25 votes

Does the earth’s rotational angular velocity change?

The Earth is not a single rigid body, but consists of at least five separate regions which can move relative to one another. These are the crust (which is the region that we use to measure day length),...
  • 36.9k
21 votes
Accepted

Is there an explanation for action-reaction law?

Yes, the explanation is the conservation of momentum. In Newtonian mechanics the third law produces conservation of momentum in mechanical systems. Later on you will see cases (matter interacting with ...
  • 71.3k
12 votes

Does the earth’s rotational angular velocity change?

To add to @gandalf61's answer: You can also look up solar time. Due to the orbit around the Sun, the Earth has to rotate a bit more than 360° for the sun to get back to the same apparent position in ...
5 votes

Does the earth’s rotational angular velocity change?

There is no “gravitational” source of external torque acting on the earth Yes, there is. The tides are caused by the Moon's gravity. That energy has to come from somewhere. The drag caused by the ...
3 votes
Accepted

How to determine where a gluon should be emitted in a Feynman diagram?

The answer is that this is just one diagram. You need to draw diagrams that cover all posibilities, so there will be diagrams where each quark emits the gluon. There will also be diagrams where ...
  • 21.3k
3 votes

Force and momentum, cause and effect, transition and state

The symmetry of the interactions is the cause for the observed effect that the momentum change is zero. There is no cause and effect relationship here. The symmetry and the conservation law have a ...
  • 71.3k
2 votes

Conservation laws (testing my understanding)

Let's start with physical principles holding in classical mechanics, and then look at your problem. Physical principles and equations second principle of dynamics (dynamical equation for translation ...
  • 2,303
2 votes

Is Conservation of Linear Momentum subservient to conservation of Angular Momentum?

I want to build on John Rennie's answer, which invokes Nother's theorem, because it actually clarifies your idea beautifully. I will assume we are considering Euclidean space throughout the entire ...
2 votes

Energy and momentum conservation using Dirac delta function

The scattering matrix elements you calculate in QFT have to be integrated over various phase space measures to yield measurable results. In this process all delta functions are expended and you end up ...
2 votes
Accepted

Are elastic collisions possible ever in real life?

The collision is inelastic. Not because of non-idealities imposed by the real world, but because even on paper the collision is inelastic. If the problem is what I am imagining, after the collision ...
  • 7,696
2 votes

Force and momentum, cause and effect, transition and state

Can one in good faith say "forces oppose each other because of the fundamental axiom that momentum is conserved", instead of the other way around? Are the two sentences equivalent? That ...
  • 39.6k
2 votes
Accepted

Conservation of linear momentum in a rotating moon

"...but the moon does not have any linear momentum due to rotation" This is not correct. Why? At the moment a tractive force is applied through the wheels, and an equal and opposite force ...
  • 34.1k
2 votes

Law of conservation of momentum

The momentum gets transferred to the atmosphere that stops/burns the meteorite and eventually to the entire planet Earth. The main force involved is the air drag (type of friction).
  • 412
1 vote
Accepted

Law of conservation of momentum

What force does the earth's atmosphere produce so the final momentum becomes 0? The short answer is the final momentum of the meteorite is not zero because it still has momentum in the form of the ...
  • 58.5k
1 vote

Law of conservation of momentum

Momentum is conserved only when net external force acting on the system is zero. However when the meteorite(system, here) moves through the atmosphere it experiences drag forces (and gravity would ...
  • 372
1 vote

Conservation laws (testing my understanding)

$M$=The ramp $m$=mass moving on the ramp When $M$ is free to slide on the ground with no friction: Momentum: When you consider $m$ and $M$ together as one system, forces like frictional forces, ...
  • 372
1 vote

Force and momentum, cause and effect, transition and state

Can one in good faith say "forces oppose each other because of the fundamental axiom that momentum is conserved"1, instead of the other way around? Are the two sentences equivalent? ...
1 vote
Accepted

Relativistic invariants of a classical field in 4D fashion: why the relation between the components of the current density holds?

The definition of $dS^i$ (Landau §6) is that it is a four-vector equal in magnitude and normal to the hypersurface element; in other words, $dS^i$ is the projection of the hypersurface element, ...

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