25,926 reputation
160133
bio website lightandmatter.com
location Fullerton, California
age
visits member for 3 years, 4 months
seen 19 hours ago

I teach physics at Fullerton College, a community college in Southern California. I have an undergrad degree in math and physics from Berkeley and a PhD in physics from Yale. Back when I was doing research, my field was experimental low-energy nuclear physics.


Oct
20
revised Underdetermined forces in a statics problem
added 701 characters in body
Oct
20
comment Underdetermined forces in a statics problem
@dmckee: Even a small deviation will break the symmetry and make the system determined again. Hmm...I sketched an analysis, and it didn't look that way to me. In the asymmetric case, it seems to me that you have four unknowns ($N_1$, $N_2$, $F_1$, and $F_2$), and three equations: $F_x=0$, $F_y=0$, and $\tau=0$.
Oct
20
awarded  Nice Question
Oct
19
comment Underdetermined forces in a statics problem
@BMS: Yes, but in that example it's clear why it's underdetermined, and how the system knows what solution to pick: it picks the solution that's imposed by the externally determined force on the rope. Here there doesn't seem to be any such external force presently being exerted on the system that could determine which solution to pick.
Oct
19
comment Underdetermined forces in a statics problem
@ja72: I don't think your analysis is right. The problem is underdetermined for any value of $\mu>0$.
Oct
19
revised Increasing matter density in the Friedman equations?
added 351 characters in body
Oct
19
answered Increasing matter density in the Friedman equations?
Oct
19
asked Underdetermined forces in a statics problem
Oct
18
comment Doesn't the second postulate of special relativity imply the existence of “luminiferous ether”?
Huh? The speed of propagation of sound is independent of the speed of the emitter.
Oct
18
revised Infinite space or finite space?
added 27 characters in body
Oct
18
comment How Earth protected from the gamma rays generated by Sun?
Note that if none of the high-energy radiation generated at the sun's core got reabsorbed and had its energy converted into heat, then the sun wouldn't be hot. (In fact, the neutrinos do escape, and don't contribute to heating.)
Oct
18
comment How Earth protected from the gamma rays generated by Sun?
That's an error in the NASA page. Magnetic fields have no effect on gamma rays, only on charged particles.
Oct
18
comment How does EM radition depend on the reference frame?
The material about the Unruh effect isn't relevant. This is a classical question with a classical answer.
Oct
18
comment Doesn't the second postulate of special relativity imply the existence of “luminiferous ether”?
I don't understand the question. What logical link do you think exists? If A has properties P and Q, and B also has property P, that doesn't imply that B has property Q.
Oct
18
comment Why does “closing speed” $>c$ not violate relativity's second postulate?
@Geremia: I still don't understand the question. You haven't explained why you think there is a contradiction.
Oct
18
comment Why does “closing speed” $>c$ not violate relativity's second postulate?
Note that even if the closing speed between material objects A and B is greater than $c$, the velocity of B in A's frame of reference is still less than $c$. This is because velocities don't add linearly in relativity.
Oct
18
comment Why does “closing speed” $>c$ not violate relativity's second postulate?
In the question you have "relativity's postulate of c being the highest velocity for light in any frame." Einstein's second 1905 postulate doesn't say that. It states that the speed of light equals $c$. In the title, you have "relativity's postulate of c being the fastest velocity in any frame." The postulate doesn't say that either. The fact that you can't accelerate, say, an electron past $c$ isn't a postulate in relativity, it's a fact derived from the postulates. Could you clean this up so that the question will be clearer and more useful to others?
Oct
18
comment Why does “closing speed” $>c$ not violate relativity's second postulate?
Thus, it is not directly constrained by the 2nd postulate. The second postulate in Einstein's 1905 treatment doesn't say anything about constraining the speeds of objects.
Oct
17
comment Why is Energy = Voltage x Charge, and how to prove that?
The question isn't about capacitors, so we don't necessarily have any charged plates. Manipulating equations like this doesn't really constitute a proof of anything unless you decide what you consider to be your fundamental assumptions.
Oct
17
revised How does the Pauli exclusion principle create a force in degenerate matter?
reconcile title with pared-down question