Introduction to motion (part 2)

[Kricardose]

I am very happy to see the vidoe More on how velocity, distance, acceleration and time relate to each other after you give this

[bebeheuy]

I Love The Video It Can Increase My Knowledge More on how velocity, distance, acceleration and time relate to each other

[Mjhond]

Steady I Really Like This Video More on how velocity, distance, acceleration and time relate to each other

[tulitech]

Mythbusters tested the penny dropping from the empire state building killing somebody myth.. It turned out to be busted...

[Masterlink981]

I'm 11, (in 6th grade) and I just happen to wonder why we don't learn STUFF like this in school... Instead of English, History, Computers, and Gym. That stuff isn't going to help us in life. Physics, math, and science will help us.

[soopa123]

at 5:10 you talk about average velocity... why not instantaneous velocty/? what i the difference?

[MrJackalex123]

thank you god,,,for salman khan

[QueenKelleyz]

at 7:00 you seemed as though you were talking fast, I would be nice if you can slow down and allow the viewers to take in the information. I really enjoyed your video and it helped me a lot.

[iamtheplauge]

this is an i, XD

[francais2121]

I wish I could redo my test!!!

[TheRayMan91]

These videos are awesome. They are so good I almost thought the guy wasn't doing them right for how simple it actually seemed. Thanks man!

[LinnieLinXD]

I might as well stay home instead of going to physics. I learn (and understand) a lot more from your videos than I do in class. Thank you so much for spending your time to make these videos.

[alearg89]

The average velocity its final velocity + initial velocity divided by 2, only when there are 2 seconds difference between final and initial velocity, if for example there is a 5 seconds difference between initial and final would that be devided by 5? or is 2 always there no matter the time difference?

[thegreendayfanable]

i thought

speed=distance/time

velocity=displacement/time

[Zenoniel]

@thegreendayfanable That's right.

[purpleleach1]

@thegreendayfanable Speed is distance/time and velocity is displacement/time

Sometimes we write v because we really are only talking about the magnitude of velocity, if there is an arrow over it then for sure it is including direction.

[lizardX11]

g = 9.8 but basic learning is 10 so yeah he is doing it right :| Not a very big deal ,He knows what he is doing .

[GamingJediPwns]

Thank you for making these videos they are really helping my game programming :)

[ghfreakwierdo123]

wait but wouldnt at/2=the speed that it ended up as/2? Because initial speed lets say =10m/s and a=lets say 10m/s^2 and t=lets say 10 seconds, 10m/s^2x10sec+10m/s=110m/s after 10 seconds(considering the initial speed was zero) (100+10)/2=55 yet Vi+at/2=10m/s+(10m/s^2x10s)=10­m/s+(100m/s)/2

=10m/s+50m/s=60m/s. But we should get 55 m/s. So there must be something wrong with your replacement formula of Vf=Vi+at. So does anybody know whats going on?

[harvard1yale2prince]

the acceleration of gravity is 9.81m/s^2! not 10!

[Skyrodude]

@ThePeracha

they give it now too, this vid is from 2007 where he didnt had it ;)

[larahassani]

(g=9.8m/s2)

[oreos4mendyu]

does the velocity differ depending on the object's mass/weight?? is weight and mass the same thing?? -- as far as i remember its not =S

thanks btw - this vids are awesome!!

[krraa]

The last part at the bottom in the pink color; why doesn't the coefficient, 2, not cancel out the denominator of 2 as well? This confuses me.

[solshine26]

@krraa You cannot do this because the term at did not have a coefficient of 2

[jonjonjon1370]

Can you please make a video w/ a bit more detailed explanation of how you got 20

at 1:38

[Famousx81]

d=v*t, is d distance or displacement? It shud represent displacement since velocity measures the rate at which position is changing. I'm confused bec in the video u keep using 'distance' for the formula.

[Famousx81]

@bringingback, accel is always 9.8m/s^2 (or 10, if yr rounding) . The sign can be determined in light of the scenario...for example, if u throw an object up n take 'up' to b positive, then down would be negative n the sign before the accel wud be negative. And if u decide to keep down as positive (g=+10m/s^2), then 'up' wud b neg.

[BringingBackTheVid]

I'm confused about something. My physics teacher told us the acceleration constant for gravity was -10 m/s (actually -9.8), because gravity causes a downward force on an object. So is it + 10 or - 10??

[niranjan808]

i have another doubt 8.42 well how come (2v+at)/2 become (v+at)/2.This is impossible did he make a mistake or something?????

