8.01x - Lect 23 - Doppler Effect, Binary Stars, Neutron Stars & Black Holes

[@xavierpoes4985]

By miles, maybe light years, this has been the hardest lecture to get a good understanding from. A lot of good information condensed in just 50 min.

[@SudeepJoshi22]

It's really amazing to watch this lecture after the first image of Black Hole released...!!🤩🤩🤩

[@KeithandBridget]

Not having formally studied astronomy this lecture is packed with insight into fascinating concepts and phenomena. Keith

[@hemkantsonavane7403]

legend of physics lecturer

[@johnbingham6355]

To complement this brilliant lecture it might be worthwhile to investigate Cygnus X1 and its companion Cygnus X2 in the Crab Nebula>

[@ゾカリクゾ]

Dear Prof. when did you firmly decide you wanted to study (and teach) physics? Regards

[@lecturesbywalterlewin.they9259]

age 17 - you can read all about this in my book "For the Love of Physics"

[@tanishqsharma4864]

It is really a very good book and chapter about ww2 is really emotional

[@randomvideosbyarandomguy2709]

Sir how to measure value of change in wavelegth of a star like the magnitude..

[@lecturesbywalterlewin.they9259]

use google

[@randomvideosbyarandomguy2709]

Thank you sir and i am very grateful to you to reply to all my queries......... 🙏

[@ju1cyy656]

@@randomvideosbyarandomguy2709 lmfaooo

[@carultch]

@@randomvideosbyarandomguy2709 Spectral lines. We know what spectrum each of the elements common to stars will produce or absorb. When there is a shift in the spectral lines from where they ordinarily are from a laboratory sample of the element, or from the quantum models of what the element should produce, that shift enables us to conclude an optical Doppler shift.

[@deepakmorwani7246]

Sir you are amazing

[@good_to_see_you_again]

Simply he is the messenger of physic

[@darrylwillard2419]

Professor Ewin, I enjoy watching your lectures here on KZbin. I also watch David Susskind's lectures too. Professor Susskind has a different teaching style and it's more mathematical in nature. Do you know Professor Susskind by chance? I have taken some courses from the World Science U online. They covered the theory of inflation in the early Universe. Do you think the Many Worlds Version or a universe that contracts and bounces starting over again. Thanks for sharing your videos, Professor. 😊😊😊

[@lecturesbywalterlewin.they9259]

Susskind is a great lecturer. I have never met him. Multiverse is at the forefront of research in cosmology. My former colleague Professor Max Tegmark at MIT is a world authority on it.

[@morgangraley1049]

41:43 Dr. Prof. Lewin, how do you feel about your description re: black holes now that we have actual pictures of them? Do you think the physics holds up or needs to be revitalized with this new information?

[@Shreshth76]

At 24:58. Sir, can you brief the reason why and how the semi-major axis is r_1 + r_2 in Kepler's Third Law?

[@lecturesbywalterlewin.they9259]

Who on Earth wants to use Kepler's third law. Nothing is more basic than Newton's Universal Law of Gravity. If that is not clear, use google.

[@Shreshth76]

I've got it sir! I was heading in the wrong direction. The two stars are revolving around a common center of mass. So the the gravitation force on each star must provide the star with a centripetal force in order for it to go around in a circle of specified radius. From which the period of revolution follows immediately. Thanks Professor for the guidance..!!! :)

[@jcr1138]

Professor Lewin, how would gravitational waves fit in this lecture? Or will they have their own lecture?

[@lecturesbywalterlewin.they9259]

I gave Lect 23 of 8.01 in the fall of 1999. Gravitational wave detection was not even at the horizon.

[@BattleBunny1979]

23:33 oh some exam drama! What did you do? :-)

[@amedeofilippi6336]

Very brilliant lecture, however it is possible to think of black spheres replacing black holes, I mean of spheres whose radius are asymptotically close to that of Schwarzschild considering the constant spinning momentum when stars start shrinking and the inevitable enormous centrifugal forces that would oppose to gravitational force and possibly avoid any singularity.

[@lecturesbywalterlewin.they9259]

no that's definitely not the way to think about a BH. If you have the time, take a course in GR.

[@surendrakverma555]

Happy to listen excellent lecture. 😀😀😀😀😀😀😀😀 Thanks and Regards.

[@lecturesbywalterlewin.they9259]

Most welcome!

[@qcislander]

About people in the hall not hearing Doppler changes from that whirling-tube-thing... It's made of plastic that's pretty transparent sonically *and* it's many centimeters long (about 1/2 the radius of the swinging tip's path). Think "dispersion". It's not, in the sense normally meant... but that's what it acts like to nearby ears. :-)

[@godson200]

But what was weirder was that it was audible to us, listening to the microphone in walter Lewin t shirt. Which means it was audible to walter lewin sir also

[@rashmi6571]

Professor Lewin, At 8:21 you said , that in the case of sound, we should not move, but in case of light, just relative motion is enough . Did you mean that if I sat in a car and raced towards a parked cop car I would not experirience Doppler effect?

