Work/energy problem with friction | Work and energy | Physics | Khan Academy

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A conservation of energy problem where all of the energy is not conserved. Created by Sal Khan.
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[@nanjo621]

Sal I don't know if you're still reading these but man I have been learning so much from you- Trig, Pre-Calc, Calc, and now Physics. You're a huge inspiration, man. I'm considering becoming a teacher and I just wanted to acknowledge and thank you. Thanks for all of the hard work you do, day in and day out.

[@ayahqasim9935]

are you a teacher now?

[@adityabhat4523]

i had no interest in maths and physics i used to waste a lot of my time on the internet but now cause of khan academy i have started liking and understanding these subjects a lot better and it becomes very interesting. thnx a lot man

[@dorukbecerall6833]

Now I am studying to my Physics Final Exam from these videos which are very useful and clear to understand. I want to say a BIG THANK YOU! I can understand physics with these videos easily rather than 1600 pages of PHYSICS BOOKS. I am very happy to see people who are making good works to help people. From other side of the world with respects!

[@khirzaydenmalapote7827]

8 years later....did you pass?

[@studtulloss45]

HAHAHA clicking through each video in 2 sec. intervals: "welcome back", "welcome back", welcome back", welcome back", ......

[@prithvivarma4755]

U still exist?

[@oluwasefunmioluyede2352]

beautiful job done.God bless u sir

[@azngirlchibi]

OHHHH, I get it now! Thanks so much - this is a big help, and you explain so much clearer than my physics teacher!

[@suleimanmaina4482]

I have learn so much, words are not enough to express my gratitudes .I pray may GOD blessed you all the Tutors whom have put their effort here.

[@bhahmz]

You SIR are AMAZING! I'm doing 12 A2 exams in January - 5 of which I need to teach myself! :( I've always not liked doing M2 since I don't understand the textbook and I don't like bugging my teacher for help all the time. But now that I've found your videos, I'm finding mechanics more and MORE enjoyable! :) Thank YOU! :)

[@elmanahmed6117]

Love that interesting problem. Great explanation as usual !

[@sushi8562COOL]

This really helped me figure this stuff out, Thanks!

[@eightstars]

i can't thank you enough for these!!

[@username6333]

A great man once said, "If you slid down a slide of sandpaper your pants would feel very warm"

[@Brenden886]

The way he said it made him sound like he was describing hot cocoa on a winter evening but it was bitterly unpleasant in comparison 😂

[@Tmum08]

Very extreme helpful! Thanks

[@academiarelatedemails9769]

Another well made video!

[@romeo.s_regret]

Wow. This channel is awesome😭 never thought I'd ever love Maths or Physics at all

[@her_honey9715]

Today I didn’t understand anything what my physics lecturer told me about work and energy but now after watching your last 3 videos Maaaan this is too much easy than I though ... thanks to you sir !!!!

[@akhockey77]

Wanted to say you saved me from failing my midterm! Thanks a lot keep it up

[@SoccerGurl8P]

WOW WORDS CANNOT EXPLAIN MY LOVE FOR YOU u helped me so much

[@amrit2232]

GREAT BASIC CONCEPTS

[@adamaqmal]

These video make my understanding better

[@gratitudeforall]

@stiffz thanks your way is quicker in exam! However I agree this video is helpful and helped me understand!

[@loik345]

U r awesome man. Good explanation.

[@wahkimoocow]

thank you soo soo soo much!

[@palmapanfu]

I didn't take last year of physics in high school and the introductory physics course at my university is way harder than a high school class would've been lol. My final is tomorrow.... and your videos are perfect for reviewing!! How do you know so much!?

[@tamerthelame]

thank you soo much you were a huge help :)

[@nfedrichyHD]

These videos actually help a whole lot. :)

[@MsRoseapple]

Thank you very much.understood than former:)

[@HooliganBMars]

Thanks a billion! have an exam 2mao

[@fireemblem2770]

Good luck!!

[@jossell911]

Thanks

[@LaurenceMosca89]

you are my heroe!

[@abdallababikir4473]

How can we support Khan academy? Youve done us good for so long!

