Quake3World

Let's say I have two objects on a surface. They collide with each other. Assuming there is no friction & air resistance, does changing the elasticity of one object affect whether momentum is conserved? There aren't any external forces so the system conserves momentum regardless if I alter the elasticity of the objects, right?

Sort of...I think.

from what i remember, yes.

do your homework ( and learn to use google!! someone should reiterate this ): http://www.glenbrook.k12.il.us/gbssci/p ... u4l2b.html

I've used google already (in fact I have 4-5 different physics related sites open right now), most of them talked about how KE is lost in that situation but don't mention the effect of elasticity on momentum. I just wanted to confirm that my assumption was correct. :icon25:

btw - if there is no friction and no air resistance every collision on your 'surface' is equivalent to a collision in 3d space. both cases are mathematically isomorph.

<img src="http://www.quake3world.com/forum/images/smiles/dork.gif" height="200" width="200">

Well thanks for the help, I'm going to play some quake now. :icon34:

Some of the potential energy goes into deforming an object of lesser elasticity. Imagine a rock hitting a clay ball, energy goes into changing the shape of the clay, which reduces the remaining momentum of both objects.

At least if I'm understanding your question.

Or I could be wrong.

on the other hand the clay ball deforms the rock as well... but only a bit

True. But both ways energy goes into altering the objects, and is then lost from the momentum.

Imagine if 2 cars hit each over compared to 2 huge marble spheres. Both in the same context, but the two cars will deform and won't bounce off each other. The marble balls will.

actually they do bounce off, depends on the speed... if the material can absorb the potential energy and all.

this is something I never understood - if the balls are perfectly elastic then no energy is lost in the collision.

so kinetic energy & momentum must be conserved right?

surely if kinetic energy is conserved, momentum is also right?

and if KE is lost, then momentum must be also lost right?

this looks like a job for smeester

[xeno]Julios wrote:this is something I never understood - if the balls are perfectly elastic then no energy is lost in the collision.

so kinetic energy & momentum must be conserved right?

surely if kinetic energy is conserved, momentum is also right?

and if KE is lost, then momentum must be also lost right?

Exactly. If 2 balls roll into each other at equal velocities, the KE will cancel out. If one ball rolls into another who's standing still, the rolling ball will give all his KE energy to the ball who's standing still.

You'll always lose a little momentum when two objects collide no matter what the material is - some energy will go into deforming or heating the objects, which will be taken away from the momentum.

So to answer your question, changing the elasticity of one object can affect how MUCH momentum is conserved. You didn't specify the mass of the objects or anything else, so it's impossible to answer how much would be lost to a certain amount of elasticity. An object with a low mass hitting a massive piece of clay will lose a lot of momentum, whereas a solid object of greater mass hitting a smaller piece of clay won't lose nearly as much.

R00k wrote:You'll always lose a little momentum when two objects collide no matter what the material is - some energy will go into deforming or heating the objects, which will be taken away from the momentum.

So to answer your question, changing the elasticity of one object can affect how MUCH momentum is conserved. You didn't specify the mass of the objects or anything else, so it's impossible to answer how much would be lost to a certain amount of elasticity. An object with a low mass hitting a massive piece of clay will lose a lot of momentum, whereas a solid object of greater mass hitting a smaller piece of clay won't lose nearly as much.

but what if it were perfectly elastic (hypothetical ideal) - would momentum & kinetic energy be perfectly preserved? I remember learning that it wouldn't (or maybe i learned that in a non-perfect collision, momentum would be conserved but not KE, or vice versa)


fuckin confused

In perfectly elastic collisions, both momentum and total energy(kinetic and potential) are conserved. There is no loss of energy through irrecoverable processes.