Projectile motion (part 1)

[jordanleigh713]

I'M going to jump off a cliff from finals this week...

[Xoneize]

oops i meant (Vf + Vi)/2

[Xoneize]

i have a question, why is average velocity (Vf - Vi)/2 and not d/t ?

[samlinkworth]

@Xoneize average velocity can be both defined by average distance travelled in time or (vf+vi)/2. No difference.

[ambarigreyson3000]

bro u cant drop an apple????

[Kricardose]

I am very happy to see the vidoe Using the equations of motion to figure out things about falling objects after you give this

[bebeheuy]

I Love The Video Using the equations of motion to figure out things about falling objects It Can Increase My Knowledge

[Mjhond]

Steady I Really Like This Video Using the equations of motion to figure out things about falling objects

[Smidz018]

this is motion basics not projectile motion..

[avd178]

....elementary school?

[hunzai001]

i dont think any single video of Sal deserves a dislike... do you?

[AlexzandrSpringvale]

@hunzai001 I don't think so either.

[dragonpie47fake]

Man, this really helped. I've been so lost in my physics class. These videos are saving my grade c:

[alfredobolanos96]

can someone help me with this problem "A cannon ball is fired horizontally 50 m/s from the top of the tower that is 60 meters tall. a) how long is the cannon ball in the air? b) how far from the base of the tower does the cannonball land?" PLEASE!

[duderme]

@alfredobolanos96 First of all, you need to write what you know Vertical Components Acceleration(a) = -9.8m/s/s(ALWAYS, unless a different acceleration is already given or something) Initial Velocity(vi) = 0m/s(Not given) Delta D/Delta X(What ever you want to call it)=60m(This is your vertical distance) Time(t) = What you want to find First, use the equation that makes use of all your givens X=vi(t)+1/2(a)t^2 60m=1/2(-9.8m/s/s)t^2 60m=-4.9t^2 Square root both sides (No neg.) 7.74=2.2t t=3.5 sec

[jensons88]

well explain. you can watch a program that calculate projectile motion before the object is released. it's on my channel. you can also download the program from softpedia.

[200YearsofSolitude]

Sal sounds like he likes skydiving 

[MyAkash95]

thnx to d whol team

u hv cleard al ma doubts

[iloveafg123]

it would be better to use 9.81 this not middle school lol

[xxlarougexx]

aweesome

[MaQWan93machanG]

why u use -10m/s^2 but not -9.8m/s^2 ??

[zhigh93]

@MaQWan93machanG

Because on the AP Physics exam, they dont care about significant figures. Therefor, it is quicker to just round up.

[gofish8195]

@MaQWan93machanG Gravity is about 10m/s^2, he uses 10 to make the working out simpler as he's working it out on the screen. It's like taking pi to be 3, it's still accurate just less precise.

[kabukulan2000]

why do you use -10m/s^2 instead of -9.8m/s^2

[200YearsofSolitude]

@kabukulan2000 he didn't wanna use complicated figures and 9,81 is almost the same as 10

[nt4life]

hey these videos arent showing up on the website khan!

[Julz333666]

Sal, Sal, he's the man,

teaches math like no one can!!

[muzifreak95]

actually he cudv used the formula 2as= v^2- u^2 ....... wudv bin much easier.

[Ghaiyst]

Damn thats awesome! I figured this out in my head! Man I got this down! :D

[Jackielovescandy]

i wish the volume was higher :(

[wemosam]

This has confused me since he is falling at 10m/s^2 wouldnt he only fall for 2 seconds? for the first second, he falls 10m the 2nd second he falls 10*10m/s which is 100m/s?

[pianodemon88]

At time 1:34 he introduces the direction of velocity and says that it is a "scalar" quantity. Wrong, it is a vector quantity precisely because it has direction as well as magnitude.

[darknesssacred]

Why not just solve it in 1 formula..?

Vy^2 = Voy^2 + 2a(X-Xo)

100^2 = 0^2 - 2(9.81)(X)

10000÷(2x9.81) = -X

X = -509 3s.f. (taking upwards as positive)

[Joeylumadip]

@darknesssacred

yeah but hes not just trying to solve it. Hes trying to explain how to derive that equation. Its great that you know the big four and can plug in numbers, but hes trying to get people to understand physics.

