Increase in PE due to increased mass

  • #1
Suppose a body is held stationary at a height h above earth, having some PE

Now if the same body is in orbit around earth at same height, owing to its velocity its mass increases causing the PE at that height to be more than what it was when the body was stationary.

Where did the extra energy come from?
 

Answers and Replies

  • #2
Ibix
Science Advisor
Insights Author
6,825
5,673
Whatever you used to accelerate the mass.
 
  • #3
Whatever you used to accelerate the mass.
But the work done by that external agent would cause increase in kinetic energy.it is not doing any work against force of gravity(as they are perpendicular). So if its not doing work against gravity how is its work converted to gravitational PE?
 
  • #4
Nugatory
Mentor
12,949
5,639
Now if the same body is in orbit around earth at same height, owing to its velocity its mass increases causing the PE at that height to be more than what it was when the body was stationary.
Your premise is mistaken - it's mass does not increase because of its velocity, at least not the way you're thinking, so the question does not arise. Confusion of this sort is why the the concept of relativistic mass is seldom used these days: https://www.physicsforums.com/insights/what-is-relativistic-mass-and-why-it-is-not-used-much/

You can safely say that the total energy of the orbiting object is greater than the energy of the suspended object. The "extra" energy is kinetic energy, and as @Ibix says, it came from whatever you used to accelerate the object.
 
  • Like
Likes ComplexVar89 and Ibix
  • #5
Your premise is mistaken - it's mass does not increase because of its velocity, at least not the way you're thinking, so the question does not arise. Confusion of this sort is why the the concept of relativistic mass is seldom used these days: https://www.physicsforums.com/insights/what-is-relativistic-mass-and-why-it-is-not-used-much/

You can safely say that the total energy of the orbiting object is greater than the energy of the suspended object. The "extra" energy is kinetic energy, and as @Ibix says, it came from whatever you used to accelerate the object.
Thank you. I understand now
 
  • #6
Ibix
Science Advisor
Insights Author
6,825
5,673
What @Nugatory said. I should read more carefully. You can't handle gravity in a relativistic universe by just substituting relativistic mass into Newtonian gravitational formulae.
 
  • #7
1,443
112
Suppose a body is held stationary at a height h above earth, having some PE

Now if the same body is in orbit around earth at same height, owing to its velocity its mass increases causing the PE at that height to be more than what it was when the body was stationary.

Where did the extra energy come from?


Suppose two bodies at different temperatures are held stationary at a height h above earth, having some PE

Now if the hotter body heats the cooler body, owing to the cooler body's increased heat energy its mass increases causing the PE at that height to be more than what it was when the body was cooler.

Where did the extra energy come from?
 
  • #8
Nugatory
Mentor
12,949
5,639
Now if the hotter body heats the cooler body, owing to the cooler body's increased heat energy its mass increases causing the PE at that height to be more than what it was when the body was cooler.

Where did the extra energy come from?
The hotter body lost energy as it cooled.
 
  • #9
1,443
112
The hotter body lost energy as it cooled.
Okay but there is still one problem: The hotter body lost potential energy as it cooled. Where did that energy go?


-----------------

Okay I answer myself: It went to the body that gained potential energy.
 
  • #10
pervect
Staff Emeritus
Science Advisor
Insights Author
9,811
1,029
Suppose a body is held stationary at a height h above earth, having some PE

Now if the same body is in orbit around earth at same height, owing to its velocity its mass increases causing the PE at that height to be more than what it was when the body was stationary.

Where did the extra energy come from?
Special relativity by itself will not handle gravity. Getting gravity to work correctly along with relativity was why Einstein created General Relativity. This was not an easy, or short, task that can be fully explained in a post that's a few paragraphs long. It gets involved, but we can say a few things. The shortest reasonable answer is that, as I already said, we do need General relativity, not special relativity to answer your question, and that there is more to GR than plugging in some different values for mass into Newtonian formulae.

