9 ms·

Stanford has an excellent physics course, "Back-of-the-Envelope Physics," based on Mahajan's work. On the first day we worked out how much to feed a baby every

Stanford has an excellent physics course, "Back-of-the-Envelope Physics," based on Mahajan's work. On the first day we worked out how much to feed a baby every day assuming they are spherical heat emitters of radius 1 meter... (also, you got points off the problem sets if your answers were _too_ precise).

baggy_trough 6y ago
That's a big baby.
- dzdt 6y ago
  A baby elephant has close to a 1m diameter. Maybe an elephant?
- saeranv 6y ago
  Calculations are so much easier when it's a unit circle/sphere. Here's my favorite trick: the radians of an angle in a unit circle is equal to the length of the arc of that unit circle.
  - baggy_trough 6y ago
    That's not exactly a trick. It's the point of the definition.
    - saeranv 6y ago
      I think you're being needlessly pedantic. Just because it's easy to derive doesn't mean it's immediately obvious. I only realized it when I watched a 3Brown1Blue video where he pointed it out as interesting property of the unit circle. That alone tells me its not that obvious to many.
      - srean 6y ago
        You did not get parents comment. Its not a derivation its a definition.
        saeranv 6y ago
        Personally, that again sounds like a very pedantic and obvious point to me. However, I could be wrong so feel free to break down your point further. My interpretation is that I think you guys are missing the fact that the intent of my original comment is in the context approximations, from which you can see the use of the 'trick' is correct. To break it down a little further: the trick I am referring to is in practice it's often convenient to scale approximations so that you can use the unit circle for calculations, since you can use the radians as a measurement of arc length. If it's not a unit circle, the angle != arc length, so that convenience is lost.
        srean 6y ago
        > Just because it's easy to derive doesn't mean it's immediately obvious is what I was responding to. My point is this -- there is no derivation happening there. If it was pedantic, obvious and simple to you as you claim, I wonder why you claimed that it was a derivation. To you the distinction between a definition and a derivation might be a pedantic one, I have doubts on whether that is an universal or even an useful position to have.
Keyframe 6y ago
Of course https://en.m.wikipedia.org/wiki/Spherical_cow https://en.m.wikipedia.org/wiki/Spherical_cow
- tarsinge 6y ago
  Made me think of that one: topologically a dog is a sphere https://i.redd.it/pqq10ddcgdc41.jpg https://i.redd.it/pqq10ddcgdc41.jpg
  - credit_guy 6y ago
    Ever since I was in high school and first read about topology in George Gamow's "One, two, three ... infinity", I always perceived dogs as tori. Now I'm too old to change that perception.
saeranv 6y ago
That sounds really fun. Do you remember what the approach was? I know that these lumped models are standard in heat transfer calcs, but the application to humans is always interesting. It reminds me a lot of thermal comfort models built to represent human comfort relative to environmental conditions. They all tend to use simplified heat balances to model the heat exchange (with net loss/gain resulting in feeling cold, hot respectively), and there's a couple (Pierce Two-Node Model) which uses two thermal lumps, one for the internal core, and the other for the skin.

baggy_trough 6y ago

That's a big baby.

dzdt 6y ago
A baby elephant has close to a 1m diameter. Maybe an elephant?
saeranv 6y ago
Calculations are so much easier when it's a unit circle/sphere. Here's my favorite trick: the radians of an angle in a unit circle is equal to the length of the arc of that unit circle.
- baggy_trough 6y ago
  That's not exactly a trick. It's the point of the definition.
  - saeranv 6y ago
    I think you're being needlessly pedantic. Just because it's easy to derive doesn't mean it's immediately obvious. I only realized it when I watched a 3Brown1Blue video where he pointed it out as interesting property of the unit circle. That alone tells me its not that obvious to many.
    - srean 6y ago
      You did not get parents comment. Its not a derivation its a definition.
      - saeranv 6y ago
        Personally, that again sounds like a very pedantic and obvious point to me. However, I could be wrong so feel free to break down your point further. My interpretation is that I think you guys are missing the fact that the intent of my original comment is in the context approximations, from which you can see the use of the 'trick' is correct. To break it down a little further: the trick I am referring to is in practice it's often convenient to scale approximations so that you can use the unit circle for calculations, since you can use the radians as a measurement of arc length. If it's not a unit circle, the angle != arc length, so that convenience is lost.
        srean 6y ago
        > Just because it's easy to derive doesn't mean it's immediately obvious is what I was responding to. My point is this -- there is no derivation happening there. If it was pedantic, obvious and simple to you as you claim, I wonder why you claimed that it was a derivation. To you the distinction between a definition and a derivation might be a pedantic one, I have doubts on whether that is an universal or even an useful position to have.

Keyframe 6y ago

Of course https://en.m.wikipedia.org/wiki/Spherical_cow https://en.m.wikipedia.org/wiki/Spherical_cow

tarsinge 6y ago
Made me think of that one: topologically a dog is a sphere https://i.redd.it/pqq10ddcgdc41.jpg https://i.redd.it/pqq10ddcgdc41.jpg
- credit_guy 6y ago
  Ever since I was in high school and first read about topology in George Gamow's "One, two, three ... infinity", I always perceived dogs as tori. Now I'm too old to change that perception.

saeranv 6y ago

That sounds really fun. Do you remember what the approach was? I know that these lumped models are standard in heat transfer calcs, but the application to humans is always interesting. It reminds me a lot of thermal comfort models built to represent human comfort relative to environmental conditions. They all tend to use simplified heat balances to model the heat exchange (with net loss/gain resulting in feeling cold, hot respectively), and there's a couple (Pierce Two-Node Model) which uses two thermal lumps, one for the internal core, and the other for the skin.