2 dimensional projectile motion (part 2)

[TheHedgehogPilot]

Thanks a lot for this excellent video but as far as I know, the 'Time of flight (T)' is determined by the formula '2Usinθ/g' , where U is the initial velocity of the particle projected, θ is the angle between the horizontal axis and tangent of the curve, and 'g' is the gravitational constant. As per this formula, on substituting the values we get T = 2(10 x sin45)/10 = 2(10 X 1/√2)/10 =√2 or 1.4 seconds approx.. But you said T = 2√2 or 2.8 seconds approx.. How come?

[Kricardose]

I am very happy to see the vidoe More on 2 dimensional projectile motion after you give this

[bebeheuy]

I Love The Video More on 2 dimensional projectile motion It Can Increase My Knowledge

[Mjhond]

Steady I Really Like This Video More on 2 dimensional projectile motion

[DrDeso1]

I think Sal made a mistake in this video with finding time

[DrDeso1]

If ⌂V=a*t, and you solved for t, it would be ⌂V/a=t, not a/⌂v=t

[WHY70122]

5:26 COOL. that's the coolest thing i've ever found out on God's green earth cuz i always thought that the difference in angles would make a difference in the time the ball stays in the air.

[Muhsin168]

Hel...Sal....There was a mistake,,in the last minutes of this video..

The actual time. of..the ball in the air should be "sqrt2"

not 2sqrt 2..ok

[sydniebon]

and in getting the time, is it accurate to use rational numbers?? i mean , in our normal conversation, we dont say " it tooks 2sqrt of 2 before it hits the gound.". its awkward right???

[sydniebon]

@ 8:44 , 10/5sqrt of 2 is= 2sqrt 2. rationalize the denominator and the answer is 2sqrt of 2/ 4. or 1sqrt2/2. i dont know if its still possible to get the lowest term.

[PuhhMuhh]

When the ball is being thrown upwards at an acceleration of 10m/s^2 at an angle of 45 degrees, then the vertical force from the throw that is pushing the ball upwards is about 7,07N/kg, but the gravity is pulling the ball downwards at a force of 9,81N/kg. Shouldn't the ball be able to fly at all?

[otakughost]

@PuhhMuhh You're mixing things up. Acceleration IS gravity. And Sal is rounding 9,8 to 10. So the acceleration here (gravity) is -9,8 m/s^2, or rounded it's -10m/s^2.

[121CH41217]

thanks man now i get this, hopefully i pass this semester, full credits to you making these videos.

[doublezee17]

THIS IS HARD O_O

[V1CanisMajoris]

ummmm......The longer the video the better i suppose, then you have more time to think in between the explanations. (LOL, if my spelling is bad just point out, haha)

[endysea]

vadersdemise93 is right. It's 5root2 over 10, not the other way round.

[kx32ta1]

I appreciate the video but if you want some constuctive criticism, the amount of stuff you cover in your videos should only be 3 to 4 mins and still have the same effect.

[3973555]

Isnt gravity 9.8 m/s2

[otakughost]

@3973555 Sal is rounding it to 10 so as to make it simpler.

[moj543]

I WISH U WERE MY PHYSICS TEACHER!!!

[azianluvbug000]

You say it so excitedly.

[guillelainez]

Gravity is 9.81 m/s not 10 m/s

[Jantles]

@guillelainez i know it sounds crazy, but my Physics professor has told us we can use 10 m/s for gravity. The reason is, you still have to do the math right, and know the concepts, but it makes the math easier to do in your head.

[purpleleach1]

@Jantles Our physics teacher has us use 9.8 m/s^2

[guillelainez]

@Jantles lucky you, me teacher makes us use 9.81 -_- more decimals :(

[kulkid40]

@guillelainez we just say it to be 10 in most or basic physics classes

[iareace]

@guillelainez @Jantles @3973555 im not sure you understand the way khan academy works. he teaches CONCEPT so you understand the process in solving the problems. he rounds to 10 to make the math easier. using 9.8... that's more of a technical aspect of learning and is somewhat irrelevant if you take into consideration what i mentioned the objective of these lessons are. CONCEPTUAL learning. i think he assumes that once you learn the process you'll go into detail such as using 9.8 instead 10. :)

[iareace]

@iareace whoa. just saw the other replies. srry for reply resposts. please replace with ** yeaa. what they said! **

[123Anythang]

wtf shouldnt t be equal to 5*sqrt(2) divided by 10?

