How come there is no conservation of force?

[polaris12]

title says it all

 

[russ_watters]

The title is not very well worded, but if you mean, 'why doesn't every force have an equal and opposite force associated with it?' then the answer is, they do.

 

[polaris12]

I'm sorry, but what I meant to ask was that I learned about how momentum and energy are always conserved, but my teacher never mentioned the same thing for force. Was that just an oversight or does it really not exist, and if so, why not?

 

[russ_watters]

I don't think I'd say it was an oversight, it was probably just explained without using the words you're trying to apply to it. This is Newton's third law and it was demonstrated to me in college in almost exactly this way (I think it was skateboards instead of skates):
http://en.wikipedia.org/wiki/Newton's_laws_of_motion#Newton.27s_third_law:_law_of_reciprocal_actions

The word "conserved", though, doesn't really apply.

 

[Phrak]

I hadn't really thought about it. Excuse me if the mathematics is too advanced for you. It's a very thoughtful question.

Where conservation of momentum over a region might be expressed as

$$\sum \frac{d\textbf{p}}{dt} = 0 \mbox{~or~} \sum \textbf{F} = 0$$

then the conservation of force is already implicit in the conservation of momentum.

$$\sum \frac{d^2\textbf{p} }{dt^2} = 0$$

We could go on and on with this and say that impulse, and so forth, are conserved.

 

[russ_watters]

I hadn't really thought about it. Excuse me if the mathematics is too advanced for you. It's a very thoughtful question.

Where conservation of momentum over a region might be expressed as

$$\sum \frac{d\textbf{p}}{dt} = 0 \mbox{~or~} \sum \textbf{F} = 0$$

then the conservation of force is already implicit in the conservation of momentum.

$$\sum \frac{d^2\textbf{p} }{dt^2} = 0$$

We could go on and on with this and say that impulse, and so forth, are conserved.

I don't really like that - I don't think it addresses the OP's question and I'm not really sure it is correct anyway. The definition of "conserve" is "to hold (a property) constant during an interaction or process". What I tried to convey is that this word doesn't really apply to force, which the OP already instinctively knows, which is why the question was asked.

Consider a common conservation of momentum problem: with a pair of billiards balls. The "interaction or process" is a collision. The total momentum before, during and after the collision is always the same. But the force starts off at zero, increases, then decreases back to zero. So too with impulse.

 

[Phrak]

In common usage a conservation law is something invariant over time.

Consider your collision of two billiard balls. During every collision dp/dt of one billiard ball is equal and opposite to the dp/dt of the other. That is to say, the sum of the force vectors is conserved.

 

[Mu naught]

I think the idea of "a force" is really where the confusion lies. Force I think is really an unnecessary, but convenient concept we use to make abstractions about the world. If we think in more fundamental terms like work, energy and potential then I think we eliminate all these confusing ideas about "forces".

 

[Andy Resnick]

I'm with Phrak- this is a very thoughtful question. Here's how I would answer:

Unlike energy and momentum which are properties inherent to the object (or system), a force represents an interaction of the system with something else- gravity, electromagnetism, friction, etc., all represent two objects interacting with each other.

Energy and momentum (as well as mass, charge, and some other properties) obey 'balance laws', which are very general expressions relating the total change (of a quantity) to the internal changes plus any amounts flowing into or out of the system:

http://en.wikipedia.org/wiki/Continuum_mechanics#Balance_Laws

'Force' does obey a balance law because it is not a property of a single system- it is an interaction.

For the case where part of the system interacts with another part (for example, considering the earth-moon system as a single object), then it makes sense to talk of equilibrium, and forces summing to zero, etc. etc just like conservation of energy. But in general, this cannot be done.

 

[Phrak]

Andy, with your last paragraph, are you saying that force is not always conserved? I can't tell.

I sympathize with Russ. This is a very alien idea, and never talked about—therefore suspect. The mathematics supports it, I'm sure, but I'm still trying to think of ways to break it. Did you find a way?

 

[Andy Resnick]

There are 'conservative' forces, but not all forces are conservative forces- friction, for example. Conservative forces are derivable from a potential.

Newton's third law F12 = -F21 should be considered a definition of *contact* forces, not as a general property of 'force'- confusing the two is what leads to the example of pulling a cart and finding that no matter how hard you pull, the cart can't move.

 

[Studiot]

I don't think the concept of conservation is applicable to forces.

Conservation of a property implies that there is a flow of that property, perhaps a fixed fund that can be 'used up' or 'transferred' to another body or location.
Forces are not susceptible to such limits.

I can connect a load to a battery and draw a current only a limited number of times.
If I bring a test mass near another body it experiences a force of attraction; I can remove the test mas and reintroduce it as often as I want, there will always be the force.

