24,182 reputation
348103
bio website arcsecond.wordpress.com
location Baltimore, MD
age 29
visits member for 3 years, 5 months
seen 18 hours ago

I'm a physics graduate student.


Feb
22
comment Entropy and probability
Pressure is a macroscopic concept. If we are considering the motion of the individual molecules in this way, pressure is not relevant.
Feb
9
comment Why does time reversibility imply equilibrium in a thermodynamic system?
possible duplicate of Intuitively, why is a reversible process one in which the system is always at equilibrium?
Feb
2
comment Why am I not burned by a strong wind?
@Anixx As the answer states, I am referring to energy density, not velocity.
Feb
1
comment Why am I not burned by a strong wind?
I was in the process of writing an answer when this question was closed. I would like the question re-opened because I find it extremely unlikely that the OP will understand the linked question to be equivalent, and also because my answer takes a considerably different tack than it would when answering the linked question.
Feb
1
comment Is thermodynamic reversibility a function of path?
@joshphysics good link. I updated the answer.
Feb
1
comment Is thermodynamic reversibility a function of path?
@Christoph Good point. I updated the answer.
Jan
27
comment Why and how is the speed of light in vacuum constant, i.e., independent of reference frame?
@JerrySchirmer To be honest I was describing physics that was beyond me. I simply remembered reading this in Feynman's QED. Looking it up, on pp 89 it says "The major contribution occurs at the conventional speed of light... but there is also an amplitude for light to go faster (or slower) than the conventional speed of light. You found out that in the last lecture that light doesn't go only in straight lines; now, you find out that it doesn't go only at the speed of light!" Maybe I misunderstand just what this means, though. I don't know quantum field theory.
Jan
19
comment Noether Theorem and Energy conservation in classical mechanics
pppqqq Is correct. Your procedure begins by assuming that for a given trajectory, the value of the Lagrangian is constant. I.E., you throw a ball through the air and over the course of its flight kinetic-potential energy stays constant. This is wrong, so of course the result you get is wrong.
Jan
14
comment Solids: is energy expended in some fashion by, say, a table to keep the top from sinking down against gravity?
That is very difficult to calculate. All I am saying is that things decay. Old buildings fall down, old statues rust, etc. Presumably a table will decay a bit more quickly with gravity than without it. There is not necessarily a constant energy input required. It is just the case that some non-zero amount of energy is required over long times in order to counter the table's gradual decay.
Jan
13
comment Solids: is energy expended in some fashion by, say, a table to keep the top from sinking down against gravity?
The energy of the table changes whenever its state changes. If the table breaks and collapses all at once, then that's how it happens. If it slowly sags with age, then that's how it happens. I don't understand the second question.
Jan
13
comment is action integral Lorentz invariant?
I'm not sure what the "first postulate" is, sorry. I'm saying that there is some $\mathbf{x}(\tau)$ which is the trajectory. It's a four-vector position as a function of proper time and is invariant. If you find it in two different frames, you'll get two different sets of coordinates describing the same $\mathbf{x}(\tau)$.
Nov
30
comment Do heavier objects fall faster?
No. The claim that heavier objects fall faster is not a claim that they have greater acceleration at a given distance from the Earth when viewed in an inertial frame. If you read the answers to the question you already linked, this is made clear.
Nov
30
comment Do heavier objects fall faster?
What about it? It's a correct statement. What do you want to know? Be more specific.
Nov
6
comment Does measuring the operator of a wave function collapse the wave function to the measured eigenstate?
If the $\phi$ are normalized, then your $\psi$ is not normalized, making this question difficult to understand.
Oct
13
comment In truth, only atoms and the void
@aufkag Yes, my mistake. Thanks!
Sep
25
comment Minimum energy for turning over a cube
I disagree that it's a "strange question". It looks pretty straightforward.
Sep
13
comment Is a spinning object heavier than a non-spinning object?
"Mass" is just energy/c^2 when viewed in a frame where momentum is zero.
Sep
9
comment Equations of motion for bob-on-a-string — am I missing some terms?
That will depend on how the fluid leaves the system. If the would accelerates as it leaves, it will exert a force that depends on $\dot{m}$. Otherwise it won't.
Sep
9
comment Equations of motion for bob-on-a-string — am I missing some terms?
force is just mass*acceleration. newtonian mechanics doesn't have such a thing as $\dot{m}$, in terms of a magically-changing mass. If mass is entering or leaving a system, you would have to specify how that happens. If a cart is rolling along at a fixed speed down a track and you dump sand into it, the sand would need to gain some momentum, so you'd need to put force on the cart to keep its speed constant. the force would be $\dot{m}\dot{r}$. But if sand is leaking out the bottom of the cart, there is no force needed to keep its speed constant; the force is zero.
Sep
9
comment Equations of motion for bob-on-a-string — am I missing some terms?
The first sentence has nothing to do with physics. It is just the observation that if h were to appear somewhere, it could be replaced by r - s. Thus, there is no need to justify the fact that h doesn't appear.