I could give you a brief rundown of some of the various sorts of masses that are defined in special and general relativity (I'm not sure I know all of the later but I could talk about the 'Big Three'). However, I'm not sure if that's interesting to you especially since it won't be sufficient to answer your question. None of these sorts of masses will allow you to plug them into Newton's formula, and get the answer you seek.
 
  • Like
Likes vanhees71
  • #11
1,895
332
The hotter body lost energy as it cooled.
It's not that easy. The hot body loses energy by emission of heat radiation and the cold body receives energy by absorption of this radiation. If the emitted and absorbed energy would be equal, the sum of the rest energies of both bodies would remain constant. But the potential energy of the system would change due to the changed mass of the bodies. The question remains, where the difference comes from or goes to and the answer is: The heat released from the hot body and the heat absorbed by the cold body are actually different due to red- or blueshift (depending on the mass ratio of the bodies). The difference comes from or goes to the potential energy of the system.
 
  • #12
1,443
112
It's not that easy. The hot body loses energy by emission of heat radiation and the cold body receives energy by absorption of this radiation. If the emitted and absorbed energy would be equal, the sum of the rest energies of both bodies would remain constant. But the potential energy of the system would change due to the changed mass of the bodies. The question remains, where the difference comes from or goes to and the answer is: The heat released from the hot body and the heat absorbed by the cold body are actually different due to red- or blueshift (depending on the mass ratio of the bodies). The difference comes from or goes to the potential energy of the system.


Let's say object number two is a sphere around object number one. This way we get rid of the distracting redshift.

And let's put those two objects as far from all masses as possible. This makes the potential energy of the objects as large as possible. We usually do this when we do not want to consider general relativity, right? So maybe we can forget the general relativity here too?
 
  • #13
1,895
332
Let's say object number two is a sphere around object number one. This way we get rid of the distracting redshift.
No, we don't. There is still a red- or blueshift in the gravitational field of object one.
 
  • #14
1,443
112
No, we don't. There is still a red- or blueshift in the gravitational field of object one.

Oh. Well then object one is two thin and light papers, one paper attached to the inside of the object #2, other one on the outside of the object #2. (Object #2 was a sphere, if anyone has forgotten)

Now I hope redshift is eliminated. I mean on the average redshift should be zero.
 
  • #15
Nugatory
Mentor
12,949
5,639
It's not that easy. The hot body loses energy by emission of heat radiation and the cold body receives energy by absorption of this radiation. If the emitted and absorbed energy would be equal, the sum of the rest energies of both bodies would remain constant. But the potential energy of the system would change due to the changed mass of the bodies. The question remains, where the difference comes from or goes to and the answer is: The heat released from the hot body and the heat absorbed by the cold body are actually different due to red- or blueshift (depending on the mass ratio of the bodies). The difference comes from or goes to the potential energy of the system.
That's not right. One way to see this is to analyze the problem with the zero point of the gravitational potential set at the height of the two-body system, instead of at infinity or the surface of the earth as we usually do. Now the potential energy of the system is zero and remains zero no matter what happens to the masses of the individual objects. Thus, conservation of energy requires that whatever is lost by one body is gained by the other; there's nowhere else for it to go.

You are right that if the two bodies are exchanging heat through thermal radiation there will be a (tiny) gravitational redshift/blueshift if their masses are different. But in this case clocks on the surface of each object are also running at different rates because gravitational time dilation is at work. The "uphill" object will receive low intensity red-shifted light for a longer time, so ends up receiving the same amount of energy as was emitted by the "downhill" object.
 
  • Like
Likes PeterDonis
  • #16
1,895
332
I mean on the average redshift should be zero.
That depends on the specific conditions. If yes, than the change of the potential energy should also be zero.

One way to see this is to analyze the problem with the zero point of the gravitational potential set at the height of the two-body system, instead of at infinity or the surface of the earth as we usually do.
What is "the height of the two-body system"?

