Transfer of KE in collisions

[Quartile]

Hi, I have a handful of questions about collisions and sound. Thanks for stopping by.

 

How fast does kinetic energy transfer between two objects in a perfectly elastic collision?

Does a perfectly elastic collision ever occur outside of a lab?

What happens to the kinetic energy that is lost in an inelastic collision?

 

Now, I dont know if these questions should be in another thread in another forum, so I will post them here as well. They are questions about sound.

 

I know that the speed of sound varies in different mediums. Does sound have kinetic energy, since it is a compression wave?

Could it be said that as a medium increases in hardness/stiffness/rigidity, the speed of sound increases in that medium?

Does sound, being a compression wave, have a particle? How does sound fit in with the principles of wave-particle duality?

 

Thanks!

[swansont]

Hi, I have a handful of questions about collisions and sound. Thanks for stopping by.

 

How fast does kinetic energy transfer between two objects in a perfectly elastic collision?

Does a perfectly elastic collision ever occur outside of a lab?

What happens to the kinetic energy that is lost in an inelastic collision?

 

Now, I dont know if these questions should be in another thread in another forum, so I will post them here as well. They are questions about sound.

 

I know that the speed of sound varies in different mediums. Does sound have kinetic energy, since it is a compression wave?

Could it be said that as a medium increases in hardness/stiffness/rigidity, the speed of sound increases in that medium?

Does sound, being a compression wave, have a particle? How does sound fit in with the principles of wave-particle duality?

 

Thanks!

 

The time it would take depends on a lot of things. Elastic collisions are realy an idealization for anything above nuclear scale, but to the extent that one can use that approximation, a satellite flyby of a planet can be viewed as an elastic collision, and take months. Nuclear ones can probably take femtoseconds or less.

 

Elastic collisions do occur, on the nuclear scale, out in the real world.

 

In an inelastic collision, you lose energy to photons (on the nuclear scale). On the macroscopic scale, you lose energy to heating, sound and deformation.

 

Sound speed varies with the square root of the bulk modulus (or Young's modulus in solids), which is the stiffness (for solids, at least)

 

Vibrations in solids are treated in QM with the phonon.

 

You can Google for details on much of this.

[Quartile]

Thank you swansont

 

a satellite flyby of a planet can be viewed as an elastic collision

I dont understand how it can be considered a collision. Are they actually colliding? Isnt most of this interaction based on gravity?

 

Maybe it would have been a better question if I had been more exact. If an iron ball with v=100m/s hits another iron ball at rest and the collision is perfectly elastic, how long after they make contact will the ball at rest begin moving?

[swansont]

Thank you swansont

 

 

I dont understand how it can be considered a collision. Are they actually colliding? Isnt most of this interaction based on gravity?

 

Maybe it would have been a better question if I had been more exact. If an iron ball with v=100m/s hits another iron ball at rest and the collision is perfectly elastic, how long after they make contact will the ball at rest begin moving?

 

Yes, a satellite is a gravitational interaction. An iron ball is an electrostatic one, though since it's electrically neutral the shielding gives this a short range.

 

The time before something starts moving is very short. The time the whole interaction takes will depend on how much the balls deform and how long they remain in contact.

[Asadullah]

dont elastic collision occur in the sling shot effect and in superconductors between electrons and lattice impurities without energy loss?

[imp]

Hi, I have a handful of questions about collisions and sound. Thanks for stopping by.

 

How fast does kinetic energy transfer between two objects in a perfectly elastic collision?

Does a perfectly elastic collision ever occur outside of a lab?

What happens to the kinetic energy that is lost in an inelastic collision?

 

Thanks!

 

I like to think, on a practical level, of nearly-perfect elastic collisions occurring on a billiard table. One could almost visualize that the time needed to effecty the collision would include that used in slightly deforming the curved surfaces of the ballso while under impact- flat areas of deformation which "spring back" elastically as the balls separate. That impact time defines the duration of kinetic energy transfer.

 

Energy is transferred, not "lost". imp

[Quartile]

Energy is transferred, not "lost". imp

 

Yes of course. But you COULD say that KE is lost if you are looking specifically at the kinetic energy of the system of two colliding particles and disregarding all other types of energy. Then it can be lost because it is transferred. This is what I meant.

 

I have another question, this time more closely related to what I am trying to understand. Imagine 10,000 iron balls hanging by nylon thread from a ceiling. They are hung in a line so that each one is touching the one next to it and they are all at rest. A sensitive pressure sensor of some sort is put at one end, barely touching the 10,000th iron ball. Then a small force is applied to the first ball in the direction of the axis that passes through the center of each ball. Wouldn't the force travel along these 10,000 iron balls with a speed much faster than c?

