Does a Moving Sphere's Kinetic Energy Affect Its Atomic Particles?

[Quarlep]

Lets think we have a sphere and it moves a constant veloctity v.So it will have a kinetic energy.Is this kinetic energy efectts spheres particle energy.(sphere made up but atoms)

Thanks

 

[Drakkith]

Is this kinetic energy efectts spheres particle energy.(sphere made up but atoms)

Sure, the sum of the kinetic energy of each atom equals the total kinetic energy of the sphere.

 

[nasu]

Not in the common definition. The KE of a macroscopic object does not include the internal KE. Not that you cannot think about it this way, too.

 

[Drakkith]

Not in the common definition. The KE of a macroscopic object does not include the internal KE. Not that you cannot think about it this way, too.

Hmm, I suppose that the KE of the atoms would be part of its temperature then?

 

[Quarlep]

Lets think another way we have a object uniform density then this object moves.then the kinetic energy will be sum of the small mass of the sphere.then we get it what's wrong in this model

 

[Doc Al]

Lets think another way we have a object uniform density then this object moves.then the kinetic energy will be sum of the small mass of the sphere.then we get it what's wrong in this model

Can you rephrase your question. I'm not sure what you are asking.

 

[Quarlep]

Lets suppose we have a sphere.Sphere has a uniform density.When v=0 the total energy of sphere will be U(lets describe it that way).Now If we move the object with speed v then total energy will be U+1/2Mv².Now we can divide M small m's like M=m₁+m₂... so then total energy will described that way.Like 1/2m₁v²+1/2m₂v²...=1/2Mv².Important think is uniform density.Whats wrong in this model ?

 

[Doc Al]

Lets suppose we have a sphere.Sphere has a uniform density.When v=0 the total energy of sphere will be U(lets describe it that way).Now If we move the object with speed v then total energy will be U+1/2Mv².Now we can divide M small m's like M=m₁+m₂... so then total energy will described that way.Like 1/2m₁v²+1/2m₂v²...=1/2Mv².Important think is uniform density.Whats wrong in this model ?

I don't see anything wrong with that model. (We are ignoring any molecular structure and internal energy, just treating it as a classical continuum.)

Why do you think it's wrong?

 

[Quarlep]

I am not telling wrong but nasu says its not correct.So I am confused.

 

[Doc Al]

I am not telling wrong but nasu says its not correct.So I am confused.

nasu was just pointing out that that would ignore the random thermal motion of the molecules. Which is often done.

If all you are interested in is the macroscopic mechanical energy (such as gravitational PE and macro KE), then you can safely ignore the details of internal energy.

 

[Quarlep]

Thank you

 

[sophiecentaur]

You can't add up the particle KE to get the bulk KE but you can add up all the momentums.

 

[Doc Al]

You can't add up the particle KE to get the bulk KE

But if you treat the object as classical rigid body, ignoring any molecular-level structure and internal energy, you can divide it into pieces and add the KE of each piece to get the total macroscopic KE of the object.

 

[sophiecentaur]

But if you treat the object as classical rigid body, ignoring any molecular-level structure and internal energy, you can divide it into pieces and add the KE of each piece to get the total macroscopic KE of the object.

Well, it would give you an answer but what could you do with number you got? I can't think it would give you a measure of how much energy you might extract from the system, for instance. (Except, perhaps, for a hot cannonball hitting a bucket of water and producing loads of hot, fast moving droplets of water. ?)

 

[nasu]

But if you treat the object as classical rigid body, ignoring any molecular-level structure and internal energy, you can divide it into pieces and add the KE of each piece to get the total macroscopic KE of the object.

Yes, if you divide it into pieces, all moving with the same velocity, the total KE is the sum of the KE of the pieces. But this is quite trivial.
These pieces are not the atoms or molecules, as hinted in the OP.

My understanding was that he asks how the macroscopic motion influences the KE of atoms. But I am not sure.

 

[Doc Al]

Yes, if you divide it into pieces, all moving with the same velocity, the total KE is the sum of the KE of the pieces. But this is quite trivial.
These pieces are not the atoms or molecules, as hinted in the OP.

It might be trivial, but it is essential to understand. And it doesn't just work for pure translation, where every piece has the same velocity (that would be quite trivial).

My understanding was that he asks how the macroscopic motion influences the KE of atoms. But I am not sure.

I'm not sure what he's asking.

 

[Quarlep]

A sphere moves a constant velocity.This sphere has a volume.Each per volume in the sphere has a mass and If we calculate all this mass kinetic energy we get total kinetic energy of object. Post 7 If object type change thinks I want you to focus galaxy.Lets consider galaxy is a sphere and stars are atoms so galaxy total energy can be the sum of stars kinetic energy.Galaxy has a uniform density.

 

[Doc Al]

A sphere moves a constant velocity.This sphere has a volume.Each per volume in the sphere has a mass and If we calculate all this mass kinetic energy we get total kinetic energy of object post 7

Sure. I assume you mean that you subdivide the sphere into pieces and calculate the kinetic energy of each piece and add them up. That will give you the total KE.

 

[Quarlep]

Yeah I mean exactly that. Can somebody tell me last decision of it ?

 

[Doc Al]

Can somebody tell me last decision of it ?

What do you mean?

 

[Quarlep]

My idea is true isn't it (I mean this cause everybody says different things)

 

[Doc Al]

My idea is true isn't it (I mean this cause everybody says different things)

I'm not sure what your idea is. (Beyond what I've already stated.)

 

[Quarlep]

Sure. I assume you mean that you subdivide the sphere into pieces and calculate the kinetic energy of each piece and add them up. That will give you the total KE.

This idea

 

[Quarlep]

Post 18 which you wrote that's my idea

 

[Doc Al]

This idea

Well, I certainly agree with that (since you are quoting me!).