Monday, July 14, 2008

Illusions, Misconceptions & Misunderstandings - II

WARNING: This series of posts should be read under Parental Sub-conscious Guidance. Not suitable for Minors (under-)grads with only trivial encounters with Physic(ist)s.

Here comes the second volume of the much-awaited (really!) IMM series of posts. I write the word 'volume' here because I now own and have read all of the manga volumes of Rurouni Kenshin. This feeling of completing a saga that you get to listen/read once in a lifetime is just too overwhelming but some of the 'typos' at crucial moments in the 'official' viz translation just make me want to scream with rage and disappointment: gaaaahhhhh!!! Ohhh wait... this post is not about RuroKen (more of it at the end!); its about the IMMs so lets get started with those...

IMM 2

Returning back from the usual shopping trip to PM, I got to see for the first time a real-life Streeetch Limousine. I was in awe: could not believe the reality of its length. By now, you should have guessed this encounter was at an intersection and I was watching it in front of me drive from left to right side. I always thought they were for use only in movies and moreover, did not consider it possible to get a glimpse of one at Stony Brook.

A limo similar to the one I saw...

Besides its length, I also noticed a raised 'hood' (not present in the above image) covering the overhead area of the first two windows. And I said to myself that is something new, something I haven't seen in the movies! And so time passed watching it drive past us at reasonable speed (nearly 5s!). As it was about to move out of my view, I caught sight of the 'hood' sliding off the limo and falling forward. I assumed the worst: man, an accident is imminent - the limo will slam on the brakes and other cars will collide with it from behind - this is Bad... These thoughts got over in 1s and when I recomposed myself, I saw the 'hood' has not fallen but had overtaken the limo!!! :)

Those who are still confused: the 'hood' was another car running parallel to the limo on the other side and I could see only a part of that car's roof-carrier (or whatever it is called) with rest of its body hidden behind the limo. So, now you juxtapose these two images (one limo & another car's roof-carrier) without using any 3D graphics emulator and you get a limo with a 'hood' which 'falls off' after about 7s.

That is all for IMM 2. Please carry on...

IMM 3

This is a bit technical IMM and involves my chat as a TA with one of the undergrads (lets call him/her M). This is an edited version of the chat but with no significant alterations. To actually get the IMM part here, pay special attention to the underlined texts below. I also insert some explanations & thoughts running through my mind during this chat (but nothing extensive!):

M: can i please ask you a quick question about the hw

D: ok...

M: okay number 44 right, so i calculated for H, and when i get W, i convert it to J/s but then what do i do
  do i convert the 200W/m^2 to Watt

D: which chapter... i've to look up the question first!!!

M: oh sorry, ill email you the question right now.

[This was the question: A circular lake 1.0km in diameter is 10m deep. Solar energy is incident on the lake at an average rate of 200W/m^2. If the lake absorbs all this energy and does not exchange heat with its surroundings, how long will it take to warm from 10C  to 20C ?]

D: so... you know k, A, DeltaT & Deltax, right...

[For the record: These are the quantities that you need to calculate rate of heat flow H and k is the thermal conductivity]

M: yes so my value for H was 479092 W
D: u've to divide by 10m also
M: i know, its delta t divided by 10m rite? and then .61*A
D: H=47909.3 J/s
M: oh howd u get that
D: A= (pi d^2)/4 where d=1000 m  k=0.61 W/m.K
M: its d squared?  oh nvm  sorry okay then when i get that
  how do i get seconds
  what do i do with the 200 W/m^s
D: watt is joules/second... multiply 200 W/m^2 by area of the lake to get total heat received by the lake... i.e. 200 *(pi *d^2 )/4
M: okay so i get, 157079632 W
D: divide this number by 47909.3W to get the time
M: so i have J/s and W  but its going to be W/W  how is that seconds?
D: wait a min...

[My thoughts now: That is a good question! What the hell am I doing??? One needs to find the total heat required and then divide by the incident solar power to get the time... Now armed with this chain of thought and believing that I cannot be wrong I replied...]

D: sorry i was wrong abt that... you need total amount of heat needed to raise the temp. of the lake which has to be divided by 1570... to get the time...   heat required is Q=m c DeltaT

M: wats m  how do i get mass
D: you know the volume of water in the lake and density of the water which gives you mass of the water
M: i dont know the volume

[Note to Reader: Do you agree with M?]

D: its a cylindrical lake...   vol=area*depth
M: ok thanks

[After 5 minutes]

D: so whats the answer?

[I wait for 5 more minutes but getting no reply move on to wash dishes or something! However, after 5 more minutes when I wasn't looking, M's answer appears...]

M: 2*10^11s? for Q i got 3.286 *10^14  i divided that by 1570
  volume i got 7853981  density was 1000 kg/m^3
  is that right, my answer is due in like 3 minutes
  what answer did u get?
  are u there?

  M has signed out.

When I got back, I found M offline as the 3 minutes were apparently over. I sort of felt bad because of the wrong answer M had and also wasting M's time earlier on when I did not think about the problem seriously and continued on the wrong track M was pursuing! But now I think it is not (exactly) my fault if the student can not realize what '1570...' actually meant! Don't you agree with me? :)

That is all for IMM 3. See you again in IMM-III...

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Rurouni Kenshin!

Disclaimer: If you recognize yourself as M, please do not get offended (try to read this in third person mode). If that is impossible for you, contact me and I'll remove this chat from here!