# Why is momentum conserved?

by tenzin
Tags: conserved, momentum
 P: n/a Why is momentum conserved?
 P: 166 Because of newton's third law.
P: 184
 Originally posted by Moose352 Because of newton's third law.
And how do you think the momentum is conserved
thanks to Newton's 3rd law in a case of lever
with different distances?

http://www.geocities.com/physics_world/mech/lever.htm

 P: 166 Why is momentum conserved? I'm not exactly sure what you are asking. Are you talking about the conservation of angular momentum?
P: 667
 Originally posted by tenzin Why is momentum conserved?
This really isn't an answer but you might be interested anyway.

http://www.emmynoether.com/noeth.htm

This link will take you to a page that discusses the connection between symmetries of physical laws and conservation laws.

Of course, you have probably already found information comparable to this in your own search.

The problem is when you answer the question, "why is momentum conserved?" with the answer, "because of the translational symmetry of space.", you have to ask the question, "why does space exhibit translational symmetry?" and on and on...

I don't think anyone really knows why this is. It is just something that has been repeatedly observed throughout the history of science.

 Moose352: Because of newton's third law.
I thought that was a good attempt at an answer. Of course if you asked, why do forces result in opposite and equal forces, someone might say, because of the conservation of momentum. It's all related.
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 Originally posted by tenzin Why is momentum conserved?
Nobody knows. It's a law of physics just like the conservation of energy.

re - Because of newton's third law. - In systems for which Newton's third law holds then the Newton's Third Law can be used to show that mechanical momentum is conserved. However it can't be used to prove that the momentum of an electromagnetic field is conserved. That is a postulate - i.e. the expression for the momentum of radiation can be found my demanding that total momentum be consever.

This boils down to saying that this is a law of physics and cannot be proven.
 P: n/a If nobody knows why how do you know it is true?
P: n/a
 Originally posted by tenzin If nobody knows why how do you know it is true?
Experimental observation
 P: n/a Have you checked every case?
 Mentor P: 7,319 Nor have we checked that every massive body exerts a gravitational force. The nice thing about the universe is that it is pretty consistent. Once you learn the proper parameters to watch it is very predictable. Physics does not address why phenomena occur that is the job of philosophers.
P: n/a
 Originally posted by tenzin Have you checked every case?
Actually it was you who used the term "true" not I. I should have clarified but I neglected to.

Something is said to be a Law of Nature when a relationship between physical quantities is assumed to hold in all possible cases. The dictionary definition is as follows
 Law of Nature - A statement of an order or relation of phenomena that so far as is known is invariable under the given conditions
That is the reason I stated that Nobody knows.

Given this assumption of nature one then performs experiments to test that assumoption. If after numerous variations of experiments no counter example can be found one gains confidence in that law. However that never means that the assumption/law is "true" since it is impossible for all possible cases to be tested. This holds for all the laws of nature.

That is why they are called "Law of Nature" rather than "truths of Nature."

If you like then just forget the notion "Total momentum is conserved" and rephrase it as you like according to the above reason. Call it the postulate of momentum conservation or whatever.

However that doesn't mean that there is some uncertainty in the minds of physicits about thins. We're as confident about the certain laws of nature as we are about whether the sun will rise tommorow. neglecting the fact that the Earth might be destroyed in the mean time etc.)
P: n/a
 However that doesn't mean that there is some uncertainty in the minds of physicits about thins. We're as confident about the certain laws of nature as we are about whether the sun will rise tommorow. neglecting the fact that the Earth might be destroyed in the mean time etc.)
Where I come from I expect more than speculation form scholars.

Whether you are smart enought to understand it or not is debatable but your comments imply that philosophy is superior to physics.
P: n/a
 Originally posted by tenzin Where I come from I expect more than speculation form scholars.
Who said anything about speculation? I certainly didn't. Speculation literally means -to review something idly or casually and often inconclusively, to take to be true on the basis of insufficient evidence. You're confusing speculation with a law of nature. They most certainly are not the same thing.