[daanncceerr1]

ur a lifesaver!

thanks sooo much!! :)

[metalmike83]

ummmm...gravity is 9.81m/s^2

[opmike343]

@metalmike83

Notice he's not using a calculator? He's rounding to 10 to make the math easier. Focus on the method, not minor details like how he decides to round a figure.

[metalmike83]

@opmike343 LOL! Dude, gravity being 9.81m/s^2 doesn't require a calculator - it's one of the constant values (at least close to the ground) that you learn right off the bat in physics. I don't think any teachers or professors would round off to 10 on a constant value like that.

[maleeham87]

@metalmike83 When you're in class you can use 9.8 m/s^2.

His goal is to show you the method and make it easy for you to comprehend. 10 m/s^2 is easier to digest when you're learning a concept.

[RebornAc3]

@metalmike83 Omg you're a genius, I wonder why MIT doesn't take you as a professor. *sarcasm alert*

[metalmike83]

@RebornAc3 You know...I've often wondered why that is.

[wiclepotter4eva]

the last part isnt right. if u write the equatino at = v then why would you divide both sides by v to get a. you would divide both sides by a to get v. the answer wouldnt be two square roots of two (2.8) it would eventually just equal square root two (1.4)

[BlackMetalWarewolf]

Heres some info that helped me not get confused at horizontal and vertical

Vertical was derived from a greek word, verticālis, meaning highest point. The term was then bodged into meaning "upright" with time and stupid people.

Horizontal is derived from horizon.

[isam1335]

g=9.8 m/s2 pls =p

[xXSabzzXx]

thanks for the videos, ive watched quite a few and they're all helpful! its free tuition without having to go anywhere :D

[shadyshaheem]

isnt sin 45 = 1/sqrt2???

[isam1335]

@shadyshaheem 1/sqrt2 = sqrt2/2

[matty1258]

horrizontal for horizon. it is straight, i always think of looking out to sea....

[LordAlda]

There was a mistake (if it wasn't pointed out)

Vf-Vi/a = t hence... -5scrt(2)/-10 = srt(2)/2 = 7.07...

P.S. Unit circles are so easy to forget. :-(

[LordAlda]

@LordAlda t = 0.707s :-P Should have calculated it instead of using the other comments for reference.

[drstingrae]

you style does make me think...at least as I want to think about physics

[kristofferinus]

thank you..:)

[miix999]

isnt gravity 9.8m/s?

[r0ckin0utIr3land]

@miix999 9.8 rounded off is 10 innit?...

[2much2teach]

some people just don't appreciate the free knowledge available to them

[TengoNoNombre]

sin (45) is not square root of 2 over 2 but i have that sin (45) equals 7.07 so i don't understand him :(

[MeesterBubble]

He is using more advanced trigonometry which uses a system called radians. Google the "unit circle" for a better idea of where he grabbed that from

[Tsardoz]

Type sqrt(2)/2 into your calculator and see what happens.

Sin 45 = 1/sqrt(2) = sqrt(2)/2 = 0.7071

10 Sin 45 = 5*sqrt(2) = 7.071

Sin 45 = sqrt(2)/2 is just common knowledge from right angle triangle geometry.

[MrSunnyis1]

@TengoNoNombre 7.07 is the same the as 5 sqrt (2)

[TengoNoNombre]

but the opposite side of the angle is 7.07 right?

i got sin (45) (10) = 7.07 and what threw me was WHY he says radical 5?

[astepintothedark]

I apologize if I'm being nitpicky, but the audio is pretty low.