[@megashani91]

These are earlier videos from the 2011 ones. ANd I know it's hard but they're just as helpful

[@m1dnightshadow]

thanks a lot for thisw mann :p appreciate it! .. all your videos

[@MrASquared100]

i think that way it's easier to stream for those with weaker internet connections. otherwise that kinda opposes the 'knowledge for all' purpose. yes it is harder to see but you still get the concept if you listen along as he writes.

[@Phagocytosis]

I think it's because these are older videos (i.e. he did the earlier concepts again later on). However, I'm not sure if I heard him say something like that once, or if I'm just guessing.

[@jonathanjulian8246]

For record you can still use your computer calculator by doing alt+2 and it will open it as a scientific calc

[@Ez-jw8xv]

Saved my life

[@battlefort501st]

Thanks man =D

[@Texas_Takeover]

i hope your gettin paid alot for these videos, because i learn more from you than from my teacher

[@sharikgorilla176]

@tngbm Yes but the ground isn't moving with the wheels. Example: Try stroking your right hand(Wheels) on to your left hand(ground) when your left hand(ground) is not moving. There is still some heat right? because the wheels are moving when there is intertia from the ground too. This creates the friction.

[@Katsumato0]

Static friction is in the direction of motion if you have a wheel (that's how we are propelled forward, e.g. walking, bicycling). Therefore, this problem is wrong. The non-conservative force should be added instead of subtracted. Very good problem nonetheless. Thanks Sal.

[@sliccburner2332]

give this man a nobel peace prize in education

[@SamRobertduty]

I like your videos. I've learned a lot.

[@ayahqasim9935]

this comment was before i was born omg

[@12manganime]

no matter what hes using to show us this info, im gonna finnaly pass physics!!!! :D

[@Daski69]

im just trying to help... if you're familiar with the E=(mv^2)/2 you'd know that v=the square root of 2gh... much easier and much less time-consuming than the way that the great Khan demonstrated. I'm NOT bashing his videos or way of thinking, he's a genius, just saying.

[@abdallababikir4473]

*conservative forces*

[@kingakhilbansal]

so if they asked you to calculate the work done, what would your answer be?

[@radiacia_3511]

I solved this in a couple of seconds with just a calculator but great explanation

[@junng6848]

Is typing on a computer keyboard consider doing work in physics sense?

[@pero828]

+Jun Ng Well, you are using some force to push a key on a keyboard and you need to move it for a certain amount of lengt so I would consider it work, yeah.

[@luongmaihunggia]

Yes because you are applying force to move some distance. Work = force x distance

[@abdallababikir4473]

You are only doing work of the object gains energy or accelerates. If you mean the springs of the keys getting tensioned and gaining evergy (which will be released after the key is stroked) you are doing work on it. But if you're applying a force and the object doesn't speed up or gain potential energy (such as by gaining height) you are NOT doing work... I just realize that u asked this question two years ago and that probably makes you a physicist by now 😂😂

[@MD-ie2dh]

@@pero828 except work requires the distance traveled to be horizontal tho

[@Espectador666]

@@MD-ie2dh No, it needs the angle between the movement and the force to not be perpendicular. It doesnt matter if it is horizontal or vertical. I think, im just learning.

[@yaboi_69]

ill think ill pass the test tomorrow

[@abdallababikir4473]

So did you pass?

[@crazyboy33398]

@toorbabray No if you push an object then you wood be pushing it in a given direction. Friction is a vector quantity so the friction would have to be opposite the direction you push because the friction would be a retarding force (sllowing the box down) . Of iit went in the same direction then relative to the box it would be adding force not taking it away which isn't possible because friction generates heat from converted energy.

[@kingakhilbansal]

dont you also have to consider the work done by the mass naturally moving down the inclined plane?

[@27DenizKaras]

NİCE

[@mousaalsaeed9410]

is the friction of rolling maximum static friction ? and why it is static friction why the velocity of contact surface is equal zero? I really want to know why.

[@animemangaluv13]

shouldnt the initial energy be in joules?

[@modularcobracustomquadcopt8127]

+Kpopluvr94 It is...

[@animemangaluv13]

+ModularCobra Custom Quadcopters oh I couldnt read it lol

[@lukey_skywalkerog]

QUESTION. why did you multiple the friction force by 500? what tells you to do that?

[@friedpotato6979]

Work due to friction = Ff times displacement cos theta the displacement is 500m so we multiply it by the frictional force to get the Work or energy loss because of friction.