[@lecturesbywalterlewin.they9259]

There is Doppler effect with sound and also with EM radiation. For EM radiation you need Einstein's theory of SR - Watch my lectures or use google

[@five5059]

Sir, at 31:32 you said that the velocity of a body falling onto the neutron star is 70% the speed of light, my question is now, since I know according to STR nothing can travel faster than the speed of light, what if we let the object fall onto the star that has such a mass and radius that will give us velocity greater than the speed of light, would then the object that is falling have constant speed c when it reaches the speed of light? Btw I really appreciate your work and what you do, your students are so lucky to have you as a professor :)

[@lecturesbywalterlewin.they9259]

the speed at which any object falls onto the surface of a ns is independent of its mass and radius.

[@five5059]

Lectures by Walter Lewin. They will make you ♥ Physics. Maybe I said it the wrong way (english is not my native language, sorry for that) I meant the mass and radius of a ns. What if the mass of the neutron star is that great, that we get v>c

[@lecturesbywalterlewin.they9259]

The maximum speed that a falling object can achieve is the speed of light as it reaches the event horizon of a black hole. Any ns whose mass exceeds 3 solar mass would become a black hole.

[@five5059]

Lectures by Walter Lewin. They will make you ♥ Physics. Thank you so much professor for your answer :D

[@youssefemad8131]

Can someone tell me how to derive the doppler effect equation and the EM waves equations

[@lecturesbywalterlewin.they9259]

I do both in my 8.02 lectures

[@youssefemad8131]

@Lectures by Walter Lewin. They will make you ♥ Physics. thanks , I am 15 years old and I am amazed by your lectures and I am trying to learn university level physics because I want to become a theoretical physicist and I will never forget your lectures

[@adithyanthrithallor6559]

25:00 professor please tell me Time period we get after applying Kepler's law is what time period, i mean which of the two object's time period. I have also a doubt in the equation you written. Why there is (r1 +r2)^3 in Kepler's equation. There suppose to be semi major axis, why is semi major axis is r1+r2

[@lecturesbywalterlewin.they9259]

in a binary system of 2 stars, both stars have *ofcoz* the same orbital period about their common COM. Plse watch my lecture again.

[@adithyanthrithallor6559]

@@lecturesbywalterlewin.they9259 I got profosser. I was bit confused at first

[@BruinChang]

In the derivation of escape velocity, the kinetic energy is equal to 1/2mv^2. But if the speed of the escaping object were reaching the speed of light, is the formula of escape velocity still workable to running away out of event of horizon?

[@lecturesbywalterlewin.they9259]

escape velocity from the event horizon is c

[@prakharbhalla9461]

sir at 11:40 you used the frequency of vibration of charge to calculate the wavelength of EM wave produced. Now since EM wave can also be produced by charge accelerating during its motion in a straight line, then in this condition how to calculate the wavelength of EM wave thus produced?

[@lecturesbywalterlewin.they9259]

watch my 8.03 lecture in which I discuss the relation between a and Power in EM radiation.

[@abdullahalsakka]

Sir in 31:25 the speed as you said is 70% of the light speed so shouldn’t we use SR to determine its energy because you said that the energy will be high because of 1/2mv².

[@lecturesbywalterlewin.they9259]

KE=(gamma-1)*mc^2 here m is the restmass. if v/c=0.7, then gamma is 1.4. Thus SR is not really needed bcoz of the many uncertainties involved. We made some simple approximations which is OK here

[@abdullahalsakka]

Lectures by Walter Lewin. They will make you ♥ Physics. Thank you sir

[@pratapsinghkanishk]

At 25:38 while discussing binary stars, finding Time period of the 2 stars from λ(t) it is said that "these better be the same". Why did we take them to be same or how was this deduced?

[@lecturesbywalterlewin.they9259]

Einstein's SR

[@pratapsinghkanishk]

At 29:49 why did we use radius of Neutron star in the formula of gravitational potential energy?

[@lecturesbywalterlewin.they9259]

accretion in X-ray Binaries

[@pratapsinghkanishk]

I didn't get that. Shouldn't it be the distance from the centre of the neutron star to the object?

[@lecturesbywalterlewin.they9259]

watch 27:00 - 40:00

[@samarthsai9530]

Sir I wanted to ask 2 questions, first that how do I choose the perfect equations for solving problems in physics and also that how do I think like physicists (I want to be a physicist), I mean I don't want to do any guess work, like if I want to solve a problem, the solution should come to me naturally not by any guess work. Thank you.

[@lecturesbywalterlewin.they9259]

>>> how do I choose the perfect equations for solving problems>>> your approach is 180 degrees wrong - Physics is NOT about eqs, Physics is about concepts. I only 20 about 20 eqs. e.g, F=ma I do know.

[@samarthsai9530]

Lectures by Walter Lewin. They will make you ♥ Physics. Thanks for your valuable reply. So now the question should be how to apply the problem solving skills learnt from concepts effectively so as to make an appropriate strategy for problems.

