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Ira
Carpal Tunnel

Reged: 08/22/10

Loc: Mitzpe Ramon, Israel
Re: A Request to the Physicists Among Us [Re: Otto Piechowski]
#5760002 - 03/27/13 11:06 AM

It is not being moved because of friction. In outer space it would move. You are imparting kinetic energy to it, but not enough to overcome its friction. The energy is converted to a tiny amount of heat where the legs touch the floor.

/Ira

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Otto Piechowski
Pooh-Bah

Reged: 09/20/05

Loc: Lexington, KY
Re: A Request to the Physicists Among Us [Re: Ira]
#5760027 - 03/27/13 11:23 AM

When I stop pushing against the same desk, the energy added has now left the desk (in the form of heat dissipated through the legs touching the floor, the surfaces of the desk radiating heat into the surrounding air, etc). Correct?

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Postmaster

Reged: 04/28/03

Loc: Atlanta, GA
Re: A Request to the Physicists Among Us [Re: Otto Piechowski]
#5760031 - 03/27/13 11:26 AM

Quote:

I am, at this moment, pushing against the desk in front of me with my hand. It has no effect (the desk is not being moved because it is so heavy). Yet, pushing against the desk adds energy to the desk. Correct?

Actually, no. If the object is free to move, then yes.

Energy = Force * Distance

If it can't move, then distance is 0, so no energy added.

In your case, you are applying force to the desk. But something (the ground, the wall, or some guy pushing the other way) is applying an equal and opposite force cancelling out the force you are applying. Net force is 0.

If you push it really hard so it starts moving, it gains energy. As it slides across the floor and slows down due to friction, that kinetic energy would be converted to heat.

Edited by Jarad (03/27/13 11:28 AM)

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Otto Piechowski
Pooh-Bah

Reged: 09/20/05

Loc: Lexington, KY
Re: A Request to the Physicists Among Us [Re: Jarad]
#5760044 - 03/27/13 11:34 AM

Let's simplify the scenario. The desk is in an interstellar vacuum. I, an astronaut in a space suit, push against the desk. The desk starts floating away from me and I start floating in the opposite direction.

Let's focus only on the desk. Because I pushed on it (applied force to it), it is now moving through the vacuum. Because it is moving through the vacuum, energy has now been added to it?

Correct?

Otto

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EJN
Carpal Tunnel

Reged: 11/01/05

Loc: Between Eigenstates
Re: A Request to the Physicists Among Us [Re: Otto Piechowski]
#5760064 - 03/27/13 11:46 AM

Yes, kinetic energy.

KE = 1/2mv^2 = p^2/2m where

m = mass, v = velocity, & p = momentum (mv).

The force applied is given by

F = m dv/dt

(The above are the classical equations and ignore special
relativity)

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Otto Piechowski
Pooh-Bah

Reged: 09/20/05

Loc: Lexington, KY
Re: A Request to the Physicists Among Us [Re: EJN]
#5760100 - 03/27/13 12:02 PM

Let us now say the desk is stopped. Let's imagine something like it running into an asteroid. At the moment it stops, the desk loses the energy that had been added to it by my application of force. Correct?

Otto

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Ravenous
sage

Reged: 11/14/09

Loc: UK
Re: A Request to the Physicists Among Us [Re: Otto Piechowski]
#5760238 - 03/27/13 01:09 PM

Some (most) of the energy is transferred to the asteroid. You pushed the desk originally, you see it go towards the asteroid and land/crash. If you have infinitely good measuring abilities, you will observe the asteroid (with the desk on it) is moving at a slightly different speed... the desk has shared its energy with the asteroid and changed its course.

When you stand on the earth and jump, the Earth bounces downwards too. (In theory - because I doubt it's ever been measured.)

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Otto Piechowski
Pooh-Bah

Reged: 09/20/05

Loc: Lexington, KY
Re: A Request to the Physicists Among Us [Re: Ravenous]
#5760248 - 03/27/13 01:13 PM

Thank you Ravenous, EJN, Ira, and Jarad,

The desk gained energy when I shoved it. It lost that extra energy when it collided with the asteroid.

On the way (after the shove and before the collision) what was that energy? Was it a something? What was the nature of that something?

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llanitedave
Humble Megalomaniac

Reged: 09/26/05

Loc: Amargosa Valley, NV, USA
Re: A Request to the Physicists Among Us [Re: Otto Piechowski]
#5760356 - 03/27/13 01:44 PM

Quote:

Thank you Ravenous, EJN, Ira, and Jarad,

The desk gained energy when I shoved it. It lost that extra energy when it collided with the asteroid.

