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Why is momentum conserved? 
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#1
Jan1704, 08:46 PM

P: n/a

Why is momentum conserved?



#2
Jan1804, 11:50 AM

P: 166

Because of newton's third law.



#3
Jan1804, 12:09 PM

P: 184

thanks to Newton's 3rd law in a case of lever with different distances? http://www.geocities.com/physics_world/mech/lever.htm 


#4
Jan1804, 01:20 PM

P: 166

Why is momentum conserved?
I'm not exactly sure what you are asking. Are you talking about the conservation of angular momentum?



#5
Jan1804, 03:04 PM

P: 656

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. 


#6
Jan1804, 06:00 PM

P: n/a

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. 


#7
Jan1904, 05:49 PM

P: n/a

If nobody knows why how do you know it is true?



#8
Jan1904, 07:51 PM

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#9
Jan2004, 05:35 AM

P: n/a

Have you checked every case?



#10
Jan2004, 05:45 AM

Mentor
P: 7,315

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. 


#11
Jan2004, 05:50 AM

P: n/a

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 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.) 


#12
Jan2004, 01:14 PM

P: n/a

Whether you are smart enought to understand it or not is debatable but your comments imply that philosophy is superior to physics. 


#13
Jan2004, 01:45 PM

P: n/a

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? 


#14
Jan2004, 04:12 PM

P: n/a

You are so stupid it is pathetic. You physics people need to learn to think. 


#15
Jan2004, 04:36 PM

P: n/a

By the way  this is a moderated newsgroup. Insults are not tolerated here. 


#16
Jan2004, 04:36 PM

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.



#17
Jan2004, 05:11 PM

P: n/a

In fact, what you are dong fits perfectly the definition of speculation you gave above. 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. 


#18
Jan2004, 05:15 PM

P: n/a

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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