Solution to Problem #60, Re-entry Satellite

[@devwalia165]

After month i m here. Its nice to see u in good health (alive) u always motivate me.

[@samersman2953]

I really should be revising for my GCSE's but I have become addicted to your amazing videos 😂. I have truly been inspired by your infectious passion for physics and now I love it :)

[@denisapg]

Very interesting! Thank you, Professor!

[@fernandomaron]

Sir, you inspire me very much. I deeply appreciate you for that.

[@lecturesbywalterlewin.they9259]

:)

[@esa062]

You need to heat the aluminum to boiling point 2467 °C, melting point is not enough. The difference in total energy is small though, evaporation takes most of it. The change in potential and kinetic energy add up.

[@jeepien]

Agreeing with kenneth wong, it seems we are missing the amount of energy needed to heat 5000 kg of liquid Al from 660 °C to the boiling point.

[@lecturesbywalterlewin.they9259]

you should watch my solution again. I did not skip any step However, the boiling point is not well known as it depends both on the temp of the Al and of the local atmospheric pressure. However, a more sophisticated calculation (computer simulation of re-entry) would come to the same conclusion as mine.

[@ashutoshprasad8494]

best teacher ever

[@tatjanagobold2810]

Thank you Mr Lewin!

[@royaldingbat1117]

i love it! Really interesting!

[@kennethwong2722]

Do we have to consider the heat needed to rise the temp from melting point to boiling point? Or it just sublime?

[@mrmgreig41]

My rough calculation using a temp rise of 1667 c to get to evaporation temperature and 1.18 kj/kgc get 9.87 x 10^10 extra joules needed. With the answer Walter has that gets 1.587 x 10^11. Odd coincidence?

[@kostasphilippides7147]

@@mrmgreig41 I get mcΔΤ=5000x1180x(2470-660)=1.07x10^10 J. To be added to Walter's 6.0x10^10 J. Still, doesn't change the final conclusion. He says in some comment that it may not even need to warm up to boiling point since the pressure might be much lower than 1 atm. I think he should have included it.

[@williameudy3921]

Professor Lewin is alway most informative and entertaining...can we still be friends with a solution of 55,000,000 BTU? (Joking).

[@nabinteemilsina6495]

Nice..

[@soubaaibrahim5729]

Thank you professor

[@johnlegros1586]

very nice analysis , but simplistic , in fact another important parameter not taken in account that explains why satellites may reenter and not be burned , like for exemple a shuttle , is the cooling effect of atmosphere during reentry , the object while exchanging energy does mostly by heating the atmosphere particules , denser and denser while approaching the earth , and hotter and hotter from - 100 to +20 C at ground level standard ( in fact density/temp is almost constant to simplify ) , and so a lot of energy is given to the atmosphere , how much ? that would be another nice analysis... you do a great work !!

[@lecturesbywalterlewin.they9259]

plse read the problem again - I stated that I had simplified matters by e.g., that no heat was transferred to the atmosphere which, ofcoz, is never the case. It was also NOT about the shuttle landing on Earth.

[@johnlegros1586]

indeed , but the shuttle is a satellite in orbit at a time, but anyway good video ! @@lecturesbywalterlewin.they9259

[@xcalibur6482]

Professor Lewin... Im still confused as some people are talking about boiling point ... which makes me ponder... as boiling point is defined to be the temp. at which the vapour pressure of liquid is equal to the atmospheric pressure.. and that is changing drastically as an object falls through a great height. It would be nice if you could make a video clarifying my doubt if you consider it important..

[@lecturesbywalterlewin.they9259]

en.wikipedia.org/wiki/Boiling_point

[@BYMYSYD]

Mathematically one would have to have a lot more information to solve that kind of problem. And I'm sure there is no exact solution. One would have to use a computer to perform a numerical approximation to see what kind of difference it would make. But maybe there is a way to approximate the kind of error you'll get while considering the pressure dependence of the boiling point.

[@lecturesbywalterlewin.they9259]

@@BYMYSYD you are right that the boiling point depends on the temp of the Al and on altitude (atmospheric pressure). As soon as the AL has melted, it's altitude may still be so high (thus atm pressure so low) that it will start to boil right away). This does complicate the problem for which I apologize. My best guess, however, is that if you solve the problem more correctly than I did that the conclusion will be the same.