On the way (after the shove and before the collision) what was that energy? Was it a something? What was the nature of that something?

That's really a relative concept. The energy it "has" is relative to whatever energy the comparative object has. The energy it has relative to your body after pushing it is equal to the force at which you pushed it. The energy it has relative to the walls of whatever external object is in the vicinity is only half of what your push imparted, because the other half is you moving away from the desk.

The energy it has relative to the target asteroid is a factor of the vector of the two objects' directions. If they are moving exactly parallel to one another without intersecting, there is no kinetic energy to impart. If they are moving directly towards one another, then whatever energy you imparted to the desk (half the amount contained in your push) will be added to its impact with the asteroid.

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Otto Piechowski
Pooh-Bah

Reged: 09/20/05

Loc: Lexington, KY
Re: A Request to the Physicists Among Us [Re: llanitedave]
#5760477 - 03/27/13 02:35 PM

Dave, thank you as always for taking on my basic questions!

There is this desk sitting motionless in the vacuum of space. It, the desk, is made up of whatever it is it is made.

Now the desk is shoved and it is moving through space. It has not yet impacted against anything. The desk is still the same desk but it is moving. Is this moving desk made up of exactly the same components of which it was constituted while it was sitting still. Or is it made up of the same components plus a "something"?

Is the force which caused it to move, somehow now a something within the object itself?

The desk is, at this moment different from the previous desk. The previous desk was a-motionless-desk. It is now a-desk-in-motion. There is nothing now in contact with it causing it to be in motion. Somehow, the originating force has been transferred into the desk, causing what was a-motionless-desk to be a-desk-in-motion. Is that something which is causing it to now (not when it was first pushed, but now) to be in motion, a constitutive part of the desk making it a desk+something? What is the something?

Otto

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Postmaster

Reged: 04/28/03

Loc: Atlanta, GA
Re: A Request to the Physicists Among Us [Re: Otto Piechowski]
#5760566 - 03/27/13 03:02 PM

It depends on your frame of reference.

The desk is made of matter, which has a fixed amount of rest mass (M0). The rest mass is intrinsic to it regardless of reference frame. To an observer sitting at the desk (at rest relative to it), its total mass is the same as its rest mass, and its kinetic energy is zero.

To an observer in a different frame of reference that is moving relative to the desk, it has kinetic energy. It also has a different total mass (M = M0 + E/C^2, where E is the kinetic energy). The clock sitting on the desk is also running slower to the observer moving relative to the desk than to the observer sitting at the desk.

So the energy is not intrinsic to the desk itself - it is a relative value that depends on what reference frame it is measured from. As far as the desk is concerned, it didn't change at all when you shoved it - you started moving away from it.

That's part of the core concept of relativity.

Edited by Jarad (03/27/13 03:04 PM)

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llanitedave
Humble Megalomaniac

Reged: 09/26/05

Loc: Amargosa Valley, NV, USA
Re: A Request to the Physicists Among Us [Re: Jarad]
#5760571 - 03/27/13 03:06 PM

Right, the idea of either you or the desk being motionless is very much limited to you and the desk. It has no real meaning to the rest of the universe.

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Otto Piechowski
Pooh-Bah

Reged: 09/20/05

Loc: Lexington, KY
Re: A Request to the Physicists Among Us [Re: Jarad]
#5760593 - 03/27/13 03:18 PM

Long ago, when I was reading things like the Principia and Galileo's Two New World Sciences, I vaguely recall reading phrases such as "impressed force". The idea I took from this was their primitive way of understanding that the force which causes something to move does create a change in the substance of the thing being moved; much like a ring imprinting an image of a seal on the wax sealing a letter.

Using their old fashioned language, the difference between the motionless desk and the desk in motion is that the second, the desk in motion is the same as the motionless desk...but with an imprint; an actual change in the desk itself.

............

Second attempt:

Just before you wrote I, too, Jarad, was thinking of that formula from relativity (M = M0 + E/C^2). And yes, I understand that the difference of mass is dependent on the frame of reference.

But the fact is that the motion causes a change in the desk; its mass increases.

