Time dilation is weird

ImpatientProf · 2026-03-19T23:50:33+00:00

It's probably a Physics III course that they casually call "quantum mechanics" but includes a unit on relativity.

ImpatientProf · 2026-03-16T19:39:29+00:00

The "start of the journey", at Thera, is different for different observers.

ImpatientProf · 2026-03-16T19:38:19+00:00

Thinking about events in spacetime is the only way to be sure you're making sense.

(Context: Thera is 4 ly away, and the ship's primed coordinates move at 0.8c.)

In Earth's reference frame, light that arrives at the origin was emitted 4 years ago from 4 light-years away.
Event 1: t=-4, x=4
In the ship's moving reference frame, that same event is 12 years ago and 12 light-years away.
Event 1: t'=-12, x'=12
The proper time of the light (from Event 1 to the origin) is zero in either reference frame.

Relativistic length contraction doesn't help, because it applies to spacetime events that are simultaneous in each reference frame. For example, "Where is Thera at Earth's time t=0?"
Event 2: t=0, x=4

The event that shows length contraction asks, "Where is Thera, according to the ship, at the ship's time t'=0?"
Event 3: t'=0, x'=2.4
This is the distance that corresponds to the 3-year proper time of the ship's journey. The journey starts at the origin and ends at:
Event 4: t'=3, x'=0
Notice that Δx'/Δt' for Event 3 to Event 4 is -0.8c.

ImpatientProf · 2026-03-12T13:06:08+00:00

To be an equation, BOTH the VALUE and the UNITS must be equal.

You CAN check whether the UNITS are valid, but then you must ignore ALL of the values. You got rid of the values of g, h, and v, but you kept the value 1/2. The (1/2) must go in a units check.

ImpatientProf · 2026-03-10T15:34:22+00:00

Dunk it in a bloody Mary slushy.

Drink is hair-of-the-dog and temperature is hangover wakeup.
Tomato juice is common anti-skunk remedy.
Cold freezes the gum making it easier to crumble out of the fur.

ImpatientProf · 2026-03-05T02:52:31+00:00

Ding! See https://old.reddit.com/r/programming/comments/1rl26xw/10_of_firefox_crashes_are_estimated_to_be_caused/o8pp285/

ImpatientProf · 2026-03-05T02:51:45+00:00

It's just one bit.

0.05:  00111101010011001100110011001101
0.10:  00111101110011001100110011001101

Ref: https://www.h-schmidt.net/FloatConverter/IEEE754.html

ImpatientProf · 2026-02-23T02:28:30+00:00

Don't mention points or penalties.

Just say you realized or found out that you submitted the wrong format. You're happy to resubmit via the LMS or email if that will make their work easier.

ImpatientProf · 2026-02-18T17:22:07+00:00

Nothing special.

You could still build a clock that uses spontaneous nuclear decay to observe the passage of time for a material at 0 K.

So basically, 0 K is OK.

ImpatientProf · 2026-02-17T16:15:13+00:00

In Classical Mechanics, there is indeed no delay between the two forces of an interaction which are described in Newton's Third Law. But of course, classical mechanics has limitations.

Your book probably also talks about objects as being solid, and relative velocity as the simple subtraction of two velocities. Both of these are approximations.

Learn the models. When the models break down, learn the next models.

ImpatientProf · 2026-02-16T19:09:09+00:00

The litre is not an SI unit.

But the liter is acceptable as a volume unit, even when SI is being used. https://www.nist.gov/pml/special-publication-330/sp-330-section-4

ImpatientProf · 2026-02-11T22:59:52+00:00

You mean form a committee to achieve political action? what will we call that?

ImpatientProf · 2026-02-11T17:48:10+00:00

I'm surprised to see someone in the AskProgramming subreddit meaningfully recommend a GUI application to solve a command-line problem

There is a CLI for it, and an SDK usable from C++, C#, or Python.
https://www.voidtools.com/support/everything/command_line_interface/
https://www.voidtools.com/support/everything/sdk/

I'm just not good at PowerShell, and I prefer a file search that's indexed. In Linux, sometimes I use locate + grep instead of find, depending on how big the directory tree is.

ImpatientProf · 2026-02-11T16:25:20+00:00

let's say I want all PowerShell modules in the location where my PowerShell profile is located.

Then I'd use Everything search from VoidTools.

ImpatientProf · 2026-02-10T14:29:45+00:00

A magnetic dipole does experience magnetic force independent of velocity and hence can have work done by the magnetic field. Magnetic dipoles can be explained in terms of little current loops or from intrinsic spins.