[niranjan808]

any one solve my doubt please

avg velocity=vf+v2/2 right so 0+20ms^ -1 /2s must be 20 ms^ -2 how come he says 20 ms -1

[TrineBeats]

You covered what we did in 2 weeks into 2 videos (20 min) in a way so I could understand! xD

[youtindia]

i think you should say initial velocity 'u' and final velocity 'v' it was in my 9th physics book like this and the velocity formula was v=u+at so i got a bit confused but got the thing later i am not at all blaming the vid just telling anyways any1 tellme what is right using v and u or vel. initial and vel. final

[zainahmed1994]

now i have my concepts cleared... my frustration has released..

[kiemul136]

If you trough a CAT from the empire state building it would survive. It would accelerant towards Earth (gravity) until the acceleration is balanced with wind resistance thus will fall at a same rate: terminal velocity. The cat also would spread its legs to add wind resistance which acts as a parachute. The impact (change in momentum) when hit the ground is not sufficient to kill the cat or even cause it sever injury.

[yas72090]

@kiemul136 the cat will die bro.... be kind to animals

[kiemul136]

@yas72090 Animals are the same decaying organic matter as anything else.

[majestic93]

Just FYI, you can't kill someone with a penny that is dropped from a building. Because of the structure, wind hitting against it shoots up the building and it creates its own climate around the top of the building. Because of this wind, the penny can't shoot downwards fast enough.

A penny at its maximum velocity, when it hit different substances, it won't make a deep (enough) impact.

[youtindia]

@majestic93 but when it will strike in anyone's head with the speed of about

100m s ^-1 then it will hurt the brain and one can die

[Master1Samsung]

YOUR A FUCKING GENIUS

[tony225]

@Master1Samsung

And YOU need to learn the difference between "your" and "you're".

[lwanatt]

its all so elegant and obvious, i feel like an idiot for not realizing some of these things on my own.

[prankcalls420]

i love physics :)

[Greyh0und]

The mythbusters tested whether a penny thrown off a building could kill someone.

nope: /watch?v=QUS89-_fbtk

[Grobannela]

Whyyy?????? This is so boring..

I love you Sal..But, I really don't like physics. :(

[prankcalls420]

@Grobannela I love it :) I guess you're a girl, they generally think maths and physics are boring

[daddycoolp5]

these videos are really helpful....... please can u do a tutorial on the graphs of vertical motion......

[MetalliDeth24]

My brain hurts ...

[onlyAerik]

I've picked up where i left off with calculus AB, I've started some linear algebra, caught up on trig I think, and now I'm reviewing physics.

Man, calculus and trig make this projectile motion stuff too easy. Love calculus.

[esteban79]

I finally understand!!!

[Ryan864]

the penny wouldnt kill someone, it would reach its terminal velocity which would be a pretty small number. correct?

[idster7]

i don't understand why the total distance traveled wouldn't be 30 meters, if the velocity was 10m/sec during the first second and 20m/sec during the second second.

[zeroimpossible]

@idster7 The initial velocity was 0 m/s, not 10m/ sec. 10 m/s was the acceleration rate of gravity.

[sk8freek246]

haha that was me who dropped the penny

[vickiormindyb]

You can kill someone dropping a penny? That's creepy, like something Stephen King should write about.

On my job as a paramedic, I've seen injuries that occur from kids dropping rocks from overpasses into oncoming traffic.

I always thought the seriousness of the wound had to do with the speed of the traffic coming toward the rock.

I didn't think about the speed it was gathering as it moved downward.

Given the added information, the suspects should get more time behind bars than they do.

[timtu22]

If you drove into a stationary rock at 80 kilometres per hour, that'd still be messy. But, yes, given the fact that the rock is already going pretty fast... People who drop rocks onto traffic are bad, bad people.

[tommyh75]

Isn't g=9.8?

[razgriz1k]

@tommyh75 it's an approximation to make calculations easier, but yes, it technically is

[onaga440]

yes but he is using 10m/s for basic learning sake

[asmodeus585]

@tommyh75: It depends where sou are at a particular moment - at the Equator g = 9.78...

[mvadim25]

@tommyh75 9.8m/s^2. It's 10m/s^2 for simplicity.

[anu427]

"Oh, i switched colors" lol :D

[deathbystarship]

"...here's the bad person who's going to drop something..." lol

[meraphox]

and if you listen carefully he says

acceleration is CONSTANT therefore its not increasing speed !