[swansont]

I have another question, this time more closely related to what I am trying to understand. Imagine 10,000 iron balls hanging by nylon thread from a ceiling. They are hung in a line so that each one is touching the one next to it and they are all at rest. A sensitive pressure sensor of some sort is put at one end, barely touching the 10,000th iron ball. Then a small force is applied to the first ball in the direction of the axis that passes through the center of each ball. Wouldn't the force travel along these 10,000 iron balls with a speed much faster than c?

 

No, the force involved is electromagnetic and is limited to c, and will in reality travel much, much slower. There are no infinitely rigid materials.

[insane_alien]

to expand, it would travel at the speed of sound in iron. fast, but nowhere near lightspeed.

[Quartile]

What if it was one gigantic iron beam

 

What would it take for there to be infinite rigidity in a solid? Infinte lattice energy?

 

I think Im having trouble understanding how it could be possible that the position of an observable macroscopic particle can be completely determined by the compression wave that travels through the particle's individual atoms. In the case that the "particle" Im talking about is the iron beam starting at rest, it makes more sense to me that when a force acts on it, its position in spacetime would begin changing a small amount before the compression wave travels all the way through it. Otherwise, I dont see the reasoning behind mathematically referring to particles of any mass as single particles. It would seem that they should be always be referred to as collections of smaller particles.

 

I appreciate your patience with me and this topic!

[insane_alien]

well, with relativity, nothing can be infinitely rigid. even hypothetical bodies within the laws of physics.

 

and 'particle' in physics(and most branches of science to be honest) does not include a huge beam made of iron atoms, made of sub-atomic particles.

 

when you get down to the elementary particles, they don't really have any sor of size in the classical sense. so there is nothing to compress. they are also not composites of smaller particles(we think) like your giant iron beam.

[Quartile]

and 'particle' in physics (and most branches of science to be honest) does not include a huge beam made of iron atoms, made of sub-atomic particles.

 

Notice I use "particle" in quotations, denoting the special definition I give it - "a unit of matter that is observable on a macroscopic level." I understand that macroscopic objects arent necessarily considered particles. I think what I am trying to say is that the iron beam is a macroscopic "composite particle," which (as far as I know) is an idea/definition that science does include.

 

when you get down to the elementary particles, they don't really have any sort of size in the classical sense. so there is nothing to compress.

 

But sound is a compression wave that travels through matter. I assume that it is caused by the displacement of atoms, which is why swansont mentions that "There are no infinitely rigid materials." If the elementary particles cannot be compressed because their size (mass) is too small, then the position of atoms in spacetime would be the only factor determining the position of the sub-atomic particles that make up atoms, similar to rocks in a bucket. Wherever the bucket goes, the rocks go. Is this true?

 

a few edits to my original post..

 

Otherwise, I dont see the reasoning behind mathematically referring to particles in collision as single particles. It would seem that they should always be referred to as complex systems of smaller, more fundamental particles.

 

Thanks again!! If this goes on much longer I might have to start paying you all for tutoring me on this

[swansont]

The collection of particles in a composite system are able to move, and exert the EM force between them. That force is finite and depends on the separation distance, and this dictates the bulk (or Young's) modulus (tells you how easily the objects compresses under a force). The speed of sound depends on that.

[insane_alien]

its the morning, my insomnia was horrible and the drugs the doctor gave me didn't work. forgive any ravings, utter a--holeness etc. etc.

 

Notice I use "particle" in quotations, denoting the special definition I give it - "a unit of matter that is observable on a macroscopic level."

 

well, in the world of science you don't just apply words to different contexts and assume everybody is going to just know. particle is very well defined and it is defined for one thing only. if you were buying a car you wouldn't ask for a compact if you really wanted a small mobile home(RV in america i'm informed by wikipedia) would you?

 

I understand that macroscopic objects arent necessarily considered particles. I think what I am trying to say is that the iron beam is a macroscopic "composite particle," which (as far as I know) is an idea/definition that science does include.

 

yes it is most definitely an idea science does include. but this does not mean that the phenomenon that occur with a massive amount of particles occur in individual particles. take football fans(soccer in the USA) with a large densely packed crowd you can get a good mexican wave going(sound wave). but if it was just one guy(particle) on his own he'd look pretty stupid. there is nothing else to interact with so he's not goingto do it.