[@yukikomatsu2447]

Sir, I have a question. At 10:14, you said f' = f(1+vcosθ/c). In lecture 8.01x - 29, you also said the same thing with sound at 34:03. However, I learned that the received frequency would be f' = f*(c±vr)/(c±vs), where vr is the speed of the observer and vs is the speed of the sound source. Since the sound source is the one who is moving this time, isn't it f' = f*c/(c-vcosθ)? You mentioned special relativity thing but the sound has nothing to do with it, so I'm confused.

[@lecturesbywalterlewin.they9259]

for me to switch lectures is too time consuming. If you have a question in Lect 29 then send me the question when you watch lecture 29 + how many minutes into the lecture. Do that for every question,

[@lecturesbywalterlewin.they9259]

the way I cover Dopper Shift for sound is correct. I cannot add to the clarity of that lecture and I suggest you watch it again. *Key is that the shift in freq depends on both the motion of the receiver and of the transmitter.*

[@yukikomatsu2447]

@@lecturesbywalterlewin.they9259 Thank you for your fast reply. I think I finally understand. It seems that my confusion stemmed from the fact that I interpreted the speed of sound/electromagnetic waves as the speed you would measure if both you and the object were stationary. I realized the speed you were referring to was the one you would measure in this situation (the speed of light is always c anyway). Therefore, my equation becomes the same as yours by substituting c±vs to c and vs-vr to vtr.

[@anirudhkailasam3505]

great lecture sir... it's great to know how we could have seen the universe if we had infra or X ray eyes.....but 1Q suppose we see the spectrum of a star then what do we do here on earth to compare it with and say that the star is moving..... my apologies since it could be a dumb Q... .

[@lecturesbywalterlewin.they9259]

Doppler shift. You are not likely to see X-ray spectral lines, thus you should use visible light.

[@anirudhkailasam3505]

thank you.... do we use prisms to split the light in universe to study the spectrum? in the labs I mean

[@lecturesbywalterlewin.they9259]

No we use diffraction gratings they have a MUCh higher resolution. Also google Fabry Perot interferometer.

[@anirudhkailasam3505]

dank u wel Sir it's so overwhelming to receive a reply almost instantly 🙌

[@somanathmaharana3219]

Professor, at 0:14 you said that the speed of sound depends a little bit on the temperature. How does it depend on the temperature? Regards, For the Love of Physics

[@lecturesbywalterlewin.they9259]

v=331.5+(0.6xTc) m/s Tc is degrees Celsius

[@somanathmaharana3219]

Thanks a lot, Professor.

[@lecturesbywalterlewin.they9259]

:)

[@syedferoz2188]

Professor can we apply basic thermodynamics to explain the temperature dependance instead of using that formula all the time?? ps: i was inspired by many of your lectures sir thank you for that

[@Nobody-y1m]

30:46 why does K = G.m.M(ns) / R(ns) ?

[@lecturesbywalterlewin.they9259]

because 1+1 = 2

[@marccowan3585]

Professor, in this lecture, you said around half of all star systems are binary star systems. I was wondering if there was a simple explanation to why they are so common and that it is not dominated by star systems containing only star such as our own. Thank you

[@carultch]

Yes. The short answer is conservation of angular momentum. As the proto-stellar nebula collapses to form the star, not all of the matter can remain in the star with stability. As it speeds up its rotation rate, some of the matter will achieve escape velocity before it can settle to the surface of the newly formed star. One way around this, is to form a planetary system at the same time, with massive Jupiter-like planets. Another way around this, is to form binary stars. Another reason for it, has to do with the fact that most of the naked eye stars you see in our sky, are the brightest of the stars in terms of total luminosity. One way the stars get that bright, is to start with enough mass to glow as giant blue stars during their main sequence lives, like Rigel. Another way stars get that bright, is by being part of a binary star system, and glowing at the collective luminosity of both constituent stars.

[@aksarora120]

Amazing

[@hanmantsartape2586]

You are genius sir

[@123MSRAO]

sir i have 2 questions on dark matter and dark energy. one of my friend said that we can easily define them with this example " if we keep 2 points on a inflated ballon and when we blow it the distance between them increases and the vol. too increases so he said that the distance between the points is dark matter and the air inside is dark energy which keeps expanding with universe " do u support this? and next one is about black holes. will black hole expand when it absorbs mass from surrounding?? thank u.

[@lecturesbywalterlewin.they9259]

I'd like to suggest that your friend takes a course in cosmology.

[@lecturesbywalterlewin.they9259]

>>the distance between the points is dark matter and the air inside is dark energy which keeps expanding with universe>>> *This is nonsense.* Your friend should take a course in cosmology.

[@123MSRAO]

Lectures by Walter Lewin. They will make you ♥ Physics. Actually me and my friend are studying 11th class. Thanks professor

[@123MSRAO]

Professor which would be a better path to choose. Astrophysist or astronaut coz I like space I want choose one from these both

[@simmiboyal7331]

Why cosmic rays are not considered as an electromagnetic radiation but still they're electromagnetic waves?