Now, recently, I have heard of a thing called the Higgs Boson which accounts for the mass of an object. If the mass of the desk has changed, there is then, so I undersstand, some difference in regard to the constitution of the desk-in-motion compared to the motionless-desk in terms of the Higgs Bosons constitutive to the two desks.

Am I correct?

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Postmaster

Reged: 04/28/03

Loc: Atlanta, GA
Re: A Request to the Physicists Among Us [Re: Otto Piechowski]
#5760642 - 03/27/13 03:45 PM

Quote:

But the fact is that the motion causes a change in the desk; its mass increases.

But this isn't a change in the desk itself.

Let me give you a slightly different scenario - you are in a spaceship coasting along, and you gently release the desk outside the airlock. It is floating along next to you, no relative kinetic energy.

Now you fire up your engines to return to earth. You are now moving relative to the desk, and from your point of view it now has kinetic energy and its (non-rest) mass has changed.

Did the desk change? It's still floating along in the same orbit as it was before you used your engines.

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Otto Piechowski
Pooh-Bah

Reged: 09/20/05

Loc: Lexington, KY
Re: A Request to the Physicists Among Us [Re: Jarad]
#5760654 - 03/27/13 03:49 PM

I'm thinking.

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Otto Piechowski
Pooh-Bah

Reged: 09/20/05

Loc: Lexington, KY
Re: A Request to the Physicists Among Us [Re: Jarad]
#5760665 - 03/27/13 03:55 PM

Is there such a thing as rest-mass?

Otto

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Postmaster

Reged: 04/28/03

Loc: Atlanta, GA
Re: A Request to the Physicists Among Us [Re: Otto Piechowski]
#5760677 - 03/27/13 04:00 PM

Yes, rest mass is what we normally think of as just mass (since we don't usually encounter objects with enough velocity to measurably change their mass).

Rest mass does not depend on frame of reference.

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CounterWeight
Postmaster

Reged: 10/05/08

Loc: Cloudyopolis, OR.
Re: A Request to the Physicists Among Us [Re: Otto Piechowski]
#5760682 - 03/27/13 04:03 PM

I think you have to be careful here in two ways. One is that the mass of the object 'contains' energy (a sort of by definition of it being composed of matter), and there is the quantum and thermodynamic aspect of that. Then there is the treatment of mass as an object and here if you are saying 'at rest' you need to have some sort of reference frame for the aggregate object and the spacetime treatment. Rather than a desk there is a good example using a spring and that the spring one wound up should somehow increase in weight as there is now stored energy. So the spring should weigh more when wound than when unwound. This change in weight is predicted to be exceptionally small, but then measuring that spring on surface of earth we now have to account for all the forces in the spacetime field the spring resides in in both states. It may be at rest with respect to the test jig and lab, but not with respect to sun and moon. But the spring is an object that is an aggregate of mass. Things change fundamentally when we consider the atoms in the spring and their individual energy.

So it's a question of scale in ways energy at the extremely small and energy at the extremely large. There are and have been a lot of attempts to reconcile the two, at least in the last hundred or so years that's where all the fun is?

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Otto Piechowski
Pooh-Bah

Reged: 09/20/05

Loc: Lexington, KY
Re: A Request to the Physicists Among Us [Re: CounterWeight]
#5760708 - 03/27/13 04:14 PM

Thank you much.

Jim, I went to that website link. In it was written..."Now when people are describing the "mass" of different objects, including particles, it's much more convenient to talk about the rest mass, also called the "invariant mass", the mass a particle has in a frame in which its momentum is zero."

Does any given object exist in a multiplicity of different frames of reference at any given time; within some it has no momentum, but relative to the many others it does?

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Postmaster

Reged: 04/28/03

Loc: Atlanta, GA
Re: A Request to the Physicists Among Us [Re: Otto Piechowski]
#5760794 - 03/27/13 04:55 PM

I think it would be more accurate to say that the particle exists, and can be measured from any frame of reference. There is only 1 frame of reference in which it is at rest.

It's not that it exists in many different frames, it's that there are many different frames it could be measured from. There is only 1 object.

As a simpler analogy let's take measuring the distance to an object. There is only 1 location where the distance to it is zero (the location where the object itself is). There are an infinite number of other locations you could measure the distance to it from, and get a different answer for the distance. That doesn't mean the object exists at many different distances. The object is unique, but the distance to it can only be defined relative to something else.

Rest mass is a property of the object. Velocity, kinetic energy and therefore total mass can only be defined relative to something else, it's not an intrinsic property of the object.

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