# Conservation of Energy and a billiard ball

The conservation of energy states energy can never be created nor destroyed, and the amount of energy in a system is always constant.

I'm just curious, say if a billiard ball just suddenly started moving without any other object making it do so, (kinetic energy was created), if it also suddenly lost some other energy (other energy is destroyed), equivalent to that of the kinetic energy it gained, is energy conserved?

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(kinetic energy was created) [...] (other energy is destroyed)
Well, energy can't be created nor destroyed . Energy is transformed to different types of energy.
is energy conserved?
Why would it not?
I'm just curious, say if a billiard ball just suddenly started moving without any other object making it do so [...], equivalent to that of the kinetic energy it gained, is energy conserved?
Well, let's make a physical example of it. Let's say you pick up a billiard ball and hold it in your hand and then suddenly just release it. Due to gravity the ball will accelerate to the ground (and gain kinetic energy). Can you describe with physics what happens, and what types of energy that are involved in this?

CWatters
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I'm just curious, say if a billiard ball just suddenly started moving without any other object making it do so, (kinetic energy was created) if it also suddenly lost some other energy (other energy is destroyed), equivalent to that of the kinetic energy it gained, is energy conserved?
I think what you are asking amounts to....

If COE doesn't apply and kinetic energy can be "created" out of nothing then does COE apply?

berkeman and weirdoguy
CWatters
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Conservation of energy only applies to what we call a closed system. That is a system in which no energy can enter or leave through the system boundary. A system boundary isn't necessarily a physical wall it's a mental construct that you draw around your system to make sure you have thought of everything that should be within it.

So for example when people think they have somehow created or destroyed energy it invariably means energy has crossed the system boundary eg they have forgotten to include a source or destination for energy that should be within it.

In your question the ball suddenly gains kinetic energy from somewhere unknown and the same amount leaves the ball to an unknown destination. As long as the energy going to and from the unknown source and destination are acounted for (eg both are inside your system boundary) then conservation of energy can be applied to that system.

Edit: I should add that you didn't explain how the KE is transformed into the other type of energy that is lost. So there is something else missing from within your system boundary/mental model. However such energy transformations are possible, for example KE to heat via friction. Perhaps the ball has back spin :-)

A.T.
say if a billiard ball just suddenly started moving without any other object making it do so,
That might violate conservation of momentum, depending on what you exactly mean.

(kinetic energy was created), if it also suddenly lost some other energy (other energy is destroyed), equivalent to that of the kinetic energy it gained, is energy conserved?
Your premise is the answer here. But even if energy is conserved, it might still violate the 2nd Law of Thermodynamics.

sophiecentaur
Gold Member
say if a billiard ball just suddenly started moving without any other object making it do so,
That is not a valid scenario. If this event was 'observed' then the witness failed to include some source of energy and was confused or mis-reporting it.

russ_watters and DennisN
That is not a valid scenario. If this event was 'observed' then the witness failed to include some source of energy and was confused or mis-reporting it.
True. And I was trying to be nice and make it into a valid, discussable scenario in post #2.

sophiecentaur
sophiecentaur
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And I was trying to be nice
A worthy approach but there is always the risk that it appears as if you believe the evidence that's been presented. I think bad evidence should always be challenged early on. (Politely when possible. )

DennisN
I'd like to comment on the original scenario too...
if a billiard ball just suddenly started moving without any other object making it do so
@TheQuestionGuy14, if we just consider that scenario as you worded it, and you read this page (http://hyperphysics.phy-astr.gsu.edu/hbase/conser.html#cons), if a billiard ball just started moving by itself, how many and which physical laws are violated if that would happen?

Delta2
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The conservation of energy states energy can never be created nor destroyed, and the amount of energy in a system is always constant.

I'm just curious, say if a billiard ball just suddenly started moving without any other object making it do so, (kinetic energy was created), if it also suddenly lost some other energy (other energy is destroyed), equivalent to that of the kinetic energy it gained, is energy conserved?
What you saying was a conjecture first stated by Niels Bohr (if I am not mistaken), that in the microscopic world some processes (for example particle collisions) create energy (out of thin air), while some other processes destroy energy. In the macroscopic world the results of these many microscopic processes balance out and we observe conservation of energy on average. But this conjecture has been proven wrong if I remember correctly.

