Lec 7 | 8.01 Physics I: Classical Mechanics, Fall 1999

[moah2012]

i am confused... shouldn't the total time for the projectile be 62 seconds? I did the quadratic formula and t =62 not 31....

[levishort36]

This video is a favorite on Honduras

[indseytate718l]

This vid is popular on Tirane

[dinotles]

@kristijan0kroflin That is not really true. light as particlevawe describes how the physical element act at all times. It is like calling a rock both hard and heavy at the same time. Depending of what you measure you might see different aspects, "hardness" or "heavyness". It is not the name of WHERE you find the light particle, like in your examples of " molecule of a tire" called "tire" and "an electron of the human body" called "human body". it has NOTHING to do with the aspects and effects

[ShrodingersPerson]

Is that a bagel in your pocket?

[ShrodingersPerson]

@ShrodingersPerson confirmed. 29:02 . it is in fact a bagel

[kevineugenius]

Even MIT can't figure out how to make blackboard erasers work very well. :)

[Scurmicurv]

@kristijan0kroflin Well, I must admit I belong to the crowd that isn't 100% convinced just yet that gravity will fit just so nicely into our current force model. I'm very colored by the image that comes out of relativistic theory; that gravity is simply an effect of curvature in space-time. That's actually the way I think of potential energy too - just a sort of relative position marker within a curved section of space-time. Though this is not something I'd argue very strongly!

[ghosty210]

damn! if i watched this earlier, i would get higher marks for my exams!

[GrowMasterGeneral]

@ispravljat Gravity is a force that isn't as well understood as the other three forces. First off, there's a huge discrepancy between the force of gravity and the rest of the forces(gravity is significantly less). Secondly, all three other forces have been proven to have force carriers or gauge bosons. For example, the photon is the gauge boson for electromagnetism. Gravity is theorized to have a boson as well (the graviton). It is believed that all particles exhibit wave-particle duality.

[kulpreetg]

i like that chalk!

[RJonStreetz]

God its so fun to experience weightlessness!! I wanna do it once in a life time!!

[Aggie98bqn]

I have no doubt he actually tried many times jumping off a high place w/ a bathroom scale attached to his feet! The man is priceless!!! Thank you MIT.

[gamemaster014]

It's non-negotiable.



[ZahidAnwar5555]

Sir I m in 8th class ....i wanna become scientist ....and I have made my Own equation which is related with Force,Mass , Distance And time .....So plzz tell me what i have to do more ....

what will be the weight of the body just before hitting the ground

[ZahidAnwar5555]

Sir I m in 8th class ....i wanna become scientist ....and I have made my Own equation which is related with Force,Mass , Distance And time .....So plzz tell me what i have to do more ....

[jackofcircles]

I wonder what was on his mind at 9:40.

Lol. Great Video overall.

[taikukaikei]

lol the experiement looks dumb... lol just flying all day long like that sounds like a pain

[purpleleach1]

@taikukaikei I think it would be fun to be experimented like that for maybe a maximum of 30min. but all day.... They're bound to have motion sickness.

[mariomaruf]

Should'nt it be "-T - mg = mac" at 14:20?

[SarahAndZaraAmadores]

@mariomaruf

You define the positive direction to be towards the center of the circle. And Tension always points towards the center. Hope this helps.

[DeZorgenkindjes]

Very good lecture, proffesor is a bit agressive :P

[elarssen91]

Why are there empty seats?

[WeShallBurnInHeck]

WEIGHTLESS BAGEL!

[tomczyce]

11:25

K i still don't get how they have equal weights. I thought weight is equal to mg. If one m is larger than the other, then shouldn't the weight be larger as well?

[69boy17]

@tomczyce because they have the same tention, if the the tension in both strings is = to lets say 1 then both are equal weight, says it at 11:35

[volk410]

@tomczyce Weight is not equal to mg. Weight refers to the force that counteracts the pull of gravity. If you are at rest, that force is equal to mg. However, as he showed with the elevator example, if you have a force pulling you upwards, that force is obviously greater than g, and hence you accelerate upwards; your weight therefore increases and is equal to m(g + a). If you're accelerating downwards, your weight decreases to m(g - a). In freefall, your weight becomes m(g - g) = 0.

[improlawl]

@volk410 very well reiterated :D

[fyckthissht]

he's a great professor but...why in the world is he wearing sandals with socks? LOL

[PaxAtheista]

Imagine a world where people dedicated themselves to art and science instead of money and power.

[pb23me]

string has relatively small ass!!!! lol 9:42

[ahsan2mahmood]

@pb23me its "string has NEGLIGIBLY small ass".



[thekhmer2507]

I love Professor Walter... I think he is the greatest physics teacher ever... I learn so much from him... Thanks MIT for the generosity...

[nextblain]

it is non negotiable tht prof walter lewin is one of the greatest teacher of all time :D

[sandeep88214]

Thank you sir. thankyou very much

[sandeep88214]

Thank you sir, thank you very much

[kunalubhi]

awesome video



[fernq88]

i love this guy, he's fantastic all siences needs people who makes it so conprensible and funny.

