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Theoretical physicist by training, PhD in spin-polarized quantum systems. Inventor of the DPD micro-hydrodynamics simulation methodology, and specialized in computational physics. Currently chief scientist for a large multinational company. Blogger at Science2.0.


1d
comment One problem on Classical Mechanics
@garyp - if you choose your system to be the rocket plus remaining fuel you have to include not only the mass loss, but also the associated momentum loss, $dp/dt = F_{net}$ remains valid.
Apr
20
reviewed Reject suggested edit on Why does a bicycle come to rest after travelling a distance?
Apr
19
comment One problem on Classical Mechanics
@garyp - in the scenario you describe $dp/dt = F_{net} = 0$ is correct. The total mass and the centre of mass momentum are all constant.
Apr
19
answered Why are we blind for the era before the recombination?
Apr
19
reviewed Approve suggested edit on Gauss' law question
Apr
13
reviewed Approve suggested edit on Huge confusion with Fermions and Bosons and how they relate to total spin of atom
Apr
13
reviewed Approve suggested edit on Deduce magnetic field based on electric field
Apr
12
revised One problem on Classical Mechanics
deleted 10 characters in body
Apr
12
comment One problem on Classical Mechanics
@garyp - thanks. Edited the text putting $dp/dt=F$ in words and phrased it more carefully. However, I am puzzled by your last remark. You seem to be saying $F=ma$ is more fundamental than $F=dp/dt$?
Apr
12
revised One problem on Classical Mechanics
added 44 characters in body
Apr
12
comment Why Don't Birds Stall?
Found this link: diydrones.com/profiles/blogs/tiltwing-stall-landing
Apr
12
revised One problem on Classical Mechanics
added 245 characters in body
Apr
12
comment Why Don't Birds Stall?
It certainly appears to me that birds purposely stall when landing.
Apr
12
comment One problem on Classical Mechanics
@Lawerance - yes, the fundamental law in classical dynamics is the statement that the change in momentum equals the net force. In case of a constant mass object this can be transformed in a statement about acceleration.
Apr
12
answered One problem on Classical Mechanics
Apr
12
comment What if photons are not the fastest particles?
@Jim - yes, I am cheating but you are cheating as well (hence my remark about local behaviors). If tomorrow I find on my desk a triangle with one side 80% larger than the sum of the other two sides, I can not simply conclude that the universe is closed, as I observe the universe to stretch far beyond my desk.
Apr
12
comment What if photons are not the fastest particles?
@Jim - that's the point: you would need different paths. A particle traveling from A to B faster than light would indicate some form of wormhole-type of shortcut. (I use the term 'closed' rather loosely.) Of course it would be better in both cases to limit the discussion to local behaviors (a particle overtaking a photon and a small triangle having one side exceeding the sum of the other two sides.
Apr
12
comment What if photons are not the fastest particles?
@Jim - right. And the same answer applies to OP's question: if tomorrow a particle is found that travels from A to B faster than light, we know we're in a closed universe.
Apr
12
comment What if photons are not the fastest particles?
Whenever a question like this is asked, I feel compelled to ask the counter question what if tomorrow we observe a triangle with one side exceeding the sum of the lengths of the other two sides?
Apr
1
comment Could CP violation be exploited to violate the second law?
For irreversibility to emerge in our fundamental laws, you would at least need CPT violation.