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location New England (USA)
age 52
visits member for 1 year, 5 months
seen 1 hour ago

I have formal training in physics, but tend to approach it very intuitively; I prefer estimation over exact calculation, principles over details. I believe many problems can be solved by making a careful sketch. Label your axes, and don't use insignificant digits.

I visit this site in the hope of sharing my enthusiasm and occasional insights with others.


2h
comment Force of a Train
When you say "exactly equal but twice as long" does it have the same mass, or same mass per unit length. Both could be considered "equal" but they are different trains... Then there's the question of "what do you mean 'hit with a certain force'?" There is instantaneous force, impulse, ... Then there are flippant answers: once you have been hit by the first train, the second one won't hurt at all, ... This could become a good question but you have to work a little harder to make it so. Please look at the links in the grey box above.
3h
comment Convert angular position to wavelength
It depends on what the angle measures, what kind of spectroscopy you are doing. Need more information.
3h
comment Space formed by dot products of three vectors
I'm sorry, I had misread the point of the question - you are asking about limits on the third dot product as a function of the other two dot products...
3h
comment Space formed by dot products of three vectors
When $u_1 = u_2 = 1$, there aren't really three vectors... there is only one since they are all aligned. That does make it a special case.
3h
comment Space formed by dot products of three vectors
I assume there is a typo and you mean $u_2 = v_2 \cdot V$. And why not use $v_3$ as your third vector? The three dot products you are talking about are the cosines of the angles between the pairs of vectors; I see no geometrical reason why these need to be at all constrained (draw two vectors, you can draw a third vector that has an arbitrary angle to each of the first two). Am I missing some subtle point?
3h
comment Time travel, could it destroy the Universe?
We are all traveling in time. Luckily we are traveling forward, and apparently at the same speed. Well - that depends on who's looking. So it would seem that the mass of the universe is changing all the time with no ill effect. I think you don't have to worry about accidentally blowing up the universe.
8h
answered Dipole approximation in solids
13h
comment Does Newton's third law apply to momentum or to forces?
@bobie Impulse has direction - one force is in one direction and the other force in the other direction. Multiplying by time doesn't change the vector nature, and the sum of the vectors is zero.
13h
answered How light speeds up after coming out of a glass slab?
1d
comment How can ants carry items much heavier than themselves?
Can you provide a reference for your "strength proportional to surface area divided by mass"? That would imply a $strength\propto \frac{1}{r}$ relationship when buckling strength in fact goes as $\frac{1}{r^2}$ - see details in my answer. Can you tell us where your $1/r$ relationship comes from?
1d
comment Why do physics students find vectors so difficult to deal with?
I suspect this will be closed as off-topic: opinion based. In my view, people don't learn this stuff early enough. "Vectors are hard" so teaching about them is put off until well into high school, if not later. Which is silly. If you learn about vectors by playing with hockey pucks on air hockey tables when you are young and can learn new concepts easily, then you will have no problems when you become "all grown up". As I said - opinion.
1d
answered How do I calculate the apparent brightness of a mirror reflecting the sun at a given distance?
1d
comment How do I calculate the apparent brightness of a mirror reflecting the sun at a given distance?
When you say "visible" do you mean "a person looking at the moon notices there is a shiny spot", or "when I am standing in the moon light I notice there is something brighter than moonlight illuminating me". I suspect you mean the former?
1d
comment How is energy converted within the body?
Google Krebs cycle, ADP to ATP conversion, glucose metabolism. This is biochemistry. Introductory video at khanacademy.org/test-prep/mcat/biomolecules/…
1d
comment How can ants carry items much heavier than themselves?
Strength is derived from cross sectional area, not surface area. Although the two will scale in the same way it is important to be accurate.
1d
revised How can ants carry items much heavier than themselves?
added 1607 characters in body
1d
awarded  Nice Answer
1d
answered Kittel solid state physics handbook - Plasma oscillation of a ball - Am I solving this right?
1d
comment What is the point of the reduced Planck's constant $\hbar$ (h-bar)? - Why don't we just have Planck's Constant $h$?
@Danu - laziness, or efficiency? If everyone understands what you mean there is no need to waste time / ink.
1d
comment What is the point of the reduced Planck's constant $\hbar$ (h-bar)? - Why don't we just have Planck's Constant $h$?
This is actually very similar to the use of circumflex ^ and umlaut ¨ which originally came about because parchment was expensive and they were shorthands for frequent letter combinations - e.g. ae -> ä, us -> û. $\hbar$ is just another shorthand.