[@lecturesbywalterlewin.they9259]

Cosmic rays are particles which have rest mass. EM radiation (like light photons) have no rest mass. *Cosmic rays are NOT EM radiation.*

[@simmiboyal7331]

@@lecturesbywalterlewin.they9259 Is it the only factor which determine electromagnetic radiation? Thanks for solving my query

[@prathmeshshukla9975]

Sir I have a random question, Like when in a big container of water, we put a hole in it, Why does the water create a Spinning motion, When doesn't the water simple come out in a streamlined manner

[@lecturesbywalterlewin.they9259]

use google If you could create perfect starting conditions, with no external forces acting on the body of water except the force due to gravity, and a perfectly symmetrical sink and hole, the water would flow straight in instead of spinning. In reality, the minor imperfections of the sink, the slight starting impulse that is given to the water by the way it is emptied into the sink, etc., all combine to give it a certain spin. From there, it gets to a stable flow which is not perfectly symmetrical, so it turns as it drains away.

[@georgen9755]

velocity 340 m/sec , frequency in the spectral band

[@prakharbhalla9461]

At 11:37 sir you told that for EM wave charges are vibrating. Is EM wave produced by only vibrating charge or charge with any acceleration?

[@lecturesbywalterlewin.they9259]

what I said is correct

[@prakharbhalla9461]

@@lecturesbywalterlewin.they9259 so sir according to your statement a charge moving in a circle won't radiate EM wave. Am I correct?

[@lecturesbywalterlewin.they9259]

@@prakharbhalla9461 when a charge is accelerated it radiates EM radiation. Thus when a charge goes around in a circle it radiates, HOWEVER it does not radiate in moving around in an atom. Classical EM does not apply to QM

[@prakharbhalla9461]

Sir that means that Maxwell statement is only applicable in classical physics? and sir how does Rutherford model was not part of quantum mechanics since it was also a study of very small?

[@lecturesbywalterlewin.they9259]

google "Rutherford"

[@turtle8558]

It got a little confusing during the lecture with the terms of electromagnetic waves and electromagnet radiation, do these equations work for both waves amd radiation?

[@carultch]

Electromagnetic waves and electromagnetic radiation are two interchangeable terms. Radiation in general means something emitted radially outward from a source. Since it also has the form of a wave, and depends on the electric and magnetic fields, it is called both electromagnetic radiation and electromagnetic waves.

[@suniljoshi5315]

Thank for these wonderful gem of lectures sir!!! I had a doubt sir, Is wavelength for a wave similar to amplitude for an oscillation?

[@lecturesbywalterlewin.they9259]

no wavelength and amplitude are unrelated

[@necatigoren9187]

Sir, i do not understand why the matter that fall towards the neutron star already heats up before it reaches and explodes. How can the kinetic energy of the matter convert into the heat before it bumps into the neutron star? You talked about this about 34-36 minutes.

[@lecturesbywalterlewin.they9259]

watch my lecture "The Pioneering Decade of X-Ray Astronomy"

[@necatigoren9187]

@@lecturesbywalterlewin.they9259 ok sir, thank you so much.

[@ydshashidhar]

We can imagine sound waves squished and stretched because of the air medium and velocity, but why Doppler effect is seen with light in space where nothing is present.

[@lecturesbywalterlewin.they9259]

kzbin.info/www/bejne/pYqbkp5tnd1-ptk

[@u2b84]

Is there any known connection to American politics, Dems (blue) and Rep (red)? Dems (gov't approaching) v Rep (gov't receding)? Even if there isn't I won't be able to forget this now. I love this course, and I'm glad I still have more to watch. Thank you professor!

[@carultch]

No connection. The history of the colors that represent the political parties does not consider the Doppler shift.

[@elmirelmir842]

Black holes are literally ideal point masses.)

[@yourboystealth]

With the Neutron star- Marshmallow problem, why is the radius of the Neutron star used for R instead of the distance between the marshmallow and the Neutron Star's center of mass? That would mean the marshmallow is falling from the surface of the neutron star to the center, does it not? But clearly it is farther away so I'm confused as to the usage of only the radius of the Neutron star. Clarification would be appreciated.

[@lecturesbywalterlewin.they9259]

watch my lectures or read some articles about accreting neutron stars

[@nithinmalladi405]

The woman at 17:39 is so pretty! But considering the year when this video was recorded, she would almost have the same age as my mum😅😅!

[@SandeepKumar-up9yl]

Sir I want to know what will happen if the two parallel plates of a parallel plate capacitor will acquire same potential or there is no potential difference between them.. Can we increase its capacitance or can we give more charges to the capacitor?

[@lecturesbywalterlewin.they9259]

watch my 8.02 lectures where I cover this

[@physicswithprakash2214]

Sir, what we can measure from the Doppler shift measurement is vcos(theta) not v. But the calculation of R requires v, which is still not known to us as we have no information of cos(theta). So, how do we calculate R?

[@lecturesbywalterlewin.they9259]

watch my solutions to relativistic Doppler shift problem #108. That tells the whole story kzbin.info/www/bejne/hZbKY2ygZ6qNn5o

[@cypriancyganek6830]

Professor, do you have a slice of melon on your chest? what for? :)

[@lecturesbywalterlewin.they9259]

:)

[@azizhunzai2755]

Lots of love and respect from Pakistan. .......would that i was in that class....

