4,544 reputation
1814
bio website none
location somewhere on earth
age 59
visits member for 3 years, 8 months
seen 5 hours ago

1d
comment Gravitation - hole to center of Earth
I don't know of another analytic one. You could certainly simulate it numerically.
1d
comment Gravitation - hole to center of Earth
You are solving a second order differential equation. It ise one of a harmonic oscillator: $\frac {d^2r}{dt^2}=-\frac {gr}R$. You get solutions in sine and cosine, but you will arrive at the surface with escape velocity and never turn around. This will come in with the constants of integration, which capture the initial conditions.
1d
comment Gravitation - hole to center of Earth
I changed to $r$. It is the second time derivative.
1d
comment Gravitation - hole to center of Earth
@Gowtham: In fact, ignoring friction, it will escape to infinity after going out the other end. While inside the earth he equation is the same. We just have a strange initial condition.
Dec
15
comment Is it possible to use a magnet to move a steel carriage, which is tied to the magnet?
Please describe the setup in more detail. Is there an external force on the system somehow or are the carriage and magnet isolated from the rest of the world?
Dec
15
comment Is it possible to use a magnet to move a steel carriage, which is tied to the magnet?
@Hypnosifl: That could be the question, and you are right.
Dec
14
comment How can we be Certain that Dark Matter Exists if we Cannot See it or Directly Detect It?
Have you read Wikipedia ? There is a long section on the observational evidence.
Dec
13
comment What's the physical interpretation of an arbitrary normal mode for masses and springs?
Yes, the common frequency is where the eigenstuff comes from. You assume that each mass moves with a common frequency, but its own amplitude and phase. Then $\ddot x_i= \omega^2 x_i$ , you get $(A-\omega^2)\vec{x}=0$ and you want nontrivial solutions.
Dec
6
comment How Earth's Gravity is more powerful than its centrifugal force?
This line of reasoning allows the earth to spin more slowly (reducing the centrifugal force to near zero) but sets an upper limit to how fast it can spin. What would be the length of a day if the centrifugal force at the equator equaled $g$? In that case, if you ran East, you would launch into orbit.
Dec
4
comment Lagging in induction coils
I think you have something in mind that I am not understanding. The inductor resists changes in the current instantly. It certainly does not make bigger current spikes. If we just connect the circuit across the supply with no inductor, the current instantly goes to steady state when we connect the supply. The inductor will make it ramp up with a time constant $L/R$. We can't say what would ruin the circuit without knowing what that circuit is.
Nov
14
comment How come Wifi signals can go through walls, and bodies, by kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?
@user3237992: the difference is that microwave ovens and WiFi operate at essentially the same frequency, so changes in absorption due to frequency are off topic here. I believe they penetrate the same, but we care about WiFi penetration and we don't recognize that the oven microwaves penetrate just the same.
Nov
14
comment How come Wifi signals can go through walls, and bodies, by kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?
@K7PEH: It looks like your comments are directed to OP, not to me. You can comment on the question for that.
Nov
13
comment How come Wifi signals can go through walls, and bodies, by kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?
@K7PEH: I don't expect the attenuation is any different. I said that a few times. The oven wall provides enough attenuation that we don't care about the signal (just as power) any more, then attenuation through walls knocks it even lower.
Nov
13
comment How come Wifi signals can go through walls, and bodies, by kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?
@K7PEH: That is (just about) the same frequency most WiFi operates at, so it is fair to expect the same attenuation.
Nov
13
comment How come Wifi signals can go through walls, and bodies, by kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?
How do you know they don't go through a few centimeters of an object. My point is that they will be attenuated by the same factor by any object. We don't normally have any way to detect them and don't care that in fact they do pass through. They are at a level (even before the attenuation) judged to be safe. Yes, there are meters that can detect the oven signals, but we don't carry them around normally.
Nov
13
comment Where does the force to stop a constant velocity object come from?
If the ball truly has a constant velocity it has to pass through the wall, because the wall cannot accelerate it and cannot apply any force. When it is slowing down it is accelerating. Constant velocity is a reasonable approximation when there is no large force acting on the object. For the ball, we ignore air friction. When the ball hits the wall, it no longer has constant velocity.
Nov
12
comment Solar vs lunar gravity: inverse square law
@garyp: I do. The question is asking for a simple calculation, but wanders in many ways off topic.
Nov
11
comment Where is the fine-structure constant in this list?
There is a note that $\alpha(m_Z)=\dfrac 1{4 \pi}\dfrac {g \cdot g'}{g^2+g'^2} \approx \dfrac 1{128}$ and that the unfamiliar value is because of the energy level.
Nov
8
comment How would one best dissipate or absorb shockwaves and pressure caused by an explosion within a confined space
Are you hoping to limit the damage from one explosion or many? In other words, does the material need to survive intact, or can it be damaged? The second provides more options.
Nov
8
comment How would one best dissipate or absorb shockwaves and pressure caused by an explosion within a confined space
I think it is hard to absorb the energy in phase transitions, but my first thought is acoustic impedance mismatches and foams are very good at that.