[@aqualife88]

Work= ForcexDistance

[@HassanMohd]

Its so hard to read these. Would you upload this video with higher resolution?

[@JavedAlam24]

+Hassan Mohd I doubt he has the original files.

[@ShebnaRoseFabilloren]

@7:45 The Work done by friction = force * displacement * cosine(angle) ? But why did he skip the cosine part? I was thinking it shouuld be Wfric = (60N)(-500m)(cos5degrees) ? Can anybody explain this to me? Big thanks :)

[@shannontaylor7980]

+Shebna Rose Fabilloren So the (angle) that is a part of the work formula that you were speaking of is actually the angle between the force and the displacement. In this case, the displacement is happening down the slope (to the right in this case) while the force of friction is acting up the slope. That would make the angle between the two 180 degrees. So, in the work formula, it would be F*d*cos(180) --where the cosine of 180 happens to be -1. That is what makes the work done by friction negative work. An example of positive work would be a push in the same direction as the displacement down the incline where the angle between the force and displacement would be 0degrees. The cosine of 0 is 1, making your answer positive. In summary, remember that the angle to take the cosine of is not the angle of incline, but the angle between the force and displacement. Hope this helps!!

[@orchoose]

+Shebna Rose Fabilloren He basicly used formula for work F*d = 60N *500m that is energy absorbed by breaks or whatever and its equal to energy lost from Ep + Ek system.... and its 30 000 J

[@mantukhan9875]

You are in big misconception cause don,t know basic definition of work done. Work is force time displacement in the direction of force from this u will able to derive dot product too.

[@toorbabray]

How can the friction (kinetic friction) be in the opposite direction of the motion or distance ? Think about it we are not pushing a box where the friction is in the opposite side of the motion. Friction comes from the sliding of the two surface in contact with each other. Here while the wheel is rolling, friction will be in the same direction as the distance or motion. This was one question I missed in MCAT! Like if you agree :) so the comment can stay on the top!

[@Abstriku]

Its intro level physics, you have to learn this before you can take air resistance into account.

[@2013dvan]

theses are the older videos , the newer ones are in hd i believe

[@screechez]

All these smart people in the comments. I need help :( I never took this in HS, I took maths I needed to graduate :'(

[@ryanlawton9666]

Why are most of your videos only in 240p? The content is great, but the quality is unbearable.

[@mattkuhry8090]

Why does it matter? Yes a freaking genius

[@mattkuhry8090]

matt J Kuhry he's

[@bingorocks123]

Complains about quality and doesn't realize this video was posted 6 years ago..

[@kalef1234]

Sean Youn still, my monitor in 08 was 1440x900. then again it was a brand new computer...

[@Reydriel]

He probably didn't have the best equipment back then :P

[@oluwashehunaina]

am feeling you sir hear in nigeria,r u a professor of physo

[@lordante89]

you killed me at 2:08 XD but thanks for the great explanation !!

[@lordante89]

+Sushil Mario i have (auto phoenix down) from Final Fantasy to revive my self :P .. XD Google it.

[@ttkid100]

SMARTER THAN 100% of all teachers at school.

[@TaylorSwifts1stfan]

He does that because force is given with respect to the incline plane. If force was in the horizontal direction then you'd be correct.

[@maricrissecarro9965]

haha who would dare to slid down on a slide of sandpaper ? :D

[@AkashPatel-ph9vz]

when you have mgh+1/2mv^2=mgh2+1/2mv^2 how do you know which side of the formula the friction goes on?

[@unrulyObnoxious]

Akash Patel The way to think about the work-energy theorem is Work done by all forces = Change in kinetic energy of the system. Following this, it is obvious which side friction has to go.

[@DABIXX]

negative work of friction? 60? where did he get 60?please explain! thank you!

[@TeenageAznBoy]

The value for friction was given in the beginning of the video.

[@bhahmz]

@Oneill9293 haha oh well, I was the one who signed up for it - thinking anything's possible as long as you put ur mind to it. xP And still believing! hehe :) Best of luck to u too m8.

[@Cryptically]

if the guy pedaled down, you would add kinetic energy to the potential energy right?

[@aayushgupta426]

what to do if the force of friction is not given but friction is there....

[@MagICskattER]

he use Wacom Bamboo Tablet and SmoothDraw Painy Program!!

