Lec 6 | 8.01 Physics I: Classical Mechanics, Fall 1999

[acetote]

WHY sometimes upper &sometimes lower string were broken

[thespetnaz101]

i used this vid to play snakes!

[perepe10]

the string that breaks depends on how he pulls it, when he does it fast the bottom one breaks, and when he does it slowly, the top one breaks, so he wanted the girl to be happy and he broke the one she said XD

I suppose that 12 years later is too late to say this... XD

[LanRous]

Can this law be proven.

NO

Do we believe in it

YES

Why do we believe it

Because all meas......

He is Epic

[lutwey]

"it's non negotiable" who the fuck is negotiating??? lmao

[AlohaBay]

Jeez that thigh massage video is such a distraction...

[AlanKey86]

At 15:04 he states that when sitting on a seat, you are pulled down (by gravity) and the seat pushes back (contact force). This seems to be inconsistent with the further 2 examples he gives. Or have I misunderstood?

[KimmyJRapeAllDaNoob]

@AlanKey86 why would it be inconsistent? The normal force is in the opposite to the applied force. that force can change. the third example shows the applied force to be his hand pushing against the wall. the wall pushes back with an equal and opposite normal force. if it did not, then his hand would go through the wall.

[termopilasify]

Obrigado Professor Wallter Lewis!!! Suas aulas sao incriveis....belo trabalho. (Brasil)

[yayyay533]

Really want to watch this but the quality sucks!

[Lucag120]

This bloke has just lost it harder than Lewis Hamilton when he took out Felipe Massa a few weeks back in Japan!!!!!! He obviously is lacking massive pace and corner vision according to the size of his glasses

[zzzs4]

@Lucag120 LOL! mate, he's even more fucked than Tacklebox going on about plane strain and plane stress elements for a whole hour

Talk about pace, push and acceleration, this guy stacks it harder than Sekhar trying to validate why he hasnt marked the assignments yet

[wideeyedraven15]

@iamsam578 except, asshat, he said he had two identical blocks of cheese. Oh...right, yeah, he did say that.

[virentoo]

interesting class, wish i had a high school physics teacher like this.

[PrettyEpicCinema]

6 people never got a proper education :D

[Mercymanful]

My first thinking is to breake the gravity in space and may think I have a clue ;)

[Zeldakitteh]

I thought Heron was a man. ;_;

[Scurmicurv]

@Zeldakitteh Hero, not Heron! Hero is a Byzantine name; dunno about the connection with the English word.

[simpatic14]

what amazes me is how great the designer of those Chalk Board :)

never seen that in real life xD

[jadx99]

He is an amazing teacher because in the last part he tested a previous concept that an object breaks if its acceleration is too high....amazing!!

[JohnnyTakeAll]

With all he knows about force, he should be able to move the chalk boards without touching them...

[romorano]

YES...he adds accelerations when pull the string, so the bottom string breaks. when he pulled the string slow (no acceleration) the one on top would break..

[Hannanstl]

cant be proven huh? if that was the case wouldnt i just fall through the earth into space? I think gravity is pretty a proven thing.

[nivu007]

doesnt 3 rd law hold for contact forces alone... if then how can earth move toward s the apple when the apple is still falling

[Zeldakitteh]

@nivu007 works for all forces.

[jackzack87]

Apparently, he controls the tension on the strings, and when he applies steady, constant tension, the top string breaks because it carries the same tension as the lower string, plus the 2kg mass in the red block.

When he quickly pullson the string, he surpasses the tension capability in the lower string before the tension can evenly spread to the upper string.

Overall, the tension on the top string is greater by 2Kg, but by a little trickery he gave these guys something to think about..

[TheBrownDoggie]

Does the bottom one break 1st because it accelerates the "massless" string easily but that same acceleration cannot accelerate the 2kg block as quickly. The string starts stretching before the 2kg mass even starts moving. It all depends on how quickly he accelerates the system by pulling the string.