[Hawkallica]

constant as in 10m/sec^2 never changes

[joninacann]

constant acceleration is increasing velocity, no acceleration would be constant velocity, but constant acceleration is just increasing or decreasing the velocity by the same amount every second

[ner2393]

@meraphox this means that the INCREASE in velocity is constant! INCREASE in velocity is constant!!!!!!!! If it is goin at a velocity 5m/s then it will increase at a constant rate- will go to 15m/s, 25m/s, 35m/s.

[asmodeus585]

Great video.

[WarzSchoolchild]

HOLD ON!....If g = 10 meters per second squared, (1 x 1 = 1) the penny will have dropped 10 meters in the first second. In the next second, it will travel more than 10 meters! 10 + 10 + more = MORE than 20 meters!

Take the example of one seconds travel, V average = Vf + Vi / 1, Vf =10 meters per sec. V i = 0, so V average = (10 + 0)/1 = 10 meters per second. GO FIGURE, The penny drops 20 meters in the next second. Total distance fallen in 2 seconds is 10 meters + 20 meters = 30 meters.

[WarzSchoolchild]

As a 'Visiting Fellow' of D.A.M.T.P. and an applied mathematician, (NOT a Physicist) I am on occasion deliberately 'perverse'.

Scientists occasionly make very expensive mistakes, even costing lives! It is very important that they are clear about the meaning of the units they use. 10 meters per second squared?...One second squared is still 'one' second! Had khan chosen 3 seconds, the average (30 + 0)/2 = 15, and 3 x 15 = 45 meters, (the distance traveled in 3 seconds), it may have been clearer.

[KimmyPossible]

you're right that would be more, but for the full first second it was not traveling at 10metre's per second, it started at zero and increased to 10metre's per second. so for the first full second, it was not traveling at 10metre's per second, it wasn't until after the first full second that it was travelling at 10metre's per second. that's why you find the average. good thinking though.

[meraphox]

CONSTANT acceleration !

means it won't speed up, listen to the video please.

[WarzSchoolchild]

Constant acceleration, also means the force acting on the object is constant. A racing car engine produces more power as the engine RPM speeds up, so the acceleration is NOT constant. That's why there is a gear box! If a Jet Plane goes into 'after-burn' it 'accelerates' even faster.

Because of 'Air-Friction', in reality a falling object reaches 'Terminal Velocity' and so stops accelerating. i.e. when the counter-force of air-friction = the force of gravity.

[asmodeus585]

@WarzSchoolchild: Actually, in his example acceleration is constant, it's the velocity that's constantly chaning due to the accelaration. But gravitational acceleration is always constant, so is the "weight" (the magnitude of the force that is applied to the object).

[WarzSchoolchild]

@asmodeus585 My point related to 'realityy', his example only applies in a vacuum. Air friction will cause the falling object to reach a terminal velocity.

[Maz906]

V_a should be (10+0)/2=5, no?

[meraphox]

yes but this is very basic motions therefore we ignore air resistance, objects increasing speed etc etc... because this will confuse many students.

[davidlazaro714]

Thats wrong if the penny is traveling at 20 m/s for to 2 seconds, you have traveled 40 meters.......not 2 meters.

[FoulplayMFer]

the penny is not constantly traveling at 20 m/s for 2 seconds. it takes Time to reach the velocity it did. the initial velocity is 0. the penny did not accelerate to 20mps without time.

[knectick]

@FoulplayMFer you sir do not understand... so please dont comment

[abdullah7891]

@FoulplayMFer so thats why the velocity is not constant! it says the body is accelerating!

[Zenoniel]

@abdullah7891 If a body accelerates over a period of time t, the velocity of the body would be increasing for the same period t. Thus, for each second - the velocity increases, therefore not constant.

In layman terms, don't you think it make sense if you're going faster if you accelerate?

[Zenoniel]

@abdullah7891 Oops excuse me. I misread your comment.

[torolf007]

yeah. i thought this guy was smart o.O

[DeflocculatedDentist]

given the size on your person, you should've used an example like "king kong is dropping somebody of the top of the empire state building". ;)

[asseeninYOURDREAMS]

Didn't the mythbuster proved this to be false?

[Nutterbutterz95]

Because Acceleration=v/t^2, and that last part of the equation is A * t, wouldn't that equation simplify into:

Velocity (average) = Velocity (initial) + v/2 ?

I'm pretty sure it does, but I'm not sure why he didn't simplify that at the end.

[jmeeks29]

hey, i was just wondering, how did he get two initial velocities for the Vf problem?

[ricikay]

because he is using the inital velocity(that u start of with) and the final velocity to find the average distance

[tupakyt]

are you God?