[@kennetchieful]

how does it became 38.5 KJ..why did you convert that..instead of joules?..tnx ^^,

[@molim722]

@7.44 why doesn't he do 60(500)(cosof angle) isn't it Fdcosangle ?

[@kotosqoposrly]

I love the hilarity put in your videos :)

[@Dee-so1rp]

Could anyone explain how he got 38500-30000=8455? Thank You!

[@herpderp818]

+Deepa Goswami The way he wrote it in purple was strange, but it's Energy initial minus the energy consumed by friction. Doing this results in the final (or kinetic) energy. Soooo he just had initial, or potential energy at first. this was found by mgh. Look at the number in yellow. I think he just wrote really messy in purple.

[@npad426]

@@herpderp818I know this is 8 years late, but thank you lmao

[@daisygirl488]

Why doesn't force due to gravity in the x direction put work in?

[@axalate4572]

the energy of the force due to the x component of the gravity is actually equal to the mgh lemme show u how work = force x distance work = mgsin(5) x distance work = 90 x 9.8 x sin(5) x 500 where 90 = mass , 9.8 = gravitational acceleration and sin(5) x 500 = height (sin (5) = h/500) so calculating the potential energy(mgh) or finding the energy due to the force is the same

[@TaraLynn218]

@TheRick309 friction?

[@RaselAhmed-jc6iu]

I don't know why but I get all the problems here but my exam problems are much harder and I get stuck.

[@ermiasmerine5304]

I don't get it, shouldn't the gravity acting on the bicyclist be 9.81m/s^2*Cos(5) because its at an angle?!?! Please help.

[@davedude1223]

+Ermias Merine There are two ways to resolve the question, one where in the mechanical energy in the system can be calculated and solved for force and the other is the method you used, either can result in the same answer, one is just faster (mechanical energy) both may be asked of you.

[@ishanujhazarika1564]

Thats will give you magnitude of force.. Energy is a scalar quantity and potential energy is always mgh

[@axalate4572]

Gravity always acts downwards it doesn't depend on the surface u are in. if u resolve it u could get the force (mgsin5) and the perpendicular reaction (which is always perpendicular to the surface) (mgcos5)

[@IreneTubeHQ1]

what is up with the quality

[@g00fy_love52]

Can someone help me out on how he got 8455J please. thanks

[@Peter_1986]

Anyway, I'll see you in the next video ♫

[@atharvachaudhari6325]

One of the best video i have seen for studying 😘🥰😍🙏🤩😎

[@atharvachaudhari6325]

Best for revision

[@AdrianVrabie]

hmmm... very unrealistic... as the speed increases, the air drag increases as well.. it would be interesting to calculate it as well

[@5hanu]

Ezzzzz

[@DaBojangler]

isn't Wf equal to F∆rcosø? because W=F•r which is a dot product...

[@Oneill9293]

@bhahmz 12? Shit, I'm bricking doing only 7 haha. Best of luck pal.

[@Daski69]

Knowing the height of the hill (43.6m), couldn't you just say that the Square root of 2gh = v? I'm confused...

[@tommyron1792]

It has friction. It's different from the usual way.

[@cadesacarlito]

stop confusing people in here, he has explained you what to do with pretty good details. plz stop

[@mikumikuareka]

Hm. Dat's strange, but seems with friction byke moves faster???? O_o

[@snarper]

THX! You just saved my sorry ass

[@aliabdal2861]

An electric motor takes a car weighing 800kg up to point A 140m above the ground. If the total resistive forces = 0.25MJ, how much energy does the electric motor use to get the car to point A?how will you work this out? will it be (800)(0.25)(140)

[@jakestimson3451]

work = force * distance so 800*140 = 112000 j Take the total work done to get the car to the top of the incline and add the frictional forces 250000 j to 112000

[@onlyAerik]

he keeps SAYING the website, but how do you actually write it out?! zeeboo? zboo? zebu?

[@richw.4447]

you have to walk before you can run

[@CrimsonSoul777]

Hahaha Slide down a slide of sandpaper... HOT PANTS!

[@naruto5538]

lol ok ok i got it you're the 5th person to comment on that :P i wasn't paying attention as I was studying for an exam at that time lol

[@Zumerjud]

energy IS conserved its just converted into heat, that iswhy you don't get the total KE equal to PE at the beginning. you should've stressed on that