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Conservation of energy only applies to what we call a closed system. That is a system in which no energy can enter or leave through the system boundary. A system boundary isn't necessarily a physical wall it's a mental construct that you draw around your system to make sure you have thought of everything that should be within it.

So for example when people think they have somehow created or destroyed energy it invariably means energy has crossed the system boundary eg they have forgotten to include a source or destination for energy that should be within it.

In your question the ball suddenly gains kinetic energy from somewhere unknown and the same amount leaves the ball to an unknown destination. As long as the energy going to and from the unknown source and destination are acounted for (eg both are inside your system boundary) then conservation of energy can be applied to that system.

Edit: I should add that you didn't explain how the KE is transformed into the other type of energy that is lost. So there is something else missing from within your system boundary/mental model. However such energy transformations are possible, for example KE to heat via friction. Perhaps the ball has back spin :-)
What I sort of meant was, since the energy in a closed system is always constant, if energy was suddenly created, if an equal amount was destroyed (both happen randomly and don't effect one another), would energy be conserved? I assume no.

ZapperZ
Staff Emeritus
What I sort of meant was, since the energy in a closed system is always constant, if energy was suddenly created, if an equal amount was destroyed (both happen randomly and don't effect one another), would energy be conserved? I assume no.
If it is something that can't be detected, why does it matter?

Zz.

CWatters
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What I sort of meant was, since the energy in a closed system is always constant, if energy was suddenly created, if an equal amount was destroyed (both happen randomly and don't effect one another), would energy be conserved? I assume no.
Ok so if energy is suddenly created (eg the energy in a closed system suddenly increases at random) that implies we are talking about a situation where the laws of physics don't apply. So how can we tell you what the laws of physics say will happen in that situation?

Its like asking... Suppose 2+2 doesn't always equal 4, then does 3+3 always equal 6? If 2+2 doesn't equal 4 then that implies the laws of arithmetic have changed so how can we answer what 3+3 means?

Energy might be conserved in the situation you describe, I mean if we can change the laws so that energy can suddenly appear we can also create a place for some of the existing energy to hide temporarily until order is restored :-)

Ok so if energy is suddenly created (eg the energy in a closed system suddenly increases at random) that implies we are talking about a situation where the laws of physics don't apply. So how can we tell you what the laws of physics say will happen in that situation?

Its like asking... Suppose 2+2 doesn't always equal 4, then does 3+3 always equal 6? If 2+2 doesn't equal 4 then that implies the laws of arithmetic have changed so how can we answer what 3+3 means?

Energy might be conserved in the situation you describe, I mean if we can change the laws so that energy can suddenly appear we can also create a place for some of the existing energy to hide temporarily until order is restored :-)
Well that's sort of my point, if energy was spontaneously created (say kinetic), but an equal amount of (say thermal) was destroyed. The overall energy never changes, but energy was just created/destroyed. Would the laws of physics be violated?

jbriggs444
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Well that's sort of my point, if energy was spontaneously created (say kinetic), but an equal amount of (say thermal) was destroyed. The overall energy never changes, but energy was just created/destroyed. Would the laws of physics be violated?
Are you saying that it just happened to have been created and destroyed in equal amounts but did not have to be? It is merely the luck of the draw that we did not happen to observe a violation? Then no law of conservation of energy exists. Eventually we will observe a violation and the law will be experimentally disproven.

Or are you saying that energy is always created and destroyed in equal amounts by some unknown and unspecified mechanism and that it is more than merely the luck of the draw that we do not observe a violation. Then a conservation law exists. We will never observe a violation and may be able to fit the unknown mechanism into our theories.

Removing thermal energy to create kinetic energy is a violation of the second law of thermodynamics, so you do have a problem there.

russ_watters and sophiecentaur
sophiecentaur
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Removing thermal energy to create kinetic energy is a violation of the second law of thermodynamics, so you do have a problem there.
Removing but not transferring thermal energy from a hot source to a cold sink. That describes a heat engine which is OK. But some mechanism needs to be specified or the question has no meaning.