[@xcalibur6482]

@@lecturesbywalterlewin.they9259 Thank you professor : )

[@gaurangagarwal3243]

It was truly intresting problem. :)

[@shivankarsingh7913]

Sir Today, I bought Label Mic for Art Quiz Solutions. I'm going to start for Art Quiz 17 now. *Art Quiz 17 :* Left : Ariadne, 1913, Giorgio de Chirico Middle : Constantin Brancusi, Untitled (Sleeping Muse), 1910 Right : The Dreamer, 1932, Pablo Picasso

[@lecturesbywalterlewin.they9259]

if you have time, plse write the solutions as you have done in the past. No need to send me the names and the years and the titles.

[@shivankarsingh7913]

❓ *Professor❤ Was **_Constantin Brâncuși_** considered a pioneer of modernism and one of the most influential sculptors of the 20th-century ??* ❓

[@lecturesbywalterlewin.they9259]

@@shivankarsingh7913 I do not want to go back to my art quizzes. You can always check online whether your soln is correct.

[@malayapaul458]

hey prof Lewin I've got a great question and it might end up being your next video topic, let get to it say there's a spring fixed vertically to the floor of spring constant k , now someone puts a block on top of it, intuitively its gonna contract right, but there's a great mystery in finding the length contracted say I take the equilibrium of forced into consideration then k . x = m . g where (m . g) is the mass of the block then we get x = m.g/k but wait doing the same thing taking the conservation of energy into consideration we get m . g .x = (1/2) k . (x^2) m.g =(1/2)kx x = 2mg/k and how on earth is is possible that we get two different values for the same quantity taking two very very valid concepts into consideration please do something with this Professor or at lease reply to me about what you thing is the missing thing that I've haven't considered thanks a lot love from India

[@ironuranium3927]

Hey young man ♥

[@hansonyuen2044]

interesting problem! would you make a video explaining/discussing how cat survive from multistory high building? has it do with its moment of inertial?

[@BYMYSYD]

Well done!

[@ulfhaller6818]

Happy Easter, professor Lewin! 🐔 🥚 🐣

[@lecturesbywalterlewin.they9259]

:)

[@kevinli5463]

Dear Dr. Lewin, thank you very much for the content. I am tremendously interested, if possible, to meet up with you for, perhaps, coffee, like some of your other viewers at a time convenient for you. How can I make this possible? Thank you.

[@kevinli5463]

I am a student from Canada, and I will be in the Cambridge area in early July, and would love to meet you at your favorite restaurant :).

[@lecturesbywalterlewin.they9259]

contact me again about 4 days before you will be in Cambridge. I will then know whether we can meet. PLEASE contact me in my last art quiz at that time. If you contact me again in this video I may not be able to get back to you. KZbin is very consumer unfriendly for video managers

[@ritwiksharma7021]

Professor, somehow can i get your autograph on your book "for the love of physics", please? you are a great inspiration for me. thanks.

[@lecturesbywalterlewin.they9259]

I will send you a signed copy of my book for $25 + postage. Postage is $8 in the US, $30 to Europe and to India. I will write a special note for you in the book. I will also include a signed colored picture of me swinging on the pendulum (with your name written on it). When you transfer the money to my PayPal account let me know your full name and your address (make sure it is correct and complete). *Also let me know very clearly which name I should use for a note in the book and on my picture.* *Please add 5% to your payment as PayPal fee is 5%.* PayPal: [e-mail]

[@abhijeetlaha3463]

Sir I am a physics graduate student. In Electrodynamics we discuss about charges. But what is charge actually? Sir please reply I did not get my satisfactory answer. I love your teaching techniques and love physics.

[@lecturesbywalterlewin.they9259]

use google

[@nielsren]

I like it

[@MegaKinder21]

Professor I have a question, why do you not considerate the temperature's difference between the melting point (660 °C) and the boiling point (2057 °C) of Al using the specific heat of the liquid (1085.98 J/(kgK)) ??

[@lecturesbywalterlewin.they9259]

The boiling temp I chose is not very precise as it depends on the atmospheric pressure. As the satellite er-enters it will heat up and there come a point that it will start to melt. If the outside pressure is much below 1 atm (which is quite possible), it may start to boil right away. However, the final conclusion that I draw in my solution will not change.

[@ranjitbhattacharjee9776]

Is it possible to create a machine or an arrangement to convert light into different electromagnetic waves and vice-versa?

[@lecturesbywalterlewin.they9259]

ofcoz that's possible. light can be converted into electricity (that's what solar cells do). Once you have electricity anything is possible, you can make radio waves (listen to your radio), you can make X-rays (X-ray machines in hospitals) . . . . . . . .

