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  • 0 posts edited
  • 24 helpful flags
  • 617 votes cast
Apr
29
comment While deriving Coulomb's Law from Gauss' Law, the power of 'r' comes out to be exactly 2 while experiment shows that it differs about 10^(-16). Why?
You should provide a link to the data that you are citing. Is this within the error bounds? One would expect it to be so.
Apr
14
comment What role does the center of mass play in this situation? (electric potential)
@NowIGetToLearnWhatAHeadIs: the figure has two pairs of masses linked by strings, so I think it is after the strings are cut. Then it says the two bottom ones are charged, but they still have a string connecting them. That is the source of my suggestion that the leftmost two are charged, presuming the string connecting them has just been cut, as has the one on the right.
Mar
26
comment Modelling an Airplane Question
The density of air is usually considered to be constant. Where I have seen drag calculations, the velocity is the current velocity, not the terminal velocity. You can keep the state as a three position matrix, location, velocity, and acceleration. Then you update each one every timestep based on your acceleration and drag equations.
Mar
26
comment How does quadcopter's battery capacity relate to it's maximum flight time?
I strongly disagree with the close vote. This is quite on topic. We want to encourage people to think about physics, which is what is being done here.
Mar
20
comment How much more “work” (or energy) is required for short people to hike uphill?
The physics answer is that the taller person needs a factor $\frac {40}{110}$ more energy to get up the hill. It seems reasonable that the efficiency of converting food to mechanical work should not be too different depending on height. The shorter person will need more steps, but each step will represent less energy output, so shorter legs are no disadvantage in this regard.
Mar
18
comment How much more “work” (or energy) is required for short people to hike uphill?
@JBentley: It is important to think about a question and what you are interested in.
Mar
15
comment Elastic collisions and 90 degrees angles
In what reference frame? In the CM frame two objects are always moving in opposite directions. They can change direction by $90^\circ$ in inelastic collisions. It would not be convention, it would be because you can't balance momentum without balancing energy. Please draw a figure to show the angle you are referring to.
Mar
13
comment Pumping water vapor into liquid water
The usual way to make pure water vapor is to boil water. That gives water vapor at 1 atm pressure and 100C temperature. If you bubble that through room temperature water, the heat will go into the liquid water and much of the water vapor will condense as the vapor pressure of water at room temperature is much lower than 1 atm. When you see bubbles going through water, say in a fish tank, they are mostly air with (maybe) an equilibrium concentration of water vapor.
Mar
11
comment Maximize time of flight and range at the same time in projectile motion
I don't think that question makes sense because $k$ is dimensional. The intuitive thing would be to take $k=1$ as giving them equal importance. If I measure T in seconds and R in mm, I get a very different answer than if I measure T in microseconds and R in km. If you set $k'=1$ you get $\theta = \arctan \frac 1{\sqrt 7}\approx 0.361367$
Mar
9
comment How the flow is distributed when the water flow from one river/drain to multiple rivers/drains?
If you are given the total flow $Q$, as I said before you have $\Delta P=(\frac Q{\sum C_i})^2$ Then you plug that in to get $Q_i=C_i\sqrt{\Delta P}$ The flows divide proportional to the $C_i$ if all the $\Delta P$s are the same. It could be that the drains flow out at different elevations, so the $\Delta P$s are not all the same, but you can still find the proper $P$ at the junction to make the flow correct. It will probably take numeric solution then.
Mar
9
comment How the flow is distributed when the water flow from one river/drain to multiple rivers/drains?
I don't know how to use the information you have in point 1 to compute the $C_v$s. Maybe somebody has some software that does that. There should be tables available online for simple shapes, like round pipes.
Mar
8
comment How the flow is distributed when the water flow from one river/drain to multiple rivers/drains?
What you are hoping to calculate is the $C_v$ for each drain, which I can't help you with. If the outlet of each drain is at the same pressure, then the $\Delta P$ is common and you can solve for $\Delta P$ as $(\frac Q{\sum C_v})^2$
Feb
22
comment How to calculate the diameter of a moving circle from afar while lacking and knowing some information?
No, you want a person at the same distance from the camera as the wheel. Then you can just scale the diameter of the wheel from their height. If they are six feet tall and the wheel is half as tall as they are, the wheel is three feet in diameter.
Feb
20
comment How to calculate the diameter of a moving circle from afar while lacking and knowing some information?
You are trying to get the scale of the photo. As you said you knew the speed I was trying to use it to set the scale. If you have an object of known size in the photo, like a person, you can use that to set the scale. Otherwise, I don't see a way to distinguish a small near object from a large far one.
Feb
20
comment How to calculate the diameter of a moving circle from afar while lacking and knowing some information?
Then you need to either take a video, knowing the number of frames per second, or record the time between pictures.
Feb
10
comment volume of the air bubble in the water
@Mphysics If the bubble is released at a depth $h$ and a volume $V(h)$, this lets you compute $V_0$, the volume at zero depth. What were you looking for?
Feb
10
comment volume of the air bubble in the water
This will let you assess the volume and therefore the buoyancy force, but I don't know how you will get the resistance. That will dominate how fast it rises.
Feb
8
comment Why is the decay of a neutral pion $\to$ electron-positron loop suppressed?
I have corrected it. I realized the angular momentum problem. It is still true that the EM decays will dominate.
Feb
7
comment Ohm's law experiment
Depending on how you wind it, you can cancel out the magnetic field it makes, reducing $L$. You did not provide any details of the winding approach, so it is hard to tell.
Feb
7
comment Ohm's law experiment
Please describe the experiment. It sounds like you are connecting a rheostat in series with a voltage source, connecting your sample across that, and measuring the current through the sample as a function of the rheostat resistance. Are you using different lengths of sample? Are you making a measurement very quickly? It would appear so as the induction effect will die away with time.