[@mohakjani6220]

sir can u explain about white holes ?? & do u think so what is on the other side of black hole ???

[@lecturesbywalterlewin.they9259]

en.wikipedia.org/wiki/White_hole

[@mohakjani6220]

Thank u professor

[@vadimtikanov]

How do astrophysicists know the original wavelength of the source? E.g. - the approaching star radiates red light, which observer on the Earth perceives as blue (due to Doppler shift). How does observer know that original light was red?

[@lecturesbywalterlewin.they9259]

all stars have absorption lines in their spectra. These lines can easily be identified. They have known frequencies in their rest frame. We observe all lines shifted by the same factor. That will give us the radial velocity between us and the star.

[@vadimtikanov]

I see. And if the star was standing still and then started moving I can compare the lines before and after. But if the star is moving for thousand years already, I can only see the shifted lines, can't I? How do I define the original spectrum to compare with?

[@lecturesbywalterlewin.they9259]

If the star is at a distance of 100 light years from us you observe the radial velocity at the time that the star emitted the light (thus 100 years ago). If the star is in a binary system with an orbital period of 45 days you will see the lines move back and force in the spectrum. If the mean radial velocity of the star is about 100 km/s and its radial orbital velocity is 15 km/sec you will observe in 45 days a change from 85 km/s to 115 km.s and back to 85 km/s. If both stars in the binary system are visible you will see two pairs of lines (they will differ as the temperature of the 2 stars will be different). If ONLY one star of the 2 is visible, you will be able to CONCLUDE with certainty that that star is in a binary system with an orbital period of 45 days. That's called a "spectroscopic binary").

[@vadimtikanov]

I see now. Thank you.

[@sanjitmukherjee7018]

sir in binary star system you chose especially circular orbit and concentric system but if it was not a concentric system like elipse non-concenrtic system how can I figure out the distance from CM??

[@lecturesbywalterlewin.they9259]

ocoz in almot all binary systems the robits of the 2 stars are elleptical. I cover the math in my books Accretion-driven Stellar X-ray sources Edited by Walter H.G. Lewin and Edward P.J. van den Heuvel Cambridge University Press, 1983 ISBN 0 521 24521 4 X-Ray Binaries Edited by Walter H.G. Lewin, Jan van Paradijs & Edward P.J. van den Heuvel Cambridge University Press, 1955 ISBN 0 521 41684 1 hardback ISBN 0 521 59934 2 paperback Compact Stellar X-Ray Sources Edited by Walter H.G. Lewin and Michiel van der Klis Cambridge University Press, 2006 ISBN 978-0-521-82659-4 Hardback ISBN 978-0-521-15806-0 Paperback

[@sanjitmukherjee7018]

Thanks sir

[@physicswithprakash2214]

Sir, if we can only measure the radial component of velocity not the actual velocity by Doppler shift then, how can we presume to measure the radius of the orbit using Doppler shift since the measurement of R requires the knowledge of v?

[@lecturesbywalterlewin.they9259]

question unclear. The relativistic Doppler shift depends on the angle of v relative to the view direction and on gamma (time dilation which always causes redshift).

[@physicswithprakash2214]

Sir, what we can measure from the Doppler shift measurement is vcos(theta) not v. But the calculation of R requires v, which is still not known to us as we have no information of cos(theta). So, how do we calculate R?

[@imaseeker100]

There are master teachers. Then there is prof Lewin.

[@good_to_see_you_again]

Where is Indian professor, see this man!???

[@suniljoshi5315]

Sir, I had another doubt. According to the Special theory of relativity the speed of light is constant irrespective of relative motion, Hence how can doppler effect be observed for an EM wave?

[@lecturesbywalterlewin.they9259]

relativistic Doppler shift has time dilation

[@lecturesbywalterlewin.they9259]

The motion of a light source has no impact on the *speed* of light, it affects the light *wavelength* which is its color (and ranges far above and below what we can see). This is similar to how the speed of a sound source affects its pitch (not the speed of sound); an approaching train has a higher pitch which changes to a lower pitch as the train passes and then recedes from the listener. This is called “Doppler shift.” It was the Doopler shift of white starlight to red called “red shift” of all other galaxies relative to our own that led to the discovery that all galaxies are moving away

[@suniljoshi5315]

@@lecturesbywalterlewin.they9259 THANK YOU SO MUCH SIR, It is an absolute honor to get my doubt clarified from you sir!!!

[@manavsivaram15]

Doesnt Doppler effect occur when the transmitter is stationary and the observer is moving in case of sound waves?

[@lecturesbywalterlewin.they9259]

yes and I cover that in one of my lectures. In the case of sound there is a LARGE difference between moving transmitter or moving receiver. That is not the case with EM radiation (astronomy).

[@mrkakotube]

Hi professor, during the lecture you insist that the observer must be in the plane of rotation of the binary system. Is this always the case if the observer and the systems are in the same galaxy?

[@lecturesbywalterlewin.they9259]

there is no need for the observer to be in the orbital plane. Simple trigonometry

[@mrkakotube]

@@lecturesbywalterlewin.they9259 why then would you stress that when drawing the system in the blackboard?