However, the content is great! ;-)

[kadry1973]

yh T should equal 5√2 /10

thanks dude ;) been a great help

[fretboardninja]

You're amazing

[tomzieeee]

it should be Change in velocity divided by acceleration if Change in velocity is = to acceleration x time. good vid tho

[masroor45]

jazakallah brother it was fun watcin and learning

[ultimage7]

Damn!

Life makes sense again :)

Thank you dude!

[emilianopickett]

Warning!! Calculation error. t should = (sq rt 2)/2

[Johnjpcoc]

I want see his face!!

[adroid28]

Thank you :)

[ffddssaavvccxxzz]

very smart, many thanks for u brothor.

masha ALLAH.

[Trigs00]

You make Physics fun to learn. I'm glad I found your vids. Thanks alot =D!!!

[lukewphysio]

awesome all sorted, thanks mate

[lukewphysio]

when you use the numbers should it read

0-7.07 = -10.t

meaning t = 0.707 secs for upwards component, and 1.414 for total time eg squareroot2?

[Kerpal2253]

The value with Sal's physics video's seems to be the guided meditation, he just gets to excited I think and makes an algebra error: Einstein once forgot the (1/2) part in a sqrt within a famous paper.. In this one, I think he made a handwriting error when he was trying to do: Δ V= A (t) 5(2)^(1/2)==-10 m/s (t) ( 5(2)^(1/2) ) / ( -10 m/s ) == (t)

t=1/(2)^(1/2)

[djinnmasterkeladry]

Wait i used sine law instead... doing (10/sin90)(sin45) and that gives me 7.1m/s.

What did i do wrong? Can you not use sine law for this?

[Kerpal2253]

10/(Sin[90 π/180]) (Sin[45* π /180])==5(2)^(1/2)=~7.07107

So you did nothing wrong.

[coccconut]

if -5sqrt(2) = -10t

then doesnt t = sqrt(2)/2 and not just sqrt(2) ?

This would mean the total time is sqrt(2) rather 2sqrt(2)

Did i miss something?

[ameenebrahim]

He's made a mistake at 8.26 mins. Read the first comment of this video where he himself describes it. Just pointing it out so that no one gets confused.

He first said 5 * [square root of 2] = 10 * t

Therefore t = { 5 * [square root of 2] } / 10

Therefore t = [square root of 2] / 2

Anyways, no teacher is perfect. Thank You Salman for these beautiful lectures!!!! May God reward you!!!

[DaleSparrow]

is anyone else watching this for gmod?

[brian100track]

keep the videos coming im learning so much!

[dudemansam4189]

what program do you use to illustrate the equations? What writting program do you use?

[SimeonBerson10101]

he uses paint.

[brooke959]

oh no sorry its because its a minus t

[Paulorific]

It's not a minus t actually. You were right. The actual answer is 2sqrt divided by 2.

[brooke959]

in this video you worked it out wrong to solve for t isnt it 5root2 over 10 which equals root 2 over 2

[bangalirussian]

can u do harder problems...like using calculus plz....

[polos505]

awesome

[ibethgc]

the other difference between the 2 balls is the distance they travel... =D

[Lewani85]

omg!! I no longer need to go to class or read the book....I understand Projectile motion now and no longer need to memorize the crazy equations our prof gave us! Thanks!!

[GoldAK47]

Thanks so much Sal. This is so gonna prepare me for next year! :)

[DeathByLight]

THANKS MAN, YOU'RE THE BEST!

[khanacademy]

There is a mistake. AT 8:25 I start with -5*sqrt(2) = -10t

The correct answer is t = sqrt(2)/2

My brain malfunctioned and flipped the answer. My thanks to Randy Lynch for pointing this out.

[erinashleyx]

yup i thought i was going nuts heheh

[PolyAthletics]

@khanacademy Thank you for correcting that mistake Mr. Khan. You saved me a lot of time. Once again your videos are exceptional. Thank you thank you thank you!!

[kulkid40]

@khanacademy that is only half the time right?

[sydniebon]

@khanacademy its not -5*sqrt(2) = -10t.. -10 is the velocity going down . thats why its negative . :)