[@ranjitbhattacharjee9776]

@@lecturesbywalterlewin.they9259 Thank you, Sir. I am actually making a project on this. This was very helpful

[@devalpatel4364]

Dear lewin sir, I want to ask you that why the light doesn't change frequency when they change medium

[@lecturesbywalterlewin.they9259]

THe energy E of a photon =hf f is its frequency. Since the energy of the photon does change as it enters glass and/or emerges from glass f does not change. There is another way of seeing as demonstrated by me with traveling waves in a rope. a rope with prop speed v1 is connected to one of prop speed v2. The ropes are connected at point A. A is moving up and down with freq f. Thus the freq f must be the same in both ropes. This is a mechanical analogue.

[@devalpatel4364]

@@lecturesbywalterlewin.they9259 But Walter lewin sir when the ray is partially reflected and partially reflected at that time the energy is distributed but then also frequency doesn't changes

[@lecturesbywalterlewin.they9259]

@@devalpatel4364 one INDIVIDUAL photon can ONLY reflect or refract - it's not possible that part of the energy reflects and part refracts. That's the great QM mystery about the double slit experiment! We can calculate the diffracted pattern by assuming that part of the wave goes through one slit and part through the other slit. That works mathematically. You may want to read up about the double slit experiment. QM is MAGIC!

[@ramandeepKaur-mt4td]

A question; if we try to put a sphere in a cone of same radius z, how much part of it will get into it

[@lecturesbywalterlewin.they9259]

use google

[@ramandeepKaur-mt4td]

@@lecturesbywalterlewin.they9259 I couldn't find the answer I believe it's 50% will get into it

[@ratnasaxena4754]

It also depends on the slant height of the cone but it can enter only half at most

[@xcalibur6482]

@@ramandeepKaur-mt4td 50% does seem to be correct

[@ratnasaxena4754]

I love you sir i want to my physics teacher to become as you

[@niranjangaur5912]

Sir what is the speed of a photon ,does it depends upon medium taken?

[@lecturesbywalterlewin.they9259]

photons have no rest mass. They therefore travel with the speed of light. which is c/n. n is the refractive index of the medium. I cover all this in great detail in my 8.02 and 8.03 lectures.

[@niranjangaur5912]

Sir may you give me the link of videos?

[@monarkthakur7066]

Sir can u tell me the difference between quantum and photon?well i searched it on Google but unable to understand it clearly...can u explain it simply with example?

[@lecturesbywalterlewin.they9259]

quantum means "discrete". All EM radiation are photons E=pc E is the energy of the photon p is its momentum

[@lecturesbywalterlewin.they9259]

en.wikipedia.org/wiki/Quantum

[@monarkthakur7066]

Thankyou so much sir:)

[@monarkthakur7066]

@@lecturesbywalterlewin.they9259 and you really made me love physics:)

[@Rishabhkumar-gp9tg]

i am a graduate in physics. now i want to do my post graduate but don't know which branch to chose because i don't know which branch to leave. please show some light.

[@lecturesbywalterlewin.they9259]

You are now doing a PhD in Physics. That means you have already made a choice in one of the many fields of Physics. It's rare for people to change fields after PhD though I did that as I was offered a golden opportunity, once in a lifetime. My PhD was in nuclear Physics - I changed into astrophysics. If you are considering a change I suggest you discuss that with your PhD supervisor and with friends. I cannot possibly make any contribution.

[@Rishabhkumar-gp9tg]

@@lecturesbywalterlewin.they9259 no, i have just completed my bachelors. But thnks a lot.

[@lecturesbywalterlewin.they9259]

so you now have to find a supervisor which is willing to take you on for PhD. That is a KEY decision in your life. The difference between becoming a Theorist or an observer or experimentalist is the difference between day and nite. In each of these 3 there are several dozen sub topics. You may want to discuss all this with your present teachers and friends.

[@raulrao9242]

Where can I find all assignments to your 8.02x course? Always only find assignment 1 :(

[@lecturesbywalterlewin.they9259]

assignments and exams are posted below the video thumbnails.

[@stargazer7644]

A lapel mic would make all the difference.

[@manardahi8409]

First seen

[@sarathchandravss812]

Greetings Sir, My name is Sarath Chandra and I'm from India. I have an idea for sustainable energy which is mostly linked to space. Kindly revert back sir, I'd love to share it with you and if it works, we can work it out together. Thanks and regards, Sarath

[@lecturesbywalterlewin.they9259]

I never discuss potential Nobel Discoveries which are someone else's ideas. This is a matter of scientific integrity. Plse find someone else.

[@c.u.v2190]

Only 362k subs Shame on us 😡

[@juliefjreide9193]

Hello Hello Hello💕

[@lecturesbywalterlewin.they9259]

Hello 2u2

[@mdeeqm7003]

I didnt understand any thing and i am in my last year in high school