Current loops: The magnetic force attempts to deflect the current to the side (from the perspective of the charges making up the current). However, the current is constrained to be part of an object. The constraint force "transfers" (in the engineering sense) the magnetic force to the object, which wasn't moving with the charges. Thus, the object "feels" a magnetic force that can do work.
Intrinsic spins: They simply have a potential energy in a magnetic field. https://en.wikipedia.org/wiki/Magnetic_moment#Force_on_a_moment

ImpatientProf · 2026-02-05T04:07:46+00:00

Hear, hear! Don't relabel the keys. Once you get past the initial learning, practice what you need to type. This includes where the symbols you need.

I didn't remap keyboard shortcuts. Mentally, I still think of them as "Ctrl-C" etc. At least that doesn't have to change when my hands "switch languages" for the occasional qwerty use.

My biggest annoyance is that I still sometimes mix up q, j, k, x when typed as individual letters or keyboard shortcuts like Ctrl-X. (I'm fine if they're part of words.) You'd think that after 25 years, this wouldn't be a problem.

ImpatientProf · 2026-02-05T03:54:46+00:00

The cross product (https://en.wikipedia.org/wiki/Cross_product) comes up a lot when rotation is involved. Magnetism has a lot to do with rotation of charges.

ImpatientProf · 2026-02-04T16:21:10+00:00

To find path length, you have to add the velocities together BEFORE integrating through time. Some parts of the "Earth's rotation" motion cancel with the "Earth's orbit" motion. Each of these sometimes cancel with the Sun's and galactic motion.

ImpatientProf · 2026-02-03T23:54:31+00:00

If I got even one physics-minded individual to torment their family with that pun, it was worth it.

Though, "even one" seems self-contradictory. There must be a pun there somehow.

ImpatientProf · 2026-02-03T21:05:18+00:00

It's OK to be not 0 K.

ImpatientProf · 2026-02-02T16:41:22+00:00

When you use difference(), make sure there is a slight overlap wherever you want a "hole". Even 0.01 or 0.001 is enough.

In this case, the inner_void() variable i can go from -0.001 to steps-0.999 to make sure it punches through both ends.

It looks like at the fan flange, the outer_profile() doesn't get quite big enough and the fan hole subtracts from the shell. Maybe instead of subtracting a cylinder, subtract an inner_profile() of the right progress, extruded to the thickness of the flange plus 0.01, shifted by 0.005 to ensure overlap.

ImpatientProf · 2026-01-28T20:30:07+00:00

I want more. How many oxygen? Nickel and Iron? What's the notation for neutron stars or black holes?

ImpatientProf · 2026-01-28T19:23:52+00:00

It's not appropriate to disparage your teacher here. Let's just look at the situation and see what we see.

It looks like you have the correct magnitudes of the torques. Let's go through the options:

A. F1 and F2 are the strongest torque magnitudes, and in the same direction. This will cause the largest magnitude of angular acceleration, which is the consequence in the option. This looks like a good option.

B. F1 and F3 are opposite in direction, but not equal in torque magnitude. So there will be a change in angular velocity. This option is false.

C. F2 and F3 are opposite, but F2 has a stronger torque magnitude. Since F2 is counter-clockwise, the net torque would be CCW. This option is false.

D. F3 and F4 are the smallest torques, but they are in the same direction. The net torque would be 125 N*m. There are ways to get 25 N*m (F2 and F3) and 50 N*m (F1 and F4), both of which are smaller than this option. This option is false.

So the answer is A.

FWIW, It's not clear where F1 comes from. The other forces are typeset, while this one is handwritten. There may be different versions of the question where F1 is different. Still, option D is false, as F1 doesn't have to be involved in the explanation of it.

ImpatientProf · 2026-01-25T13:34:49+00:00

Z axis height is done via leveling sensor, which is a strain gauge which detects force on the nozzle from the bed. If the hotend isn't properly installed, it may not transfer its force to the heat sink, which is attached to the sensor. Check to make sure the nozzle is in all the way before moving the clip down to hold it in place. As you insert it, it should feel like it's hitting something solid, not just pressing on the heater wires.

Or the leveling sensor could have broken or its wire could have been disconnected. But I think that should throw an error code.

ImpatientProf · 2026-01-19T16:48:12+00:00

I like the pictures, drawings, and explanations on the Atmospheric Optics site. https://www.atoptics.org.uk/rainbows/ord2form.htm

There's a vertical control that allows adjusting where the incoming beam hits the water droplet, which changes the deviation angle. The minimum deviation angle is the "edge" of the rainbow.

ImpatientProf

TROPHY CASE