[Ghaiyst]

@darknesssacred Thats exactly what I did in my head!

[CRISNCHIPS12398]

Oh my god! Projectile motion is independant from mass of objects!

[TheXAnathema]

Dropping a rocks off a cliff. This is exactly the kind of reincarnation math has been asking for since Carl Sagan died. FUck I wish I could afford real college.

[dvnstvn9]

khan u r funny! lol!!

[abz2000123]

a mathematical calculation of the acceleration of wtc 7 collapse, height = 174m t = 6.5s d = Va X t Va = 174 / 6.5 Va=26.7 V1 (intitial velocity) = 0 Vf (final velocity) = Va x 2 Vf = 26.7 X 2 = 53.5 ^V (change in velocity) = Vf-V1 ^V = 53.5 - 0 = 53.5m/s ^V = a X t 53.5 = a x 6.5s a = 53.5/6.5 a = 8.2 m/s/s which would imply that the lower floors were absent as the upper floors descended. so given air resistance - why the resistance of less than thin air??? abz

[cathan75]

- 500 metres ??

Thought u said distance is a scalar quantity and as suc always has to have a positive value??

[cathan75]

- 500 metres ??

Thought u said distance is a scalar quantity and as suc always has to have a positive value??

[sniper0245]

@cathan75 The negative is only there to show the direction of the object. but i understand where you are coming from with your question.

[larahassani]

lol what happened to terminal velocity

[buttali100]

it is gr8

[111000may]

You are truly amazing Sal, I learned it after watching this video just once. Thanks a bunch for sharing your knowledge!! :)

[iceblitzyt]

You better not die when you jump off i need to pass my a levels :P

[SuperSlayerX]

Average velocity is "Vav" not "Va" and the velocity equation should be associated with displacement (which is the same as distance in this case). The symbol for displacement is delta "d" with a line over it

[Ryanxclarke]

how can u travel downwards faster than gravity? what other forces act on you?

[kimbo1601]

@Ryanxclarke gravity is a very weak force

[paintdataint]

you were going 100 m/s and nothing bad happened?

[sansiddhjain]

change in distance will not be -500m but the change in displacement will be -500m as distance is scalar, displacement is vector

[200YearsofSolitude]

@sansiddhjain, but and then he says change in distance, which is the same as displacement ;)

[pasqualified]

your terminology is lame. why not speed, displacement, etc...?

[kaak1212]

the acceleration of gravety is 9.8m/s

[200YearsofSolitude]

@kaak1212 Yes, but 9.8 is almost 10 if u round it off, he didnt wanna complicate the calculation wit decimals dats y :)

[kaak1212]

@200YearsofSolitude i know i am just being techinacal lol anyway it is ACTUALLY -9.8 cause it is going down

[purpleleach1]

displacement would be a better name than distance. Displacement is actually a vector unlike distance. The definition of displace is "change in position". The variable used for displace ment is "x".

[TheFuad92]

if i hear a man is dead due to a fallen penny, im guessing the guy who threw the penny has been watching khan videos

[baeronautics]

@TheFuad92 lol good one

[Aurelien48]

good video...but we dont learn physics until high school haha. i wished we talk about velocity in elementary school

[chadsani]

or u could say Dy= (1/2)GxT^2

[notnowchief8]

@chadsani assuming that the V1y is equal to zero, I wouldn't quote me on that though. Its been a long sumemr

[91jgphonecall]

or Khan is going 216 mph when he hits the ground, hehe.

[91jgphonecall]

the penny is going 216 mph when it hits the ground, fu....!

[webmastertool]

mythbusters actually proved that.

[cubbbbbbz]

i love the part were u say " im not going to give any bad ideas if ur a bad person"

[jenjen0308]

why not directly use the equation v^2 - u^2 = 2gd? =3

[deeagnol]

Good video but he should refer to the actual figure of acceleration due to gravity (9.81) instead of 10 IMO

[Divjot04]

8:35

good idea or bad? :P

BTW these concepts are not in Syllabus of our (India) 11th grade , however i feel these are some really vital concepts...