[@lecturesbywalterlewin.they9259]

@@mrkakotube *to derive the physics you obviously place yourself in the plane of the orbits.* To move yourself outside that plane is simple trigoniometry (circles become ellipses).

[@mrkakotube]

@@lecturesbywalterlewin.they9259 I understand. Thanks a lot Sir!

[@123MSRAO]

professor i learned that the kinetic energy or any thing is conserved will be transferred to another form of energy, so can we say that when rotating a body due to this we get sound as the transformed energy

[@lecturesbywalterlewin.they9259]

The conservation of energy is universal. Heat and sound are forms of energy.

[@123MSRAO]

can u explain it clearly pls

[@lecturesbywalterlewin.they9259]

google conservation of energy and take it from there

[@123MSRAO]

Professor which would be a better path to choose. Astrophysist or astronaut coz I like space I want choose one from these both

[@lecturesbywalterlewin.they9259]

that depends on someone's parameter space. ONLY you can decide.

[@frozenglobe]

would like to ask a question: (sort of related because black holes and hawking radiation) are all virtual particles created on either side of the event horizon virtual photons? do they always leave the black hole as photons or can there be other particles?

[@lecturesbywalterlewin.they9259]

use google. Hawking radiation is throughly discussed on the web.

[@frozenglobe]

aye professor

[@Mypersonalyoutube123]

Hello professor Walter Lewin, Are you aware of a binary star system that is easily visible from light polluted skies, such that we are in the same plane as the two stars? If so, can you please let me know it's name and coordinates. I have a telescope at home and would love to see if I can identify the velocity, period, and radii of the two stars using a spectrometer and the equations that you presented in this lecture. Thank You!

[@lecturesbywalterlewin.they9259]

it depends on the power of your telescope - use google and I am sure you will be successful.

[@Mypersonalyoutube123]

@@lecturesbywalterlewin.they9259 the telescope is a 5 inch reflector. I hope it will work.

[@lecturesbywalterlewin.they9259]

a 5 inch is plenty!!! have fun www.skyandtelescope.com/observing/celestial-objects-to-watch/pretty-double-stars-for-everyone/

[@SoraSteenvoort]

First of all, many thanks for sharing your wonderful lectures, they sure make physics a lot easier to understand! However, this video raised one question about the tuning forks - I noticed that the "440Hz" fork was rather off, vibrating at about 424Hz, which is closer to a G# than an A... at first, I thought it might have to do with technical details of the recording, however, the 256Hz tone was on the mark, so obviously the recording isn't to blame... so right now I'm wondering - is there a simple physical explanation for this tuning fork to be off by more than half a semitone? (My apologies if an explanation for this is given in a later lecture, I have only watched the other 22 8.01x lectures so far.)

[@lecturesbywalterlewin.they9259]

I have no clue. It's baffling that if it really is424 Hz that 440 is edged into the fork. I could have easily measure the frequency with an oscilloscope but I never did.

[@pranjaljain6762]

at 46:00 you say that a black hole(15 solar masses) sucks a star of 30 solar masses. does the mass add up to make a black hole of 45 solar masses.If not then where the mass go

[@lecturesbywalterlewin.they9259]

yes it becomes a 45 solar mass bh

[@atharvas4399]

Professor? do you have a lecture explaining special relativity? Relativistic corrections come up a lot throughout many of your lectures. I was thinking of getting a deeper understanding of special relativity and also general r. if possible

[@lecturesbywalterlewin.they9259]

SR is covered by many lectures on the web.

[@atharvas4399]

Professor, Have you taught thermodynamics, or optics or waves?

[@atharvas4399]

oh i see you have taught waves and some optics in 8.03

[@tapankumardas3292]

3:22 oh!!! u should remember that some of ur viewers may using headphones 🤯

[@louisesuter6889]

Jeez that hurt my ears lol I laughed though

[@vishwajeetsingh9267]

I have seen in a lot of lectures why do u always wear a food item badge??

[@aneeshmalapaka8363]

sir, i was reading chapter motion in straight line and i came across a formula which included air resistance and is constant. then it is given that time of ascent of a body projected vertically up is mu/mg+R and time of descent is mu/mg-R, where R is air resistance and now iam getting a doubt that while going up or at time of ascent the velocity is gradually decreased by air resistance and same while the descent. is this correct ?? or different . second is if vel. is decreased while ascent should there be a negative sign in formula of ascent??

[@lecturesbywalterlewin.they9259]

how many minutes into the lecture?

[@aneeshmalapaka8363]

i didnt get that

[@iamcomputernerd1239]

@@aneeshmalapaka8363 Because in ascent the air resistence is downwards so is gravity.But in desent gravity is downwards but air resistence is upwards.

[@CM-Gram]

You said stars cant have more than 3 solar masses before collapses how can a 30 solar mass star exist?