[cowboyweightlifter]

When he applies tension to the bottom string slowly, the acceleration is negligible. So there is no force going through the string, he is essentially just adding more mass to the system, so the top string breaks. However, when he pulls quickly, there is acceleration so there is a force with a short impulse moving through the bottom string. This force then affects the inertia of the block, the inertia however is more than the strength of the bottom string, so it breaks.

[marts1233]

this,guys, is the basis of EVERYTHING in science

[x1x2x3ct]

lulz @ trivial math

[AdmiralCaso]

The professor has tested two different properties of the strings. The first time applied the explosive force, that for reaction applied the strings too, and got broken the one below because it underwent all the force, being closer to the prof. Then the upper one got broken cause of the resisting force, which in the above was already under strain because of the red block.

[SupremeUltimateone]

why did the bottom string break?

[behnamasid]

So, can any one tell me why the bottom string broke first?

[santa6791]

dammit i thought i clicked on the english version of this video

[kakayum]

Thank you MIT for posing this,i wish i can attend MIT ,but i can't..:(..but this really help me a lot since my prof right now is confusing me so much...THANK YOU!!!

[Cifrocco]

"It's non-negotiable!"

[Cifrocco]

He does the dotted chalk lines by holding the chalk at an angle bent away from the direction he is sliding it across the board, it causes it to jam and skip over itself as he does that.

[podgytube]

Hero of Alexandria was a real greek male mathematician and inventor.

Hero and Leander was a greek myth.

With all due respect professor you are very wrong on this one. Though they do sound similar.

[PmJ1515]

i take high school physics and this lecture is great although it moves at a slower pace than my class.

[conneljp]

The tension in the bottom string is greater than that in the top string - when he pulls quickly

[marwanxyz123]

@conneljp thats what i was just thinkg !! :)

[conneljp]

The tension in the bottom string is greater than that in the opt string - when he pulls quickly

[antikill1235]

Love the last demonstration. Using the inertia of the weight to screw with the results.

[CrumbLuke]

There needs to be a lecture on how to do that chalk thing...

[afghanplayr20]

he didnt explain why the bottom one breaks .... is it just random? first the top one broke then the bottom one ....

[Mirorn4]

@afghanplayr20 Physics is not random XD, when he did it quickly, the bottom one breaks because there wasn't enough time to affect the top one. When he did slowly the top one breaks.

[thecomanche1]

@Mirorn4 is this right -he pulled on the string with different forces

the bottom string broke on fast pull because the magnitude of accel just broke that string before the force could be transfered to the 2nd string

2nd string the accel was slow so force within breaking point of string

since the weight + pulling force are in same direction for the top string- when the force gets to high the top one will break

+ something to do with the direction of the forces close?

[Mirorn4]

Hi, I'm not sure about what you said exactly. He applied a force in a particular point, this force needs to be transferred through the medium, if he pulls slowly the force can travel throw the medium. If he pulls fast (talking about velocity, not acceleration), the force just affects the bottom one before the top one, and it take all the force which it can’t stand and finally breaks. Nothing to with direction of forces, it’s a reason of energy transfer.

[thecomanche1]

@Mirorn4 yes thats it,

Im saying

he exeeded the breaking strain of the bottom string

before the force could effect the top string

if he pulled on a metal chain instead, he could never exeed the breaking strain of that chain, so the top string would break every time.

[Mirorn4]

@afghanplayr20Physics is not random XD. When he did it quickly the bottom one broke because there wasn't enough time to affect the top one. You can notice that when he did slowly the top one broke. Physics is not random XD

[MrSnackization]

well he lost me when the chalk came out.

[srjearth22002]

This guy blows my physics professor away... Damn hes good I wish I was taking his class!!!! Thank god for Youtube!!!

[bloomingdedalus]

I love this professor - he is an excellent and superb teacher.

[hkpopfan4lif3]

at 34:31...for the magnitude of T1...why do you use cos 60 for the x component?

[joeybenn]

@hkpopfan4lif3

Well he used 60 degrees because that is the angle. For some reason he didn't label it. He said it earlier. He used cosine because with respect to the 60 degree angle, the "x" component is the "adjacent" side.