Thanks a lot.

[pompilot12]

actually i believe that 10 (9.8) is positive because going against gravity would be negative.

[FreezingSun14]

its a miracle you didn't get hurt after jumping off that high of a cliff, but then again your so godly its not very suprising you didn't get hurt

[osvaldo0407]

This is so f*k**g easy, lol!

I call you all noobs at math, and logic.

It's simple.

[gan2ws]

@osvaldo0407 then why did you watch? another noob? haha

[PhysicsSnitch]

@gan2ws I was looking for some talent on the web, don't call me noob.

[k00lkid333]

this is what i done: i used X=ut (1/2)at^2 then subbed in (x= (0 x t= 0)) x=0.5*10t^2 x=5t^2 then use v=u+at 100=10t t=10 then sub 10 in the 1st equation x=5*10^2 x=5*100 x=500

[salx1000]

they are makin us do this in pre calculus and i missed the class today. now i have no idea. lol?

hope this helps

[MrEireboy]

i thought g was 9.8 and that throwing something upwards was -9.8

[TheGolbe]

You can set the direction either way you like. As long as you clearly state that the upwards direction is negative and the downwards direction is positive then what you said is correct.

[wiibert]

@MrEireboy : 9.81 but we don't want to use a calculator cuz we r lazee

[Hawkallica]

@MrEireboy g is ALWAYS -9.8m/s^2. saying its not negative means that gravity pulls upwards.

[THEPRODUN1VERSAL]

... no wtf... what makes you think going upwards means it's negative

[CrackerXZ]

It depends on what you define as negative and what as positive direction. If "down" (you usually think that as the direction of gravity) is positive then that is the case.

[onlyAerik]

Actually, the acceleration due to gravity you use is dependent on your elevation from the center of the earth, and the density of the earth below you relative to the rest of geography.

It's fine in life to just assume the earth is a perfect sphere with uniform density. So your elevation is sea-level. That's about 9.8 m/s^2.

But there are regions on Earth where it's a little higher, and a little lower. both at different and equal elevations.

the point: the error in 9.8 is very acceptable.

[douknome]

its usually a=-g in projectile problems since that is the only force acting in the y-direction. its negative because gravity is facing downwards ( towards the negative y axis). but g is just 9.81 m/s^2

[Xcen27]

thought most people used -9.8 m/s for gravity? (perhaps its just my crazy physics teacher)

[RuthNPC]

@Xcen27 In one of the earlier videos, he said it is 9.8, but for the purposes of simplifying the calculations, he is using 10 instead.

[krumpinAnnanth]

nah it's 9.8m/s^2

[sonofsun4]

It is really interesting,how some morons try to stamp their problems onto the others by posting idiotic comments...If you do not get the first time (because you have problem with attention or what not), then replay it again and again(and perhaps go back to the 2 previous videos), until you get it and do not disturb the class with your admissions of being stupid!

[Forestgimp1]

@ kaladost

he dropped 500 m not 50

maybe you should listen?

[btkw]

i got confused when u used negative numbers lol but i got it anyways..

[kaladost]

Oh I get it :P Thanks!

[kaladost]

So...can someone explain me how you can drop from a cliff at a 100 meters per second, and ten seconds later, only dropped 50 meters?

[Spungomage]

@Kaladost, Initial velocity is 0, half way down it accelerates to 50m/s (or -50m/s) and final velocity, at impact, is 100m/s (-100m/s).

If you're asking about a seperate problem which isn't in the video, assuming the cliff isn't only 50m high, Initial velocity = -100m/s, distance = -50m and t = 10s

Using formula: S(Displacement) = Vi.t + (at^2)/2

-50 = -100.10 + (a.10^2)/2

-50 = -1000 + 50a

50a = 950

a = 19m/s per second

So you would need an upwards acceleration of 19m/s.s

[Johnjpcoc]

Thanks sal!

U are the best.! :D

[boredman443]

What program do you use for this? I'm sure you mentioned it in one of your videos on trigonometry but I can't seem to remember.

[wiseravi2000]

next time make your own video bkisme.

He's teaching physics.  You're not.

