669: Experiment

Explain xkcd: It's 'cause you're dumb.
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The other two are still lost on the infinite plane of uniform density.
Title text: The other two are still lost on the infinite plane of uniform density.


Problems in the study of kinematics often idealize the environment of the problem for the sake of simplicity. Specifically, it is assumed that objects are moving in a vacuum and that there is no friction. Then the complicated effects of air resistance and surface frictions can be ignored, and the more basic principles of momentum and energy can be explored. In more advanced physics, it is often easier or necessary to ignore friction if the process being studied is very complicated. So it could be said that "physics professors like working in a frictionless vacuum".

In the comic, Black Hat and Danish have interpreted that statement to mean that physics professors like doing their work while they are in a frictionless vacuum, instead of liking to work with problems which are set in a frictionless vacuum. Apparently, they have drugged a physics professor and put him in a glass dome (with his laptop so he can work) which they can evacuate and make frictionless. The professor wakes up confused from the drugs, and as the air is pumped out to make a vacuum (presumably slowly enough to prevent explosive decompression from coming into play), his words fade to silence because sound waves requires a substance such as air to travel through. As he starts to panic, he tries to run, presumably for the door we see in the last panels. However, without friction on the floor, he cannot exert any force to move forward, and his feet skate uselessly on the ground until he loses balance and falls. At this point he is probably suffering from asphyxiation. Black Hat and Danish are observing from outside the dome, and decide that physics professors have lied about liking to work in frictionless vacuums.

The title text refers to another common idealization, of an infinite plane of uniform density. An infinite plane extends forever in two dimensions, which makes calculations easier because surface-related properties are identical everywhere. "Uniform density" could refer to the mass density of the plane, or more likely an electric charge density, which makes a common problem in basic electromagnetism involving calculating the electric field. The "other two physicists" that Black Hat and Danish are experimenting on are lost on the infinite plane, since there are no edges or landmarks anywhere to give them direction.

It should be noted that, although a vacuum can be approximated using a vacuum pump, frictionless surfaces and infinite planes are only imaginary constructs and do not exist in our universe.[citation needed][actual citation needed]


[Cueball is standing next to a laptop, looking groggy.]
Cueball: Ugh...
Cueball: What happened?
[Cueball begins to regain some awareness, with his speech eventually fading out.]
Cueball: Where am I?
Cueball: Help! Someone help me (unintelligible speech due to loss of air in dome)
[Cueball holds his hands to his mouth.]
[Cueball looks shocked.]
[Cueball tries to run, but has no traction against the ground.]
[Cueball falls down.]
[Cueball lies prone and motionless.]
[Black Hat and Danish watching from outside the dome, with Black Hat holding a clipboard as if to take notes.]
[Black Hat and Danish outside the dome. Black Hat is now holding the clipboard down to his side.]
Black Hat: Huh. Looks like physics professors don't like working in frictionless vacuums after all.
Danish: They're such liars.

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I'm a science grad student, so these kinds of comics in particular I'm having fun writing for... (now if only I were writing my dissertation instead...) Bplimley (talk) 10:18, 7 April 2013 (UTC)

You're turning these out really quickly. This is good stuff! Davidy²²[talk] 10:19, 7 April 2013 (UTC)

One can throw something to change one's motion in a frictionless vacuum.Username2 (talk) 00:40, 27 July 2020 (UTC) If the black hat REALLY cared about physics, he would have put a spherical cow inside the frictionless vacuum room. 20:43, 7 March 2023 (UTC)