[rarsenue]

Online teaching (video) is very good to cater to more students with different learning pace. Slow learning students can replay this session that they cannot do in a traditional classroom.

[mlkage0822]

we learn about the main points of it ..newton's laws and its application basic acceleration and gravity formulas by age 14...is it gud enuf for getting into MIT ????

[boldstrides]

I think physics is amazing. I wish I wouldn't be so afraid of it.

[rampriyadarshini]

can anyone explain me about the string experiment...and why the below and next time the upper string breaks,is it has to do with the force he applied.

[tuscaaa]

The third Law is only valid in two separated objects.

[fedetipit0]

Muy bueno. Tomar clases así sería increible!!! Gracias por compartilo.

[falcord]

Is there any way to make the captions last longer? They're very hard to read.

[macro312]

For the string to break. Is it because the speed that he pulled the string that mattered??? (Don't flame only year 12 student here...)

[killintymm]

force unit after great newton

temp unit after lord kelvin

charge unit after coulomb

any other units named after scientists ?

[Hattes]

Most of them, Celsius, Tesla, Volt (Alessandro Volta), Watt, Ampere etc. Probably easier to name ones that aren't (meter, gram, second) at least in SI.

[nkgr01]

I agree. In Greece, the exams for entering a university are far more difficult... Anyway, the lectures are excellent and Mr Walter Lewin has a great way of simplifying things! I really enjoy his lectures.

[Diemedes]

@nkgr01

Αυτό για τις εξετάσεις ξαναπές το...

[linuxterminaI]

@Diemedes έχεις δώσει πανελλήνιες?

[Diemedes]

@linuxterminaI

Ναι, είμαι φοιτητής στο τμήμα φυσικής του πανεπιστημίου Κρήτης τώρα.

[johndoe1667]

Where I come from, we were taught all this in primary school. Isn't it the same in the US? Don't they assume that people that went to MIT of all places would already know all this?

[kdum8]

That's not the point. Many people do indeed learn this much earlier, including those in the US. First semester courses at universities all over the world do this. It is a review to bring everyone from different backgrounds up to the same level.

[johndoe1667]

Yea, sorry. I get it now. People bitch about this in every single video in the series. Just ignore me.

[chvanable]

@kdum8 In russia all this course was in 5 lections and then real phisics starts. Maybe that's why we first in space.

[kdum8]

@chvanable Lol, OK!

[wideeyedraven15]

@chvanable the space achievements of the Soviets are truly amazing. However, the science education in the United States is second to none. MIT does this course for ALL students, not just physics students. Lewin is a brilliant teacher. You have to understand that.

[killintymm]

but you need to start with basics and build on the concepts in undergraduate course. its the same principle allover the world. in general us education isnt great at school level high school and earlier, but are very good for undergraduate and higher studies masters phd post doc. he is explaining the concepts in his style which may not be the case who hjave learnt it earlier by rote memorisation and just accepting it

[TiGUUR]

ehh exciting!

[jamesdellay]

26100 is the room that the lecture is taking place.

[thebravestdave]

Thanks! I googled' for a hour to find out what is 26100! :D

[yagneshmsc]

the class room they are in (the room number is 26100 at mit)

[chevy3098]

lol, 17:03 - 17:06, is she copying notes?? hmmm??

[riderdude10]

k what is this 26100 hes talking about?

[vanquigt]

Massachussets post code

[Magicman53911]

The room number.

[kahgarak]

This teacher talks very clearly, I like that.

He has a bit of an accent- is it me or is he Dutch?

[BraveZombie]

he is Dutch ... but hes very good

[kahgarak]

Aye... he is.

MIT graduates ftw

[gr0mithtimon]

'but' ? :p

[kdum8]

what do you mean, 'but'? So what if he is Dutch..

[TXUSA23]

Why does the bottom string break? btw this proff is great at explaining physics concepts.

[amrosik]

i give you a example: (sorry for my poor english) you know, the fastest speed of signaltransmission is with light signals. what if you have a very long string ( about the distance, that light travels in 2 seconds). at one end of the string there is a person and at the other end you. now you pull the string, when does the other person recognize your pull? not immediately, the signal have to travel with the speed of sound in the string material. do you understand what i mean?