Edit:

After reading your post again I have an idea what you mean. Are you talking about the potential energy of the two-body system in the gravitational field of a third body only? That is indeed constant, but that's not the full story. There is also a potential energy due to the gravitational field of the two bodies itself and this part of the total potential energy is changing.
 
Last edited:
  • #17
1,443
112
In this thread a poster has a problem with disappearing potential energy:
https://www.physicsforums.com/threads/potential-energy-and-annihilation.936094/

And here in this thread a poster has a problem with appearing potential energy.

So maybe the solution to the disappearing potential energy there is the appearing potential energy here, and vice versa.

(Seems obvious to me that it it so)
 
  • #18
1,895
332
So maybe the solution to the disappearing potential energy there is the appearing potential energy here, and vice versa.
Yes, that's what I mean. The potential energy of the system is increased and the energy of the radiation is decreased by redshift (or the other way around with blueshift). The total energy remains constant.
 
  • #19
1,443
112
Yes, that's what I mean. The potential energy of the system is increased and the energy of the radiation is decreased by redshift (or the other way around with blueshift). The total energy remains constant.
Do you mean this redshift:

Light climbs away from the annihilation point and redshifts, because light is climbing away from other light. The amount of redshift is maybe a few microjoules.

Or this:

Matter-antimatter fuel is lifted from the ground to a position high above the earth. The lifting causes the light of annihilation of the fuel to have more energy, maybe a few megajoules more.
 
  • #20
1,895
332
@jartsa,

Of course the first one. In the second case there is no redshift involved and there is no reason why the energy released during annihilation should depend on the gravitational potential.
 
  • #21
1,443
112
That's not right. One way to see this is to analyze the problem with the zero point of the gravitational potential set at the height of the two-body system, instead of at infinity or the surface of the earth as we usually do. Now the potential energy of the system is zero and remains zero no matter what happens to the masses of the individual objects. Thus, conservation of energy requires that whatever is lost by one body is gained by the other; there's nowhere else for it to go.

You are right that if the two bodies are exchanging heat through thermal radiation there will be a (tiny) gravitational redshift/blueshift if their masses are different. But in this case clocks on the surface of each object are also running at different rates because gravitational time dilation is at work. The "uphill" object will receive low intensity red-shifted light for a longer time, so ends up receiving the same amount of energy as was emitted by the "downhill" object.

No. Anything that climbs loses energy. When we ask where the lost energy goes, the answer is: "it goes to the system". I have seen that answer many times here.

If two things climb away from each other, both things lose energy. The system gains potential energy.
 
  • #22
281
52
No. Anything that climbs loses energy. When we ask where the lost energy goes, the answer is: "it goes to the system". I have seen that answer many times here.

If two things climb away from each other, both things lose energy. The system gains potential energy.
How can two objects both climb away from each other unless they are on opposite sides of the larger body? Also I'm pretty sure in Nugatory's example the height of the objects with respect to the designated origin is not changing.
 
  • #23
1,443
112
How can two objects both climb away from each other unless they are on opposite sides of the larger body? Also I'm pretty sure in Nugatory's example the height of the objects with respect to the designated origin is not changing.
When an object moves from lower potential to higher potential, it 'climbs', except when something else 'lifts' the object. To 'climb' is to 'self-lift'.

When space contains two objects, there is a potential difference between any two points of the space, except when the two points happen to be on the same equipotential surface.
 
  • #24
1,895
332
How can two objects both climb away from each other unless they are on opposite sides of the larger body?
Earth and Moon climb away from each other without a larger body between them. During this process the kinetic energies of both objects decrease and the common potential energy increases.
 

Related Threads on Increase in PE due to increased mass

Replies
5
Views
669
Replies
1
Views
2K
  • Last Post
Replies
5
Views
2K
  • Last Post
Replies
6
Views
5K
Replies
3
Views
4K
  • Last Post
Replies
5
Views
2K
  • Last Post
2
Replies
49
Views
34K
  • Last Post
2
Replies
35
Views
13K
  • Last Post
Replies
6
Views
3K
Top