 

[D H]

There are 'conservative' forces, but not all forces are conservative forces- friction, for example. Conservative forces are derivable from a potential.

Correct. This has nothing to do with Newton's third law, however.

Newton's third law F12 = -F21 should be considered a definition of *contact* forces, not as a general property of 'force'- confusing the two is what leads to the example of pulling a cart and finding that no matter how hard you pull, the cart can't move.

The Earth and Moon are not in contact with one another, yet the gravitational force exerted by the Earth on the Moon is equal but opposite to the gravitational force exerted by the Moon on the Earth.

Using Newton's third law to draw the erroneous conclusion that no matter how hard you pull the cart, the cart can't move, is a common misinterpretation of Newton's third law. Newton's third law applies to two different bodies. The force exerted by object a on object b is equal but opposite to the force exerted by object b on object a. The forces exerted by object b on other objects has nothing to do with the motion of object b.

The weak form of Newton's third law follows from conservation of linear momentum. Add in conservation of angular momentum and you get the strong form of Newton's third law. The derivations implicitly assume that no momentum is stored in the fields that generate these forces. This is not always true; Newton's third law does not hold in such domains.

Ignoring this, if you want to look at Newton's third law as meaning force is conserved, fine. That is a rather non-standard view of the meaning of the word 'conserved'. I'm going to side with Russ, "The word 'conserved', though, doesn't really apply."

For a deeper explanation of why conservation of force doesn't really apply here, one must look to Noether's theorems. (Wikipedia reference because I don't have time to find a better one.)

 

[physicsworks]

Actually Newton's third law is not always true, so the "force conservation" due to the third law is something odd to me, the same goes for the concept "force conservation" itself.

UPDATE

The weak form of Newton's third law follows from conservation of linear momentum. Add in conservation of angular momentum and you get the strong form of Newton's third law. The derivations implicitly assume that no momentum is stored in the fields that generate these forces. This is not always true; Newton's third law does not hold in such domains.

Sorry, I've missed this point.

 

[Andy Resnick]

<snip> I'm going to side with Russ, "The word 'conserved', though, doesn't really apply."

<snip>

I was unaware that there is a debate, especially since everyone is in agreement...?

 

[Phrak]

Well, I'm not in agreement with some posters. Force really is conserved. It's just not very interesting to say that in homogeneous space

$$\frac{d}{dt}\sum 0 = 0 \ .$$

This doesn't make it false or an unreal symmetry. In fact, it should be a more general statement where translational symmetry is not an rule and momentum is not conserved within an interactive system.

 

[Andy Resnick]

My mistake.

Force is not a conserved quantity, for the reasons enumerated above.

 

[Hologram0110]

Force is really more of an instantaneous thing. At any given moment if there is always a force in equal and opposite directions but the size force can change.

To me, conserve implies is constant over time, but as people have said, forces can and do change with time so this is not really the case.

 

[Phrak]

My mistake.

Force is not a conserved quantity, for the reasons enumerated above.

How do you justify this wand waving without showing that the mathematical proof I presented above to be false or not appicable?

 

[polaris12]

I just want to mention I lost track of what you guys were talking about a while ago. Yea, I don't get what you guys are debating anymore.

 

[D H]

How do you justify this wand waving without showing that the mathematical proof I presented above to be false or not appicable?

The only hand waving was in that post. You assumed Newton's third law to prove Newton's third law. Newton's third law does not always hold. The Biot-Savart law, for example, violates Newton's third law. That shouldn't be all that surprising; Maxwell's equations are inconsistent with Newtonian mechanics. Aside: The original intent of the Michelson Morley experiment was to disprove the claims of those upstart E&M physicists. As we all know, that was not the outcome of that experiment.

 

[gabbagabbahey]

Phrak, what if the system contains momentum-carrying fields? What happens to $\sum_{i} \textbf{p}_i$ when momentum is transferred from a field to a particle?

 

[Andy Resnick]

How do you justify this wand waving without showing that the mathematical proof I presented above to be false or not appicable?

Work it out yourself- find a function f = dp/dt that satisfies both conservation of momentum

$$\frac{Dp}{Dt} = \nabla \cdot T + F_{ext}$$

and conservation of force

$$\frac {Df}{Dt} = \nabla \cdot Q + G_{ext}$$

Where T is the stress tensor, F the external force, Q and G the analogs to T and F.

I think you will find you cannot, unless f = 0.

 

[russ_watters]

In common usage a conservation law is something invariant over time.

Consider your collision of two billiard balls. During every collision dp/dt of one billiard ball is equal and opposite to the dp/dt of the other. That is to say, the sum of the force vectors is conserved.

