < 1

Perfectly Inelastic Maximum Possible Loss e = 0

Hyperelastic Energy Gained e > 1

 

[chopper]

Simply put, . . . . .

hahaha fonzy - are you trying to help them understand, or is this a sneaky trick to make others screw up the test on Sat?

I actually think your explanation is pretty interesting, but WAY to in the weeds for those poor souls getting ready for the test on Saturday.

sorry - didn't mean for this to be snotty. I just laughed my a*& off when I read your explanation following 'simply put'.

 

[Monkey12]

What situation would lead to an object approaching another object at rest and then both objects reacting with an equal and opposite veloctity? some case where its perfectly elastic with half the mass?

 

[fonzy]

hahaha fonzy - are you trying to help them understand, or is this a sneaky trick to make others screw up the test on Sat?

I actually think your explanation is pretty interesting, but WAY to in the weeds for those poor souls getting ready for the test on Saturday.

sorry - didn't mean for this to be snotty. I just laughed my a*& off when I read your explanation following 'simply put'.

You are right; what I said is probably not needed for solving numerical problems on MCAT.

 

[GBFKicks]

How do you know if a collision is Inelastic or Elastic when the passage does not state it??????

anyone have any strategies for this????

I think that as long as you know that KE is conserved in an elactic collision, you should be ok. (well, if you at least somewhat know the formulas).... The way that I first used to remember that elestic conserves KE is this... If you drop a ball which strikes a surface in a perfectly elastic manner, the ball would return to the exact height of the drop. (preserving kinetic energy as well as momentum)
The opposite is true of inelastic as it only conserves momentum. (the ball would not return to it's original place)
I don't know if that made sense to you, it's just the way I first learned it...

 

[cycloethane]

I've thought of this really gross example to illustrate elastic vs. inelastic (DON'T READ IF YOU GET OFFENDED EASILY). So, imagine you are taking a dumb in a forest (because there is no bathroom nearby). Your excrement will fall down and stick to the ground. This is inelastic collision. This excrement's momentum is conserved, but kinetic energy has been lost (thus it is not bouncing up). If the turd was elastic, it would bounce back and... Ok, well at least you'll never forget which one is which.

 

[GooseWing]

Your graphic response is ~7 years late, I'm afraid.

 

[cycloethane]

haha, but still relevant

 

[SN2ed]

Thread closed. Please do not bump years old threads.