Well that's sort of my point, if energy was spontaneously created (say kinetic), but an equal amount of (say thermal) was destroyed. The overall energy never changes, but energy was just created/destroyed. Would the laws of physics be violated?
I’m not sure if you feel like your question was ever answered to your satisfaction. Unfortunately, I think the way that you framed the question is going to remain problematic for you. It’s simply a question of causation. In a physical system... particularly from a macroscopic perspective... things happen for a reason. Your hypothetical billiard ball will not (can not) spontaneously obtain kinetic energy in the absence of a “causative” source of that energy (kinetic or otherwise). I think it’s probably sufficient for you to understand that “IF” a billiard ball appeared to spontaneously exhibit kinetic energy (via motion, momentum, etc) it would be a very interesting investigational challenge to determine the source of that energy, and how it was transferred to the billiard ball. But rest assured, there WOULD actually be a source, and the total energy of the system would be conserved. If not, then yes, the “laws of physics” would be violated.
Things get much more complicated on a subatomic level, but that’s not a rabbit hole you need to dive into.

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russ_watters
sophiecentaur
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total energy of the system would be conserved. If not, then yes, the “laws of physics” would be violated.
The history of Science, so far, has continued to confirm to us that 'things' don't change, wherever or whenever we look in the Universe. The expression "Law of Physics" was used way back when pretty well all people believed that the Universe had been set up by a deity and that it ran according to unvarying divine Law.
Physics is a pretty atheistic pastime but the continued use of the word 'law' has the effect of causing people to protest too much about what it means. This seems to be caused by the almost religious belief that religions are wrong. Strangely, or perhaps to confirm my point, the same doubt / objection / discomfort doesn't seem to come up when the word 'Theory' is used when predicting or explaining things. Perhaps it's time for a (slow) change here.

The history of Science, so far, has continued to confirm to us that 'things' don't change, wherever or whenever we look in the Universe. The expression "Law of Physics" was used way back when pretty well all people believed that the Universe had been set up by a deity and that it ran according to unvarying divine Law.
Physics is a pretty atheistic pastime but the continued use of the word 'law' has the effect of causing people to protest too much about what it means. This seems to be caused by the almost religious belief that religions are wrong. Strangely, or perhaps to confirm my point, the same doubt / objection / discomfort doesn't seem to come up when the word 'Theory' is used when predicting or explaining things. Perhaps it's time for a (slow) change here.
Conceded. In this case, I was referencing the terminology in the OP’s question, but I simply meant that it would be a violation of established scientific principles.

sophiecentaur
Gold Member
Conceded. In this case, I was referencing the terminology in the OP’s question, but I simply meant that it would be a violation of established scientific principles.
Actually there is nothing to "concede". I was only suggesting that the choice of the word "Law" (not your fault; we all do it) could make people antagonistic and to look for a loophole.

The conservation of energy states energy can never be created nor destroyed, and the amount of energy in a system is always constant.

I'm just curious, say if a billiard ball just suddenly started moving without any other object making it do so, (kinetic energy was created), if it also suddenly lost some other energy (other energy is destroyed), equivalent to that of the kinetic energy it gained, is energy conserved?
You are phrasing two things as though they are unrelated, when they have to be related. So a ball starts moving and now has kinetic energy. And somewhere else unrelated, that amount of energy disappeared. That disconnect is not possible. If the ball started rolling, something caused that. There has to be a direct connection.

In your example, energy is conserved. An amount is lost, and an equal amount gained. But your example is not a realistic depiction of how energy is converted from one form to another in a cause-and-effect way.

I think that is confusing. The ball started rolling ... why? Sure energy gained and lost balances (satisfying conservation of energy), but there is more to a system than that.

Say the ball was hollow, and had an asymmetric weight distribution, that was exactly balanced by frozen ice stuck on one side of the hollow sphere. Upon the ice melting and falling, some rotation might happen. Or maybe that ice was holding back an internal mass and spring, and the ball rotated in response to that. There has to be a cause-and-effect between the energy used, and the movement of the ball.