[stressanalysis]

can any body help me to know how the earth will make a reaction with being in contact with the apple

[Fittykent]

Normal Force. Opposite of the weight of the apple.

[stressanalysis]

can any body help me to know how the earth will make a reaction with being in contact with the apple

[purplepeacock1234]

i think he just said that?

[purplepeacock1234]

how many chalk boards do u want

[Pudersepp]

n+1

[fearghuskeeble]

I wish I knew how he does that thing with the chalk.....

[ArabicPilot]

haha really

[CanisLupus1987]

practice

[CanisLupus1987]

practice...

[Topguntube]

@fearghuskeeble did you ever find out how he does it?

[WarrenChu000]

@fearghuskeeble He angles the chalk so that he's pushing on it and it skips on the board. Newton's Third Law in action!

[GuyMcpersonLol]

@fearghuskeeble OMFG I THINK I KNOW HOW HE DOES IT :D

he holds the chalk perfectly perpendicular to the board so that the friction causes it to kind of "bounce" on the board which makes the dotted line! he holds it with a pretty loose grip too.

[yonatanalemayehu]

@fearghuskeeble Learn Calculus

[EJNewbury]

@fearghuskeeble I think he's just putting the upper most part (the flat head) on the board. Maybe?

[feveria]

@fearghuskeeble

for -> direction he sets up the chalk like \ this, so it fastly bounces off blackboard and makes dotted line.

[JohnnyTakeAll]

@fearghuskeeble I am in high school, (going into my senior year) and after watching these I was recently at a local university for a summer physics academy and we were lectured by a few different professor's and grad students and I asked every single one at the end of their lectures if they knew how do to it. Sadly I am still in the dark on how to master this trick...=(

[mcslushy1]

i don't want to be rude but this is highschool physics in Romania :|

I learned all that in the 10th grade.. and i do mean ALL of that

ow and the experiment at the end is obvious.. due to the large mass of the cube if he pulls quickly the bottom one breaks, if he pulls slowly & steady, the top one breaks

[unarmed1234]

yea same in UK although i believe its highschool physics in Romanian is much hard lol.

but like most other university they like to go thru high school stuff just to make sure everyone is at the same level =P

[jeffareid]

Every real world object deforms. In order to increase the overall tension in a string, that string must be stretched. If the bottom string is pulled quickly enough, the momentum of the relatively large mass between the strings reduces the rate of stretch of the upper string, so the bottom string breaks first if it's jerked hard enough.

[jeffareid]

This is easier to visualize if the upper string is replaced by a spring, and then noting the amplitude of oscillation of the spring large mass versus the jerk and force applied to the lower string.

[nosojdjos]

I demonstrated Newton's laws back in my mother's womb

It was really boring in there so I decided to get out and explore the world

[sameapple]

I think the physics behind the final demonstration, that the outcome of whether the top string or the bottom string breaks still doesn't violate our common sense: the string that which experience a greater force (tension) break first. And this depends on the acceleration that professor WL gave to the string. pulling it fast the first time means higher acceleration while pulling it slower, like he did in the second attempt, means merely adding tension to the first string to the point it breaks.

[kepler98]

only 49:16 minutes. i yet saw a video with one hour and 20 minutes, was a eternity, but this video is very nice, i like so much of fisics

[seanmPWH]

notice he accelerated the pull when the bottom string broke, but he pulled slowly when the top string broke.

[oubhat33]

that teacher should come to morocco!!!!!!!!

[Missluvangel]

i know right !!!! lol

[lakshddiva]

nice..amazing clarity of concepts.

[cdumeohyeah]

Why are you being such a douche?

[VinzJoker]

i want my physics teacher like that... hes so clear - - not like mine..

[thevidfather]

Same. I think I'm gonna go through all these videos over the summer to brush up on my physics since my physics teacher can't really explain stuff.