[@lecturesbywalterlewin.they9259]

I NEVER said that - you misunderstood and took things out of context. If neutron stars get a mass larger than 3 solar masses (in core collapse of very massive starts) they become a black hole. Please watch this lecture in which I discuss stellar masses and supernovae and core collapse of very massive stars. kzbin.info/www/bejne/ZqizZnqGe6Z_eck

[@CM-Gram]

Oh now i see I think misunderstanding can happen when i watch 8 videos of your one shot :D very addictive indeed!

[@lecturesbywalterlewin.they9259]

:)

[@prakharbhalla9461]

Sir is frequency of vibration of charge and frequency of EM radiation same or different?

[@lecturesbywalterlewin.they9259]

question unclear. watch my 8.03 lectures where I cover how an accelerated charge produces EM radiation.

[@mrkakotube]

Houston we have a problem.. I am doing the assignments of 8.01. I do not have the same book used for some problems (Ohanian version 1? 2?) but I have the third version. The issue is that the number of the problems have switched, so I had to go to the MITOPENCURSEWARE to look at a table were there was a correlation of the problems between the different versions. Well.. Your lectures on 8.01 as of 19999 have just dissapeared from this site. Do you have that correlation by any chance? Could you tell me what version were you using of Ohanian in those assignments? I will write to MIT as well to see if they can help me.

[@lecturesbywalterlewin.they9259]

8.01 was Ohanian 1.

[@lecturesbywalterlewin.they9259]

8.01 Physics Hans C. Ohanian Physics 2nd edition W.W. Norton & Company ISBN 0-393-95748-9

[@mrkakotube]

@@lecturesbywalterlewin.they9259 Thanks a lot Sir.

[@FlAristone]

fmax over f cannot be equal v, something is wrong about that.

[@lecturesbywalterlewin.they9259]

how many minutes into the lecture ?????

[@FlAristone]

@@lecturesbywalterlewin.they9259 just around (before) the minute 6, when you say that f'max / f gives the value of the sound velocity

[@FlAristone]

@@lecturesbywalterlewin.they9259 BTW, congratulations on all these excellent lectures. I use them as inspiration to give my lectures down here.

[@lecturesbywalterlewin.they9259]

@@FlAristone >>>something is wrong>>> *what is wrong here is that you did not listen to what I said!*

[@theblueberry8585]

really awesome lecture, learnt a lot MY EARS FRIGGING HURT ASFDHSJKGSKJDHFGKJAHGFKJHG

[@kishanSingh-qd3zr]

Love from India

[@nithinmalladi405]

Which year was this lecture recorded?

[@lecturesbywalterlewin.they9259]

1999

[@louisesuter6889]

Whoa I was born 5 years after that lol

[@riteshkumarsingh6473]

Sir is this the swahcherchild equation

[@juliankrassnitzer8772]

no

[@manassrivastava7448]

Sir I want some help from you. I recently made an observation but I am not having confidence on it at all. Please help me. I fixed a stopwatch on disc and attached with a very powerful motor. When I stated this system and ran it for about 0.5 hour at very -very high frequency of rotating disc the I observed that time recorded by stopwatch attached was about 29.98 min but according to stopwatch in my hand it was exactly 30 min. Sir can you cross check my observation if possible. Sir I want to know its experimental error or something else.

[@lecturesbywalterlewin.they9259]

What were the uncertainties in both measurements?

[@manassrivastava7448]

Sir I do not know as its an automated system programmed by me . Sir can it be sort of time dialation

[@lecturesbywalterlewin.they9259]

time dilation IMPOSSIBLE. By the way any measurement without knowledge of uncertainty is meaningless.

[@manassrivastava7448]

Lectures by Walter Lewin. They will make you ♥ Physics. Sir how can we know that time dilation is impossible if we want to prove it on paper

[@123MSRAO]

What's ur opinion on dark matter and dark energy prof

[@lecturesbywalterlewin.they9259]

use google

[@jnanadas1250]

Sir I have a problem which I wasn't able to solve. It would be nice if you could help or atleast give me some hint.

[@lecturesbywalterlewin.they9259]

what's the problem. Tell me in few words.

[@jnanadas1250]

+Lectures by Walter Lewin. They will make you ♥ Physics. A sound wave of certain frequency was send through blood vessels. when it was received after reflection the frequency was less. speed of sound in blood is given. speed of blood is asked to find out.

[@lecturesbywalterlewin.they9259]

Doppler effect! The radar freq used by Police is received after reflection off cars. It increases as the car approaches, it decreases when the car receeds.

[@jnanadas1250]

+jnana das I was looking for like a formula... I will try with this...

[@jnanadas1250]

+Lectures by Walter Lewin. They will make you ♥ Physics. In my case neither the source nor the observer is moving. Just the medium of sound is moving, but in case of police car the source is moving. This makes the difference.

[@123MSRAO]

professor if shout aloud in an open space i mean with trees ets or in a city with peoplre and i shout to y friend its not heard to the person at 100m why. inspite of keeping hands near mouth like how all others do he cant hear . he has no ear problems and i have the loudest sound in my family( hehehehe)

[@lecturesbywalterlewin.they9259]

this is an ideal question for Quora

[@lecturesbywalterlewin.they9259]

I do not answer Quora questions.