[thecolouramanda]

I love his lectures, they're the best way to destress after a long day! And this is so much better than what I'm learning in high school. Good to see that there are still earnest teachers in the world. What's up with the doughnut on his right pocket? He also had a strawberry in other videos, if I'm not mistaken. Cute.

[gokusonic92]

the theory is showed practically............creativ­e lecturer.......hope mine is like that......



[TestECull]

@vrekman32 Not really. People will just go to the next video site and watch the dog videos. Note how this lecture only has 47,000 views...most car crash videos have more than that.

[7kholoud]

1999 is too old ,, However, I like him !!

[ThirdSon13]

Also, the engines (of the 'zero-gravity' airplane) are not cut-off, but are gradually throttled back on the climb and throttled up on the descent. This is to counteract the atmospheric drag on the airplane so the airplane continues to follow a free-fall path. Otherwise the airplane would slow down from air drag.

[ThirdSon13]

Minor point: The "parabolic" path of the 'zero-gravity' airplane is actually not parabolic but a segment of an ellipse. If truly in free fall during that operation, the airplane is following an elliptical orbital path with the center of the Earth as one focus. It is close, but only if the Earth were flat would it actually be a parabolic path.

[david0aloha]

@vrekman32

I think it would be much like the M.I.T. site that hosts these videos as it is now. We would view them on that site, and everyone else would watch videos elsewhere. Maybe that's not totally true due to branding and how much people associate with youtube - maybe more people would check them out - but I don't think that it would be a great number... unfortunately.

[SonGuko69]

i love him <3

[hemanthaew]

"God Bless".....??? ... whose god are we talking about this blessing of MIT ???

[Gintable]

I like the whooshing sound at 17:40

[KillaUsman786]

i wonder how long his class is

[boeing747200lr]

Awesome lecture. God bless mit open courseware!!!!

[masroor45]

he's really a super star but he confuses me @ certain times

[JustWonderingHowToDo]

When i was young i actually did that experiment. We put a bathroom scale in the elevator of a hospiltal. As it went up are weight went up.

[alexlee67]

is that a donut on is shirt?

[AlexDisaster6348]

I am taking physics in my high school currently and I wish this man was my teacher in person because he is right now and the teacher at my school is an insult to both math and physics i do not know what i would do with out these

[Hantroxy]

Great Lecture! It helped me clear a concept. iIts very engrossing to listen to his lectures.

One misses such dedicated people in instituitions.

[painxtreme]

@Hantroxy For me, Lewin is as engrossing and effective as both Feynman and Gell-Mann. If you close your eyes and just listen to his words and cadence, plus his style, and student rapport. It is similar to listening to the Messenger Series of Lectures by Richard Feynman, but with a more modern lingo on occasion. No, I don't his voice is similar to Feynman, he had a loud, deeper commanding tone. Gell-Mann has a similar mid-register voice. Hearing their voices in his teaching is a lovely reminder.

[selahia]

great experiment with this scale, very smart!

[DIME10NASA]

Also, after 42:04, when he speaks of the engines being "cut off", the engines on the airplane are actually gradually 'throttled' back going up and then 'throttled' up going down. Engine thrust is important to balance the air drag normally encountered by the airplane to maintain a true free fall trajectory.

[DIME10NASA]

At 42:04, you need to realize that the shape of the path is not really a parabola, but an ellipse. The path is simply the top portion of an elliptical orbit around the center of the Earth. It would be a parabola only if the Earth was flat (which for this distance is a reasonable simplification). But the path is actually elliptical.

[Diemedes]

A wonderful Physics lecture! Physics student from Heraklion here!

[dopestghost]

i learned all that in highschool

[DoktorStranamore]

Very beautiful physics course!! ( i study physics in Italy, Pisa)

[thevidfather]

Very interesting lectures...

I wish I had a physics teacher like this.

[WeShallBurnInHeck]

Hmm... Is he wearing a white or a purple shirt... the color is kinda blocky and shiny and I cant tell o.O

[themightydumbass]

intruiging

[BuJianBuSan83]

Why are this kind of people never at school, where you need them?

[WeShallBurnInHeck]

BuJianBuSan83: They are at the schools which deserve their skills.

[KarlPhys]

Weight is defined as the force W acting on a mass m in a gravitational field, i.e Force W = mg. By this definition, a mass falling freely in a gravitational field still has a weight. It can only change if g changes! The professor's definition is wrong.

He really should know about Newton 3(rd Law) pairs. This is sloppy and misleading!!

[QuintenFGC]

He should point out that he's talking about the system of the elevator. make that your system of reference and you will indeed be weightless if it is in free fall.

[KarlPhys]

I fully agree! In this case we have an non-inertial frame. However, it seems to me that the Prof is taking the Earth as his inertial frame. If the students are doing the same then I am sure you can see it is misleading.  He is also right if he takes a space capsule in orbit as a non-inertial frame. I am also sure you know that dealing with non-inertial frames involves the introduction of fictitious forces. However jumping of the table? Surely the inertial frame is the Earth!

