Conservation of Momentum - true?

[El Hombre Invisible]

I still disagree, and I'm not just trying to be difficult here. I will illustrate.
Take 2 Earth masses. Separate them apart at a distance. You are on Earth 1, I am on Earth 2. We are both on the side facing each other. At t<0, they are fixed in the center of mass frame. At t=0, some Divine creature let go of both masses. Would you now notice a difference than when t<0? I would! I would feel an extra gravity emerging all of the sudden.
But take this a step further. What if you have a brother on the OTHER side of Earth 1. Not only will he suddenly lose weight all of the sudden, but even if the forgot about t=0 (maybe it was 50 years ago when both Earth's were let go), both of you can still figure how you are accelerating by surveying all over the planet the weight change of a fixed mass. You will clearly see that your planet is accelerating towards something. You will not be fooled, like droping a book, that you are at rest and not moving.
Zz.

Yes, you can complicate the system by adding more bodies to it and asking different questions about those different bodies and we will see things that will advise us as to what's really going on but nothing changes the fact that something appears to be at rest in its rest frame. However many different examples you choose, however many additional complications you include, this one truth will always prevail and will always be the general answer to all the variants of the author's question.

Again, LOOK at the question. It is a question about reference frames.

In fact, it's a very simple question with a very simple answer. (My answer was over-complicated due to the slight intoxicated state I was in when I wrote it.) The question is really: must Newton's laws (from which you can derive the law of momentum conservation) hold in non-intertial frames? The answer is simply: no, an inertial frame is defined as one in which Newton's laws hold true. There is no obligation for momentum to be conserved in non-intertial frames. There you have the author's question and the approprtiate answer. Your specific answers to special cases such as the OP's example and my counter-example are perfectly true, but they are not relevant to the actual question being asked which is about Newton's laws in non-intertial frames.

End of story. Refer to previous paragraph for any argument you have because it sums up all that is relevant in this dispute. Enjoyed the argument nonetheless.

 

[TiBaal89]

El Hombre you are absolutely correct that the simple answer to my first post is to point out the fact that it is a non-inertial frame of reference. However I do appreciate everyone's input!

There are, as we see, many arguments that off-shoot from this simple example. Interesting stuff, thanks.

 

[pmb_phy]

I disagree. The principle of conservation of momentum, Newton's First Law, applies to a body, not to a closed system of bodies:

Therein lies the mistake.

re "A body remains at rest or in uniform motion unless acted upon by a force." - That's true with no doubt. But you've changed beats here. An individual body's momentum will change as F = dp/dt. But the total momentum of a system is what is being conserved, not that of a single body. Please read

http://scienceworld.wolfram.com/physics/ConservationofMomentum.html

Pete