The sum of the force vectors is equal and opposte, but not conserved. Equal and opposite doesn't include the concept of invariant over time - as I pointed out, the forces do vary over time.

Heck, the equations you are using to argue it don't include a time range! Where is time in this equation?:

$$\sum \textbf{F} = 0$$

Well, I'm not in agreement with some posters. Force really is conserved. It's just not very interesting to say that in homogeneous space

$$\frac{d}{dt}\sum 0 = 0 \ .$$

Again, the concept of "over time" doesn't appear in that equation and without it, the word "conserved" doesn't apply. What you are really saying is just that that equation is always true over a period of time - as opposed to 'this time-dependent quantity is always constant'.

The way you are trying to use the word, every algebraic equation could have the word applied to it. Does 1+1=2 mean 2 is conserved? Does a=f/m mean acceleration is conserved? Similarly, does E=.5mv^2 mean kinetic energy is conserved?

 

[mikeph]

The sum of the force vectors is equal and opposte, but not conserved. Equal and opposite doesn't include the concept of invariant over time - as I pointed out, the forces do vary over time.

The forces vary but their sum does not. Applying Newton's 3rd law at every instant should give sum(Force vectors) = 0 at every time. Isn't this a stronger statement than normal conservation?

 

[Studiot]

If I pump up a bicycle tyre I am adding to its internal energy.
This energy is still there when I stop pumping and can be recovered at some later time.

That is what is meant by conservation of energy.

If I whirl a stone around my head on a string, there are forces acting and there is euqilibrium in the balance of tension and inertial force.
If I cut the string the forces cease and cannot be recovered.

There is no conservation of force.

By the way what about D'Alembert's principle and moments?
Forces have both direct and indirect effects.
Would you also conserve these?
There is no coresponding pair for energy.

 

[mikeph]

If I whirl a stone around my head on a string, there are forces acting and there is euqilibrium in the balance of tension and inertial force.
If I cut the string the forces cease and cannot be recovered.

There is no conservation of force.

Adding all the forces together you arrive at 0 N before and after you cut the string,
d/dt of the sum of forces = 0 so the net force is conserved.

This is what they are saying.

 

[D H]

But what they are saying is wrong. Newton's third law is not a universal law. It can fail in electromagnetism.

If force is conserved, what is the symmetry of nature that is responsible for this?

 

[mikeph]

Noether's theorem seems to suggest that a symmetry leads to a conservation. You are suggesting the converse: that a conservation requires a symmetry. To me that isn't covered by the theorem as it appears on Wikipedia. (maybe I need to get a better source!?)

 

[Phrak]

But what they are saying is wrong. Newton's third law is not a universal law. It can fail in electromagnetism.

If force is conserved, what is the symmetry of nature that is responsible for this?

Now, this-all is what I've been talking about. Why can't we have a civil discussion of the nature of physical law without it being initiated by the smell of blood? (Yours truely, in this case.)

The context in this thread is Newton's laws. I don't see any reason to stay within this context, and I find it actually more interesting than the context, though I wish you would have brought this up from the beginning rather than landing it later.

I wasn't aware that electromagnetism violated Newton. And I'd still like to see how. But where in this forum is the home for this sort of discussion? Can we have the admins make us one?

 

[Dadface]

How does Newtons third law fail in electromagnetism?When we put Biot and Savarts law to the test, for example with two parallel current carrying conductors we find that the force on conductor A is equal and opposite to that on conductor B.Are there any conductor geometries where this is not the case?

 

[mikeph]

How does Newtons third law fail in electromagnetism?When we put Biot and Savarts law to the test, for example with two parallel current carrying conductors we find that the force on conductor A is equal and opposite to that on conductor B.Are there any conductor geometries where this is not the case?

If an electron radiates, it has a force on it, but where is the opposing force? Either (1) you attribute this to the radiation, and I don't see how, or (2) you assume the radiation ultimately interacts with a second electron, but the radiation has a finite travel time so you no longer have that net force is instantaneously conserved.

With the Biot-Savart law, what if the two conductors are moved apart?

 

[Studiot]

Conservation means the same total before and after.

There are forces acting before, there are none after I cut the string.
These forces have suddenly been removed from the universe.

There is no balance of force for a body falling freely under gravity.

$$\sum {F \ne } 0$$

Nobody need violate Newton or dream up complicated examples.

 

[mikeph]

There is no balance of force for a body falling freely under gravity.

$$\sum {F \ne } 0$$

Nobody need violate Newton or dream up complicated examples.

That's only half the system- there must be at least two bodies interacting with the gravitational field or nothing will "fall".

 

[Studiot]

That's only half the system- there must be at least two bodies interacting with the gravitational field or nothing will "fall".

Where in Newtons first or second law does it say that there have to be two bodies?