Violation of newtons Third law

1. Aug 15, 2011

TheBlackNinja

2. Aug 15, 2011

D H

Staff Emeritus
It is violated, either that or F=ma is violated.

I beg to differ. What is true is that when that momentum is included (an advanced topic), the conservation laws are again obeyed.

One way to look at Newton's third law is that it is a special case of the more generic laws of conservation of momentum and angular momentum. The circumstances under which Newton's third law derives from the conservation laws are when forces are central in nature and depend on position only, and when the field that mediates the force does not itself store momentum or angular momentum. Various electromagnetic forces fail on all three accounts, so why would you expect Newton's third law to hold in such a case?

Last edited: Aug 15, 2011
3. Aug 15, 2011

TurtleMeister

If this is true then why don't we have reactionless drives?

4. Aug 15, 2011

D H

Staff Emeritus
Because momentum and angular momentum are conserved quantities.

5. Aug 15, 2011

TurtleMeister

Probably too advanced for me, so I'll just take your word for it. I've always thought of the third law and the conservation of momentum to be simply two different ways of viewing the same thing. Violate one then you violate the other.

6. Aug 15, 2011

atyy

Newton's third law is true for Newton's gravity - the sun attracts the earth and the earth attracts the sun at the same time - even though both are far from each other. How can each know how hard the other is pulling immediately? In Newtonian physics, there is no upper limit to the speed of things, so there's no problem there. But this is obviously doesn't fit in with special relativity - so Newton's third law doesn't hold in special relativity.

In special relativity, the local exchange of momentum is between fields and particles. A particle feels a force due to a field. But there isn't a simple way to say that a field feels a force due to a particle. So to keep the spirit of Newton's third law for fields and particles, we ascribe momentum to both, and say that momentum is conserved.

7. Aug 16, 2011

TurtleMeister

Oh, I see. Thanks for the simple explanation.