Cause..God, i love these videos!!

[niranjan808]

@tupakyt yap i am GOd

[trc777]

great lesson, very good explained. thank you alot for making these videos!

[Studiology]

fun fact. if you drop a penny of the empire state building. you would not kill someone. because of air it has a maximum velocity and it velocity i dont remember but my calculations rendered that it would only put a small bumb on their head. did a project on it in the eighth grade.

[EminentStar]

well he originally said a rock then he changed it to a penny...

[SimeonBerson10101]

I cannot believe how kind you are to give up so much of your time making these videos. Thanks so much!

[EminentStar]

I agree thanks! Your videos are so helpful!!!

[yadsik]

Wow, great. You are right. Physics is very intuitive, to appreciate it you have to internalize the equation.

[wangpangu]

no accelerate 10 after drop equals

[master254sy]

that was my 1st physics lesson and if thats the most basic equation... i had dificulties to understand that

[k00lkid333]

if the acceleration is 10 metres per second, doesnt this mean the first second would be 10 metre drop, and then because the 2nd second would be 20 metres per second this means the drop would be 30 metres over the two seconds. unless the first second was 20 metres per second, and the acceleration was 20 metres per second

[shkellyp]

He started with zero velocity so yes after the first second it is accelerating at 10m/s per s. The average velocity over the 2 seconds is 0m/s plus 20m/s divided by 2s with is 10m/s per s. The vector of the penny Earth system is towards the center if the Earth.

[shkellyp]

What I meant was the sec on the bottom will cancel out one of the sec on top which mean what you have left is 10m/s.

[thelastwords]

the acceleration is 10 metres per second squared. so 10 m/s^2, meaning that every second, the speed increases by 10 m/s.

[chiggs92]

ive learned more in this video than i have in my entire physics course at school ahah

this is amazing thanks so much:)

[manoj0072006]

why g is not negative ...is not it droping down it drectionis down so it should be negative thnaks correct me i am wrong

[ajifebri]

Because in this case, we take the direction downward to be positive. That is the direction for d, v, and also g (the penny is going down).

[Pooploops666]

Killing someone by dropping a penny from a building is actually false. The penny will reach terminal velocity in a relatively short amount of time, and due to its mass being small, the force of the penny hitting someone or something is insignificant

[Xurbia0]

But it'd still hurt like a biatch

[gimmeurdosh]

ROFL yeh it could crack ur skull, but wouldnt kill you.

[HotActionProductions]

introduction to dero

btiptipcally, a Kalckalator can do it.

do you know anything about diesels?

[amannvig]

salman khan

[Winsunn307]

Thank you

[girlinsanfrancisco]

are you a teacher?

[thesikniss1777]

i had trouble with algebra in school bc they did such a bad job explaining it, it just is dont ask why. you explain why.. thaannkk yyyooouuuu lol

[iluvjiyongx3]

wow. is it just me, or do u explain EVERYTHING so clearly and understandably? thank u~

[Lad22nd]

I understand it so much better now thank you!

[jtlangel]

awesome

[rleroygordon]

Decent presentation, but it's a shame you don't use a lightpen because the bouncing cursor, and shaky handwriting are very distracting.

[LitheonAV]

Dropping a penny from the empire estate building would not kill someone, its terminal velocity is not even near great enough, it however would give you a bruise or a cut.

Awesome tutorials anywho.

[khanacademy]

You're right. Someone actually performed the experiment (with wind). The penny would impact at about 60 mph. Could easily make someone blind though ;)

[redjr242]

@khanacademy Wouldn't the penny have fallen 30 meters after 2 seconds? After one second it would be going 10 meters per second and after the second second, it would be going 20 meters per second which ads up to 30 meters... I'm confused.

PLEASE ANSWER!

[ninjajesus81]

@LitheonAV Depends on who you hit. Someone with hemophilia or thrombocytopenia could be killed from anything.

[carlosyt2]

@LitheonAV i think that was just an epxression...

[iAmPriss]

very great :) but isn't gravity on earth 9.8m/s squared? thats what they taught us in my physics class o.o

[Rain0nM3]

it is but some just say 10 to make things easier.

[QuantumCoulomb]

@iAmPriss

he rounded off to 10m/s squared for easier calculation.

[panda00303]

Thanks, your videos are very informative. Only thing is that terminal velocity is disregarded.

[personsame]

i despirately need videos that based on TORQUE can you please post some videos especially SEESAWS

[sabakimsg]

Excellent!

[neuroneuster]

Nice presentation, teacher.

[thunyarat]

you're great!!!