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Aug
31
comment How far up an object is its gravitational potential energy taken from?
It is also only true if the body can be considered rigid and in the same orientation before and after, or symmetric enough that you don't care. Otherwise, you split up the body into lots of small bits, compute the change in height of each bit, and add them up.
Aug
26
comment Electromagnetic radiation from AC circuit?
And every circuit where conditions are changing is AC. A simple digital gate radiates when the inputs change in such a way that the output changes. It may (very likely) be within the allowable limit, but it does radiate. +1
Aug
13
comment What should the brake force in this problem be?
No, I was saying there is another term, the braking force. Just add it in. Imagine you hook a spring to the particle and pull it enough that the spring applies 260N. That is the brake. I would just project on the hill, so gravity supplies $+50 g \sin 25^\circ$. Friction supplies $-50 g 0.05 \cos 25^\circ \sin 25^\circ$. The brake supplies $-260$. Add them up and you have the resultant.
Aug
13
comment What should the brake force in this problem be?
The braking force is applied to the particle. There is no coefficient to apply. (What is CoE?) The friction coefficient is used to convert the downward force due to gravity to lateral friction force, but if the brake apply 260N to the particle that is the force.
Aug
13
comment What should the brake force in this problem be?
If you are using $\mu$ for the coefficient of friction, as I read the problem it should not apply to the braking force. You say the brake exerts $260$N, so that is the force. It doesn't say how it is applied, nor does it matter.
Aug
12
comment Thermodynamics about turbines
@apnorton: This is a Physics question. You are correct that it will be closed in a heartbeat if migrated. It will only land back here if OP is intransigent. I participate on both sites and prefer our homework policy, but this question supports the motivation for theirs-they get many more plug and chug than we do.
Aug
7
comment Is most of the matter in the observable universe within galaxies?
This NASA page quotes 4.6% atoms, 24% dark matter, 71.4% dark energy which has much less baryonic matter than suggested here. I believe this is more in line with current consensus science.
Jul
25
comment How can the orbit of Jupiter's moons be used to calculate the speed of light?
@Javier: No. We are not looking at the time between eclipses, we are looking at the absolute time of each eclipse. If the distance from Earth to Jupiter did not change the times would be a nice arithmetic progression. The motion of Earth imposes a sine wave on the time with amplitude the light time over the diameter of the Earth's orbit around the sun (assuming Jupiter's orbit is circular). We asses the effect and find the light travel time over the diameter of Earth's orbit.
Jul
23
comment What is the work done?
@JohnM: I believe showing the calculation of the PE change in the barrel is enough to satisfy this. OP has isolated the question to which value is being asked for.
Jul
8
comment Tritium decay is spontaneous even if the binding energy of tritium is higher than the binding energy of 3He. Why?
This is spot on. The point is that the masses of the atoms include the electrons, so the decay is favorable from an energy standpoint.
Jul
6
comment Why do we ignore rotational energy in monatomic gases?
Once it contributes, you have three axes for rotation, so $\frac 32kT$ becomes $3kT$. This is the same for molecules larger than diatomic. For diatomic molecules near room temperature we ignore rotation around the long axis, so there are two instead of three degrees of freedom. The motivation is the same. The MOI is so small the DOF is not excited.
Jul
6
comment Why do we ignore rotational energy in monatomic gases?
Then the MOI is decreased because now you have a bare proton. That means the temperature where the degree of freedom becomes important is higher-I suspect much higher for the reasons in my last comment. Other things may be going on by then-increasing number of electrons contributing the $\frac 32kT$ would be my first guess.
Jul
6
comment Why do we ignore rotational energy in monatomic gases?
Aside from hydrogen, an ion has almost the same moment of inertia as an atom. I think (but didn't do the calculation) that the electron cloud has most of the MOI of an atom in the classical picture. The electrons are $2E3$ times less massive than the protons, but the radius (which gets squared) is $1E5$ times greater or more.
Jul
3
comment Drag in low earth orbit
If you don't include gravity gradient, then there is nothing to prevent the arrow from flying straight. The damping comes from transverse drag. If the arrow is spinning, there will be drag to slow the rotation down. That may be in your step 4.
Jul
3
comment Radiation from home heaters
@CuriousOne: It is true you can get burned by a heater, but the wording convinces me that OP believes all radiation is harmful, not can be harmful.
Jul
3
comment Radiation from home heaters
This assumes all "radiation" is equally dangerous, which is clearly false. Different types of radiation (note that sound is included as well as electromagnetic) have different damage pathways and different danger levels. -1
Jun
26
comment Cylindrical capacitor in an electric circuit
If you think about having $R$ connected to the cable and then connecting the battery/$R0$, the capacitor starts completely discharged. As the capacitor cannot change voltage instantly, the current will flow through $C$. With a time constant of $RC$ the current will shift to the resistor. As $t \to \infty$ the current in $C$ will go to zero and you will have a constant current in $R$.
Jun
26
comment Cylindrical capacitor in an electric circuit
Yes, that is the capacitor. Once you calculate $E(r)$ you can use that to compute the capacitance of the coaxial fiber per unit length.
Jun
26
comment Unable to understand relative motion
You probably wouldn't see the same effect if everything outside the van is the same distance from your route of travel. I made a long drive yesterday with mountains in the distance on both sides of the road. It looked like the mountains were moving with us in the direction of travel with everything nearer to us than the mountains moving rapidly backwards.
Jun
26
comment Cylindrical capacitor in an electric circuit
As I reread it, I think you are correct that the wrapper is connected at both ends. Your circuit is correct as well. They say that at large time no current flows through the capacitor, which I misread as the wrapper. In your circuit, you have current flowing around the outside loop permanently in just the amount stated in the problem. Now C charges according to the voltage divider formed by the two resistors across the battery.