[bkisme]

I won't allow the propagation of false information.

I would expect fallacies like that to be fixed by a grad of Harvard and MIT.

[cgxuvimr1]

@wiseravi2000 shahrukh khan

[KimmyPossible]

he mentioned in the video before, that he was ignoring wind resistance for this area, so he was already. but you're thinkin :D.

[sifs4]

@bkisme dude in physics at this level we don't even worry about air resistance!

[RobStyl]

@bkisme At this basic of a level, a vacuum is not discussed. It is easier initially to forget about wind resistance for the purpose of understanding the formula. This allows us to not have to factor in the net force of air, or the concept of terminal velocity, until we are comfortable handling gravity, weight and mass. Physics is always taught this way at this level asshole.

[hhschool]

you are a life saver! i thought i was going to fail physics..but not anymore=]

[ineed2peeat6]

its funny how he talks as if we are next to him . thanks for taking the time to do this video . ive missed alot of school days and this is helping me keep up :)

[jrberry13]

9.80m/s^2 is gravity...to more exact...lol

[Aali8Britney]

9.8 ms-2 in Scotland

[jaybirdr007]

I think in a previous section he stated it was 9.8 but then said he'd make it easier by having it be 10. It shows the concepts, and allows those just learning to understand what is going on a bit easier.

[jrberry13]

in physics all things need to be exact....it keeps for a cleaner problem and sharing answers won't be different...thats why there's signficiant figures....

[Kanashto]

@ 8:43 i thought u were gonna say "physics allows us to figure how to kill people " :P

[PartVIII]

8:34 lol

thanks for these grade-saving videos.

[wahkimoocow]

you made this so easy to me.

thank you

[apricotsnms516]

thanks sal your the best

[game1208]

I don't get what delta means?

[nickicary]

Delta means "Change in". So, delta T, means change in time. It's the greek letter for "D". Greek letters are used in science and math for abbrivations.

[Aali8Britney]

Greek is bad ass:P

[Paradoxical003]

change in, the difference between the final and initial values of something

eg. final distance minus(-) the initial distance = delta(difference of) distance.

[Aali8Britney]

change in an oblect

[EricLindros]

Aerex12: Yes, it is.  Sal's just rounding to 10m/sec^2 for simplicity's sake.

(technically, it's -9.8m/s^2 since the acceleration is downward, and thus negative)

[Faustesh]

I'm probably wrong, tell me if i am because im not very good at physics. But Since gravity is directed towards the center of the earth wouldn't that make things traveling upward (-)? just wondering.

[Aerex12]

wait....isn't g= 9.8 m/s^2.

[vedranhoe]

yes it is, but my guess he is making it 10m/s^2 because its easier to learn away with 10

[RionoCly]

Best Physic teacher ever.... alot better than my

[megaelliott]

I have calculated also, that a penny that was dropped from that height will have about 35Joules of kinetic energy as it hits the ground which is about the same kinetic energy as a 1300kg truck moving at 4.31m/s or 9.64miles/hour which could surely kill you if it hits you on the head so seriously don't do it.

[alex49carson]

did you factor in air rsistance

[megaelliott]

Dude right below my comment it sais that I did not because a penny has very little air resistance.

[alex49carson]

i know

iwas joking

[nitsuan1]

physics is a fantasy.

[SimeonBerson10101]

THANKS SO MUCH

[pewz13]

Could you use a kinematic equation?

[SlimShuvro]

"i don't want to give you any bad ideas, if you're a bad person" ROFL. Salman Khan is hilarious and a genius

[lukasgomes1428]

so if time is ever negative does it imply time travel to the past?? lol

[ussernameunavailable]

Time CAN be negative, it just depends on what do you call "T=0" as, t=0 might be the moment the item starts going down,so, i can say "at time=-1, the ball was rolling through the table with a velocity of 2(m/s)" XD...

[monterey194]

so basically yea coz youre travelling back from the moment that you defibe as zero

[ussernameunavailable]

not traveling to the past, just traveling IN the past, you can't go through time just cause u defined t=0 when it starts falling... if u define whenever u grow up (let's say 100 years) t=0, rite now u aint traveling to the past, u R on the past, which is different... (maybe time traveling IS possible, but, that isnt what i was trying to say...)