 

But sound is a compression wave that travels through matter. I assume that it is caused by the displacement of atoms,

 

yes, atoms are held in equilibrium by the EM force, they're not really touching(you could imagine this as a bunch of spheres bonded by springs to the neighbouring spheres. you can get a wave going (and that would be a sound wave) if you make it long enough you'll notice that the other end doesn't move till the wave hits it. thisis essentially what happens.

 

which is why swansont mentions that "There are no infinitely rigid materials." If the elementary particles cannot be compressed because their size (mass)

 

size doesn't equal mass. elementary particles don't have a size really, most do have a mass though. but all of them, behave like infinitely small points under the right conditions.

 

then the position of atoms in spacetime would be the only factor determining the position of the sub-atomic particles that make up atoms, similar to rocks in a bucket. Wherever the bucket goes, the rocks go. Is this true?

 

no, see it's more like a pile of rocks. the position of the pile doesn't define where the rocks go but the position of the rocks defines where the pile is. not the other way around.

 

Otherwise, I dont see the reasoning behind mathematically referring to particles in collision as single particles. It would seem that they should always be referred to as complex systems of smaller, more fundamental particles.

 

appart from the utter hideousness of your interchanging custom vocabulary( i think i've figured it out) this is generally true. in the real world(not the classroom though, any resemblance to reality there is purely coincidental and probably a mistake) beams are not considered to be particles. we have a different set of equations to govern them and generalize their behaviour. this can get rather complicated and messy so we leave it up to computers most of the time. look up finite element analysis. if anything it's an interesting read.

 

Thanks again!! If this goes on much longer I might have to start paying you all for tutoring me on this

 

as grumpy as i am, its free and sort of fun.

[Quartile]

That force is finite and depends on the separation distance, and this dictates the bulk (or Young's) modulus (tells you how easily the objects compresses under a force). The speed of sound depends on that.

 

Could it be said then that the intensity of sound varies with the amount of displacement caused in a composite system by kinetic energy transfer? This displacement would then be high when the difference in speed between the two objects in collision is high because they are transferring large amounts of kinetic energy? When a ball at 15m/s hits a ball at 10m/s, much less kinetic energy is transferred than if that ball at 10m/s were actually at rest. I think this has to do with relativity, especially considering that two objects undergoing uniform acceleration will appear to one another to be still, and when one object begins increasing its acceleration and thus experiences jerk, it will be accelerating from the other object's pov until it returns again to a state of (increased) acceleration and will appear to have a uniform velocity with respect to the object that doesnt change acceleration. Yes/no?

 

yes it is most definitely an idea science does include. but this does not mean that the phenomenon that occur with a massive amount of particles occur in individual particles.

 

But certainly the movement of a composite system determines the movement of every particle within it at all times? If a ball is moving with uniform v=30m/s, every particle within it is at rest relative to every other particle. But if the ball gains or loses KE, it produces a compression wave with speed proportional to the structural rigidity of the composite system. If it gains or loses KE in a smaller amount of time, the intensity of this compression wave is higher and thus more audible.

 

size doesn't equal mass. elementary particles don't have a size really, most do have a mass though. but all of them, behave like infinitely small points under the right conditions.

 

Ah yes thank you. Gauss' Law has interesting implications, then, when it is applied to large bodies of mass and their related gravitational field. Their gravitational influence also behaves as though it originates from an infinitely small point, even though these large masses are obviously not infinitely small points. I tend, for whatever reason, to see the same sort of behavior in sound mechanics. It is difficult, however, to reconcile the two in any way without considering a composite system as a "particle."

 

no, see it's more like a pile of rocks. the position of the pile doesn't define where the rocks go but the position of the rocks defines where the pile is. not the other way around.

 

I see your point. Why can't it work both ways - movement of the pile determines position of the rocks within it and at the same time the position of the pile relative to other "piles" around it, and movement of the rocks determine position of the pile as well as position of the rock relative to the other rocks within the pile. I tend to think of existance as a giant, multidimensional, self-similar fractal.

 

appart from the utter hideousness of your interchanging custom vocabulary( i think i've figured it out)

 

Is it so hideous if you can figure it out so easily?

 

as grumpy as i am, its free and sort of fun.

 

Good to hear!

[swansont]

But certainly the movement of a composite system determines the movement of every particle within it at all times? If a ball is moving with uniform v=30m/s, every particle within it is at rest relative to every other particle.

 

No, if the center-of-mass is moving at some v, you can often ignore that when looking at the behavior of the constituent parts. For a ball moving at 30 m/s, the individual atoms are most certainly not at rest, for they have thermal motion. The mean velocity will end up being zero, but not the rms, i.e. the velocity averages to zero but not the speed. You could even have oscillatory behavior (i.e. i could be "ringing") and not affect the COM motion.