[PerfectandHumble]

He surely makes a Davy out of himself, he is more of a showman to me. But we are just being picky about the wording. You know, using 10m/s² for g is kind of an introduction to physics rather than a real one. (I used 9,8 in sec5, Idk what is that in the us, but it's my last highschool year, before CEGEP)

[KarlPhys]

Well he is a lecturer. As a teacher I have to put myself in the position of my pupils. Actually, being picky about the wording is important if you really want to understand Physics. I assume this course would have been taught pre-university in the US. so maybe the student arn't too confused. I teach this course to pre-university students in th UK.

[RtGFuSiOn]

He used 10m/s to let the students understand the concepts without worrying about decimals. Of course, in actual calculations, you would use 9.8m/s for instances on earth.

But yes this is more an introduction to concepts of weight (g forces).

[Gintable]

It's not 9.8 inches/second^5,

g = 9.8 meters/second^2

Or even better units:

g=9.8 Newtons/kg

[kerbyt]

meters/second^2 and Newtons/kg are the same :p

(kg*m/s^2)/kg --> m/s^2

[Gintable]

@kerbyt

Yeah, I'm well aware.

I just think that Netwons/kilogram give a better intuition about what g actually represents.

g isn't necessarily an acceleration unless no other forces come in to play.

g is the gravitational force acting on an object per unit mass of the object.

[kerbyt]

@Gintable

Right you are ;D

[Gintable]

Well, the earth is far from being a true inertial reference frame, but it is much more inertial than any object in motion on its surface.

[PerfectandHumble]

Actually, no. We use the same definition in my class, you are misled, it is mainly the counter acting force. If you don't move and is perpendicular to the plane, of course the normal force is Mg. But if you are actually inclined. your weight is simply mgcos (the angle with the plane) if you are not stopped your weight will remain this value. Go on Wikipedia, look at the sources, the weight is strictly related to the force applied to support another object , Therefore: freefall means no weight

[KarlPhys]

I doubt that I am misled. I have an honours degree in Mathematics and have taught Physics for 25 years. From Wikipedia - For a body supported in a stationary position, the normal force balances the Earth's gravitational force, and so apparent weight has the same magnitude as actual weight.

[PerfectandHumble]

Ok, Well, Apparent weight : free fall = 0, Actual weight, if we use your definition = mg. But in actual physic classes I attended, weight was usually apparent weight rather than mere mg.

[PerfectandHumble]

Actually you may be right, From Oxford's dictionnary, in physics : Weight is "the force exerted on the mass of a body by a gravitational field." .. I am sorry, I have been misled. This is still strange that my two physic classes, + this one tells me that weight is dependnt on the normal force...

[KarlPhys]

No need to apologize. I have had to correct other physics teachers! If you read Wikipedia under apparent weight it should clear the matter up. You may also like to read my other comments to Quinten FGC. Actually, in a later lecture the Prof uses the term "apparent weight" although somewhat quietly and quickly.

[Gintable]

What he means by weight is the force required to constrain a body at rest in its immediate environment.

"My" definition of weight would be the force of gravity acting on an object.

Both are valid, but one must be more precise when asking students to calculate either. OR define which standard definition is to be used unless otherwise specified.

[Gintable]

Well what you've stated is certainly "my choice of definition" of weight, but you can also define weight as the following:

The force of constraint required to hold a body at rest relative to its immediate environment.

[uphillcastle]

without nerds you wouldnt have youtube to call people nerds. Can you see the irony?

[wyz135]

I'm always confused with mass and weight, after listening this lecture I completely understand it. His explainations is very crystal clear. I love this teacher... er... I mean lecturer

[explos1ve13]

mass is the amount of stuff in an object

weight is basically just gravity pull on u

lol im sure after 2 months uve figure it out xD

[aznhelena]

"and this string has negligebly small ass......mass"

[uttara]

good one!

[oregonbeerman]

10 is easier to calculate in your head during a lecture than 9.8...

[Gintable]

I remember an interesting problem, where we need to calculate how thick of a layer of dirt we'd need to add to Earth in order to make g=exactly=10 N/kg.

On Saturn, it is slightly greater than 10 N/kg

[golafs]

when concetrating on 1 fundamental law it´s better to rule out other things you are not focusing on (when teaching, not in real applications), air friction, massless string (doesn´t exist in real world), rounding g to 9.8 etc.

why use g value 9,80665? only to make all calculations more confusing.

also g depends on where you are on the planet (ranges from ca 9.79-9.82) it´s where i´m now, in iceland, around 9.82.

so 9.8 is a fine number for g when teaching students from all around the world.

[MuleXC09]

He was more trying to get the concept across to the students than to find an exact numerical answer. Professors at a university always have a reason for what they do. He knew full well that acceleration is 9.81 m/s/s and probably knows by heart a few digits past that.

[AlephNine]

Has the OP actually taken a physics course? I think not. This kind of approximation is standard practice, and helps avoid numerical clutter when concentrating on the content for the first time.

[maheryagub]

This is to make you have a feeling to the Fundamental ideas my friend, be able to understand what you are calculating, it is a concept adopted by great teachers, like Mr. Richard Feynman

[ideasrule]

Um, isn't this grade 11 physics material? And isn't MIT supposed to be one of the most prestigious universities in the world?