[twistedclown08]

If someone were to drop a penny off of a skyscraper people on the ground need not worry the penny will reach a termanel volcity an stop acclerating.

[gimmeurdosh]

It can still go fast enough to kill you. It wont go sraight through you, but it could still kill you.

[megaelliott]

but if you think about it, a penny has very little air resistance and that is what gives something a terminal velocity so the terminal velocity will still be very very high.

[Sn3kz]

I remember the mythbusters guys dropped pennies from a helicopter or something like that while a high-speed camera watched the pennies to see how they moved, and actually, they were flopping around so much because of that tiny bit of air resistance, that some guy on the ground actually caught a few in his hand because they were moving relatively slow.

[megaelliott]

My statement was just theoretical, and obviously it is impossible to deduce the patterns of air resistance every single time you drop a penny unless you drop it in a controlled area or a vacuum. But that is fascinating that someone actually caught a few.

[Vendresse]

The shape of a penny cause it to slow down too much, but if you drop a more stable and aerodynamically shaped object like a bullet then there are good chances it will fall vertically (as long as the side wind speeds aren't too high I guess). It is even said that bullets shot vertically upwards happen to kill people on their way down

[erinashleyx]

you definitely have a talent for teaching! thank you so much for this. i'm a university student applying to medical school but i have always had trouble with physics - if you can help me succeed on my MCAT i will be forever indebted to you!!! :D

[Pitdark]

thanks a lot... 2mro im goin to have a quiz and this is gonna help a lot..

:-)

[BackyardSkunkworks]

You gotta remember air resistance on the real world version of the penny example.

Mythbusters tested that very myth and found the penny at a terminal velocity of around 20 m/s and would do little more than leave a small cut on the head of the unlucky person it was dropped on.

[gklrajan]

SIMLY GREAT!

[gklrajan]

but my teacher taught me to take g always negative in such prblms cuz using sign conventions wil confuse us it seems...??

[enoughtimeto69]

man are you serious!!?? a penny can go up to 360kmph. i had no idea it goes that fast, lucky i live in a small town where we dont have sky scrapers lol, oh and awesome vids man, they are really helpfull

[moslempersiangirl]

oh thank you what a good teacher Im going to have exam for entering the university and it really helps me.

[cyberdems]

great tutorial, except distance is a scalar. you're actually talking about displacement which can have a negative value. also, a more applicable formula for this particular experiment would be v^2 = u^2 + 2as, which would allow you to obtain the displacement in like two steps.

d=v.t is also only applicable when accelleration remains constant.

[calvinhobbesliker2]

when the only source of acceleration is gravity, it is constant

[coolpopk]

Thanks man! This really helps me out, i'm about to take physics next year and i want to get a head start and so far, you've helped me understand a lot! thanks!

[JiGsAwMoFo]

No if you got pushed off a cliff then who would teach us about physics and calculus! I would be lost without cha, I might as well throw myself off that cliff too. XD

[ktecaya]

Thank you so much for doing this! My teach sucks so i have to learn in other ways. I'm so glad i found you. I signed up with utube just to say thanks!

[polos505]

you are an excellent teacher ! thanks

[butterfly9255]

i think ur sexy

[anthonychemistry]

wow..amazing...

[GaryKoji]

Brilliant, Thanks for all your vids!

[avyion13]

you can use the DAFIT formulas, one being a 'dont care' condition. then plug in and good to go. works for kinematics, free fall, projectile etc.

[xxnaruto1234xx]

Thanks, it helped a lot. ^^

[blacksunkent]

it si free fall

[Shunli11]

so does it mean that mass doent matter??

or is it in this example mass doent play a part?

[Yungclutch]

all things accelerate at the same rate in regard to gravity, nomatter the mass, only friction makes a difference and that has to do with surface area.

[keybladewielder999]

How would one calculate air resistance?

Will the matter and surface area count?

[khanacademy]

Yeah, then the shape of the object (and the air pressure) matters. It can be done experimentally and/or through a computer model.

[djbasic]

you are amazing. thank u for all your videos

[sHLaVa]

GReat!