Suppose I go into the kitchen and fill a pan with water and place the pan on the stove. I then turn the gas on and light it so that there is a flame under the pan. It is not speculation to assert that the pan will get hot or to assert that the water will boil. Even a child knows that to be a fact of life. How can it be proved that this will always happen? It can't be proved. But that doesn't mean that we don't have any idea of what will happen.

Do you know what a law of physics is or when/how they come came to be?
That's not a meaningful statement. You're trying to compare apples an oranges. Philosphy and physics are different things but are related to each other. In fact physics used to be refered to as natural philosophy.
P: n/a
 However that doesn't mean that there is some uncertainty in the minds of physicits about thins. We're as confident about the certain laws of nature as we are about whether the sun will rise tommorow. neglecting the fact that the Earth might be destroyed in the mean time etc.)
I thought blind faith was something scientists criticized religion for.

You are so stupid it is pathetic. You physics people need to learn to think.
P: n/a
 Originally posted by tenzin I thought blind faith was something scientists criticized religion for. You are so stupid it is pathetic. You physics people need to learn to think.
Nobody is going by blind faith. You simply don't understand the scientific method. I've tried to explain it to you but you seem more interested in insulting me that learning

By the way - this is a moderated newsgroup. Insults are not tolerated here.
 Emeritus PF Gold P: 8,147 Physicists didn't make up a pretty story about momentum conservation. They observed it to happen over and over for hundreds of years. And every attempt to say, "Hey, I've got a situation where momentum isn't conserved" turned out to be false. Science has nothing it can prove as math does. Everything is always on the table. Not everybody agrees with Karl Popper, but he said science is the collection of propositions that can be falsified. You think you can do better, good luck.
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 Originally posted by Arcon Who said anything about speculation? I certainly didn't. Speculation literally means -to review something idly or casually and often inconclusively, to take to be true on the basis of insufficient evidence. You're confusing speculation with a law of nature. They most certainly are not the same thing.
You may not have said anything about speculation but the fact that you don't see that you are speculating shows how little you know. Since you can not establish pervasively that momentum will be converved in all cases you are speculating. Speculation is the opposite of certainty. It is making a statement without the support of a valid reason.

In fact, what you are dong fits perfectly the definition of speculation you gave above.
 Suppose I go into the kitchen and fill a pan with water and place the pan on the stove. I then turn the gas on and light it so that there is a flame under the pan. It is not speculation to assert that the pan will get hot or to assert that the water will boil. Even a child knows that to be a fact of life. How can it be proved that this will always happen? It can't be proved. But that doesn't mean that we don't have any idea of what will happen.
This can be established if you understand logic, reason and relations.

 That's not a meaningful statement. You're trying to compare apples an oranges. Philosphy and physics are different things but are related to each other. In fact physics used to be refered to as natural philosophy.
How can you compare forces to momentum? They are different things but related? See how stupid your logic is. Since you need it explained to you here is what I meant.

Both philosohy and physica are concerned with what exists. Since philosophy understands what is necessary for something to exist it is superior. You physicists don't even know what criteria something must have to exist. You think that everything that exists must be detectable with one of the 5 senses. You have no idea how limited your knowledge and methods are.
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 Originally posted by Arcon Nobody is going by blind faith. You simply don't understand the scientific method. I've tried to explain it to you but you seem more interested in insulting me that learning By the way - this is a moderated newsgroup. Insults are not tolerated here.
I didn't realize stating the truth would be considered an insult. You are insulting me with your ridiculous comments. You are going on blind faith because there are two kinds of faith. That which is based on valid reasoning and that which is not based onvalid reasoning. You people do not have any understanding of valid and invalid reasons. Just because you observe it does not necessitate that the observation is valid. In fact, the senses are known to be deceptive.

The downfall of physics began when sense based observation became more important than logical consistency. If it exists it can be established by a valid reason, if it doesn't exist it can not be established by a valid reason.

Take your time replying here. I am sure this is all new for you and you are having a hard time understanding it. That is ok though, you are a physicist.

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