[@livintolearn7053]

You mentioned whirling a source of sound in a circle and placing the observer somewhere outside the circle in the plane of the circle, and then you said that f' will be at its maximum when the source's velocity is parallel to the line connecting the observer to the center of the circle and in the direction of the observer. I would point out without any explanation (because I am too lazy to explain everything) that the above would be the case only if the angle between the line joining the observer and the source of sound (when the source's velocity is parallel to the line connecting the observer to the center of the circle) and the line joining the observer and the center of the circle the source makes as it goes around = approximately 0 degrees. You did not mention that in the lecture... Pls write back if you require the explanation (you will not need it....)

[@lecturesbywalterlewin.they9259]

>>>> then you said that f' will be at its maximum when the source's velocity is parallel to the line connecting the observer to the center of the circle and in the direction of the observer.>>>> I never said that. When the source of sound is moved in the direction of the observer the observer will hear an increased frequency. When the velocity component in the direction of the observer is a maximum, then the observed freq is the highest. End of story!

[@livintolearn7053]

Oh yeah....thank you very much for the clarification.

[@shrinivasaprabhu8931]

what is that on your shirt?!

[@louisesuter6889]

Water melon slice lol

[@sreevathsk9906]

Hello professor I'M from India You have given this information in class but you have told the mistakes only to us ...... Did your students recognise the mistakes? If no please inform them because they may loose the concept.. I love your teaching sir . I love to join in MIT and try to come there. Regards Sreevaths

[@lecturesbywalterlewin.they9259]

there is NEVER a mistake in my Physics, however, there can be slips of the pen and slips of the tongue. Any one who watches my lectures should be able to understand that those 2 kinds of slips have NO CONSEQUENCES becoz in all what follows slips play NO role; slips cannot propagate! THINKKKKKK

[@sreevathsk9906]

Thanks very much for your reply sir. There was a misunderstanding . love your teaching sir .

[@sreevathsk9906]

I understand it sir .

[@sreevathsk9906]

Sorry sir for the message please don't mind it😊

[@AngraST]

Using the same formula to calculate the escape velocity and the radius of the event horizon, could I not use the same formula for any body with mass? I don't see why it would only apply to black holes, since the escape velocity formula does not change with different bodies, and as I understand we could equate V=c to any body (independent if it's a Black hole, earth or tennis ball). So logically since the center of mass behaves as if all the mass is concentrated onto that point, wouldn't a event horizon form around the center of mass? I can see that physically that doesn't seem to make sense, but what I don't understand is why would a general formula permit the calculation of a event horizon for a black hole, and not for a center of mass? The formula does not know whether the object is a black hole or not, all it does is equate V=c while calculating escape velocity. Oh, and thank you for answering my previous questions, I am still confused about some things but enough to continue pushing on and grasping the key concepts! =)

[@lecturesbywalterlewin.they9259]

We now know that any body for which the escape velocity, according to classical physics, is c or larger than c is a black hole. Without any knowledge of SR and GR, it was already pointed out in the 17th century what M/R would be for the escape velocity to be larger than the speed of light.

[@fashelkhan5212]

Sir can we calculate the age of star ? And if yes .then how ?

[@lecturesbywalterlewin.they9259]

read articles about stellar evolution. Our sun is about 5 billion years old

[@fashelkhan5212]

Lectures by Walter Lewin. They will make you ♥ Physics. But sir how can I believe on it .

[@georgen9755]

the CEO of ethiraj college has accomplished so much that there is no need for ♀/♂ to learn all these stuff , ............ it is not possible for us to locate ashoka trees in deserts .... .....

[@aaaab384]

These students will never appreciate subtle differences in frequency if they don't know the rudiments of music theory.

[@lecturesbywalterlewin.they9259]

+aa You underestimate my MIT students

[@imaseeker100]

I think it's the other way around.

[@dione57]

Dear Professor, did you read this news about the NICER x-ray telescope just installed on ISS to study neutron stars? It already has exciting observations of x-ray binaries when it's calibrating the instruments! phys.org/news/2017-07-nasa-neutron-star-mission-science.html

[@lecturesbywalterlewin.they9259]

Yes I knew about this. Its great news. The article mentions GX 301-2. My group discovered this X-ray Source and we gave it its name (1969 balloon observations from Australia).

[@nihaal7750]

Is this supposed to be a funny video?

[@BroadeningHorizonsos]

Do physics have any evidence of god.......

[@lecturesbywalterlewin.they9259]

There is no scientific evidence for the existence of a god. However, keep in mind: "the absence of evidence is not the evidence of absence". Apart from this, 25% of the people in the US are atheists and the number is rapidly growing.

[@BroadeningHorizonsos]

If I want to do research in this ....I have my own scientific theory then, will that come under physics

[@lecturesbywalterlewin.they9259]

PLEASE do not waste your time.

[@ashishkumarpemmaraju2491]

loved your reply sir....loved it

[@abdullahalsakka]

Sir in 31:25 the speed as you said is 70% of the light speed so shouldn’t we use SR to determine its energy because you said that the energy will be high because of 1/2mv².

[@youcube1]

I agree

[@talentlogic1512]

Amazing.