[yourusernamehere2]

@VinzJoker seems a lot of people feel the same :P

[Monkeyradar]

What does he mean that Newton's first law cannot be proven? Whatever happened to objects that move at terminal velocity. Sufficient friction or resistance will eventually yield a constant velocity although there are centripetal accelerations (the sum of which are convergent) also present on the object moving at constant velocity.

[bezerker4828]

He is refering to the fact that under any frame of reference you will be experiencing some acceleration relative to a larger frame of reference, thus making it impossible to generalize your force-momentum relationship.

[kdum8]

Nothing can ever be proven in science. Ever. It is a fundamental tenant. Things can only be dis-proven. That doesn't mean they are correct though. (And remember technically that Newton's Law are all wrong, they are just an approximation of what is really happening). Still they are good enough for almost all purposes including NASA.

[equinox3o0o]

its all about the impulse which is the product of the force and the time taken for the force to be applied, a smaller time means a smaller impulse remembering impulse is equal to the change of momentum, smaller the impulse, smaller the momentum change

[FlameChilled]

good teacher

[thebicentennialman]

finishim touch worth ponder

[BeingSomeoneHuman]

its all about the speed he pulls on it... The 2kg block is at rest and wants to remain at rest. So depending on how fast he pulls on the string determines how much energy gets transferred through the block.

[allentazwell]

Rockets do not push against air to move forward.

we could not go to the moon if this were true.

[eXX0]

He's right :) Recoil is not dependent on the air.

[thebicentennialman]

by sayin that balloon pushes on air he implies the air inside the balloon ,but not outside.Air gushes out of the balloon due to elastic nature of balloon,during this the air also pushes balloon with same force inopp direction.

[opiumgland]

Air gushes out of the balloon because the pressure outside is less than the pressure inside.

[opiumgland]

A rocket in a vacuum can still generate thrust by expelling the fuel it carries. mv=mv

[Saskachewan]

The answer is simple if you think about the acceleration of the red block. When he pulls quickly, the red block can't be accelerated that fast, so it opposes the force making more tension on the bottom [opposing force + pulling force], tha to the top [only the opposing force's reaction].

But when he pulls slower, the box can accelerate and then the force on thread 1 is bigger [block's weight force + pulling force] than on the thread 2 [only the pulling force].

[sentriclecub]

Suppose the tension will break the string at 50 newtons.

If you pull 40 Newtons on the bottom string, then that causes the top string to be 40 Newtons plus the weight of the 2 kg block (weighs 20 newtons).

So the tension of the top string is always 20 Newtons plus the tension of the bottom string.

If you pull up 5 newtons, neither string will break.

If you pull less than 50 newtons, the bottom string will not break, regardless of what the rest of the system does.

[piratemcdaddy]

ok so someone explain the last bit.

I think he obviously applies more force the first time by quickly pulling down and using more strength than he does on the second attempt where it seems he pulls down much more slowly until the top string actually breaks. The last time he pulls down quickly like he did the first time and once again the bottom string breaks.

I assume he is applying much more force on his first and third tries, but it still doesn't make much sense.

[piratemcdaddy]

on second thought

On his first and third attempts he is accelerating his pull down faster than acceleration due to gravity, so the bottom string would break first since it is experiencing a greater force than the top string which is experiencing the force of gravity, thus the bottom string is experiencing more tension. It breaks first.

On the second attempt, he pulls down much slower so the top string experiences more force and breaks first.

Or something like that. I still don't get it.

[omaz]

The impulse of an action is equal to the force applied multiplied by the time it is applied for.

So when a short time is used, the impulse is small and the inertia (difficultiness of moving an object) is not overcome. So a force acts in the opposite direction to the Prof's pulling force - bottom string broke

When a long time is used, the impulse is larger, the inertia is overcome and a force acts in the same direction as the pulling force - top string broke.

Hope that helps and is correct

[sameapple]

I too thought along the line of impulse after confounded myself in visualizing the effects of various accelerations . But your statement about overcoming the inertia is incorrect. Because a body of any size will have its inertia overcoming at the presence of an external force of whatever magnitude. In simpler terms, the impulse that the block experiences only implies that the string has been pulled down over a duration. The force is either greater or less than the tension a string can withstand.