# Introduction to torque | Moments, torque, and angular momentum | Physics | Khan Academy

Welcome to the presentation
on torque. So, if you watched the
presentation on the center of mass, which you should have, you
might have gotten a little bit of a glancing view
of what torque is. And now we’ll do some
more in detail. So in general, from the center
of mass video, we learned, if this is a ruler and this is the
ruler’s center of mass. And if I were to apply force at
the center of mass, I would accelerate the whole ruler in
the direction of the force. If I have the force applying at
the center of mass there, the whole ruler would accelerate
in that direction. And we’d figure it out by
taking the force we’re applying to it and dividing
by the mass of the ruler. And in that center of mass
video, I imply– well, what happens if the force
is applied here? Away from the center of mass? Well, in this situation, the
object, assuming it’s a free floating object on the Space
Shuttle or something, it will rotate around the
center of mass. And that’s also true, if we
didn’t use the center of mass, but instead we fixed
the point. Let’s say we had
another ruler. Although it has less height
than the previous one. Instead of worrying about its
center of mass, let’s say that it’s just fixed at
a point here. Let’s say it’s fixed here. So if this could be the hand
of a clock, and it’s nailed down to the back of the
clock right there. So if we were trying to rotate
it, it would always rotate around this point. And the same thing
would happen. If I were to apply a force at
this point, maybe I could break the nail off the back of
the clock, or something, but I won’t rotate this needle or
this ruler, or whatever you want to call it. But if I would apply a force
here, I would rotate the ruler around the pivot point. And this force that’s applied a
distance away from the pivot point, or we could say from the
axis of rotation, or the center of mass. That’s called torque. And torque, the letter for
torque is this Greek, I think that’s tau, it’s a curvy T. And torque is defined as
force times distance. And what force and what
distance is it? It’s the force that’s
perpendicular to the object. I guess you could say to
the distance vector. If this is the distance vector–
let me do it in a different color. If this is the distance vector,
the component of the force is perpendicular to
this distance vector. And this is torque. And so what are its units? Well, force is newtons, and
distance is meters, so this is newton meters. And you’re saying, hey Sal,
newtons times meters, force times distance, that looks
an awful lot like work. And it’s very important to
realize that this isn’t work, and that’s why we won’t
call this joules. Because in work, what
are we doing? We are translating an object. If this is an object, and I’m
applying a force, I’m taking the force over the distance
in the same direction as the force. Here the distance and
the force are parallel to each other. You could say the distance
vector and the force vector are in the same direction. Of course, that’s
translational. The whole object
is just moving. It’s not rotating or anything. In the situation of torque,
let me switch colors. The distance vector, this is the
distance from the fulcrum or the pivot point of the center
of mass, to where I’m applying the force. This distance vector is
perpendicular to the force that’s being applied. So torque and work are
fundamentally two different things, even though their
units are the same. And this is a little
bit of notational. This distance is often called
the moment arm distance. And I don’t know where
that came from. Maybe one of you all can write
me a message saying where it did come from. And often in some of your
physics classes they’ll often call torque as a moment. But we’ll deal with
the term torque. And that’s more fun, because
eventually we can understand concepts like torque
horsepower in cars. So let’s do a little bit of
math, hopefully I’ve given you a little bit of intuition. So let’s say I had this ruler. And let’s say that this is its
pivot point right here. So it would rotate around
that point. It’s nailed to the wall
or something. And let’s say that I apply a
force– Let’s say the moment arm distance. So let’s say this distance,
let me do it in different color. Let’s say that this distance
right here is 10 meters. And I were to apply a force of 5
newtons perpendicular to the distance vector, or to dimension
of the moment arm, you could view it either way. So torque is pretty easy
in this situation. Torque is going to be equal to
the force, 5 newtons, times the distance, 10. So it would be 50
newton meters. And you’re probably saying,
well, Sal, how do I know if this torque is going to be
positive or negative? And this is where there’s just a
general arbitrary convention in physics. And it’s good to know. If you’re rotating clockwise
torque is negative. Let me go the other way. If you were rotating
counterclockwise, like we were in this example, rotating
counterclockwise, the opposite direction of which a clock
would move in. Torque is positive. And if you rotate clockwise
the other way, torque is negative. So clockwise is negative. And I’m not going to go into
the whole cross product and the linear algebra of torque
right now, because I think that’s a little bit
beyond the scope. But we’ll do that
once we do more mathematically intensive physics. But, so, good enough. There’s a torque of
50 newton meters. And that’s all of the torque
that is acting on this object . So it’s going to rotate
in this direction. And we don’t have the tools yet
to figure out how quickly it will rotate. But we know it will rotate. And that’s vaguely useful. But what if I said that the
object is not rotating? And that I have another
force acting here? And let’s say that that force
is– I don’t know, let me make up something, that’s 5
meters to the left of the pivot point. If I were tell you that this
object does not rotate. So if I tell you that the object
is not rotating, that means the net torque on this
ruler must be 0, because it’s not– its rate of change of
rotation is not changing. I should be a little exact. If I’m applying some force here,
and still not rotating, then we know that the net torque
on this object is 0. So what is the force
being applied here? Well, what is the net torque? Well, it’s this torque, which
we already figured out. It’s going in the clockwise
direction. So it’s 5– Let me do it
in a brighter color. 5 times 10. And then the net torque. The sum of all the torques
have to be equal to 0. So what’s this torque? So let’s call this f. This is the force. So, plus– Well, this force is
acting in what direction? Clockwise or counterclockwise? Well, it’s acting in the
clockwise direction. This force wants to make the
ruler rotate this way. So this is actually going
to be a negative torque. So let’s say, put a negative
number here times f, times its moment arm distance, times
5, and all of this has to equal 0. The net torque is 0, because the
object’s rate of change of rotation isn’t changing, or if
it started off not rotating, it’s still not rotating. So here we get 50 minus
5 f is equal to 0. That’s 50 is equal to 5 f. f is equal to 10. If we follow the units all the
way through, we would get that f is equal to 10 newtons. So that’s interesting. I applied double the force
at half the distance. And it offsetted half the force
at twice the distance. And that should all connect, or
start to connect, with what we talked about with mechanical
advantage. You could view it
the other way. Let’s say these are people
applying these forces. Say this guy over here is
applying 10 newtons. He’s much stronger. He’s twice as strong as
this guy over here. But because this guy is twice
as far away from the pivot point, he balances
the other guy. So you can kind of view it
as this guy having some mechanical advantage or having
a mechanical advantage of 2. And watch the mechanical
advantage videos if that confuses you a little bit. But this is where to
torque is useful. Because if an object’s rate of
rotation is not changing, you know that the net torque
on that object is 0. And you can solve for the
forces or the distances. I’m about to run out of
time, so I will see you in the next video.

## 94 thoughts on “Introduction to torque | Moments, torque, and angular momentum | Physics | Khan Academy”

1. siddharth tiwari says:

why is that, if we push a door from the corner most side we have to apply less force in order to rotate it by an angle ?. I know that the torque will be more but it is way to mathematical torque = force x length of the arm . I don't want to know it mathematically but physically .

2. Mary says:

Torque sounds like Twerk idek

3. ItsMeJeff08 says:

it's just like Equilibrium

4. Sean Cordry says:

Think about the distance your hand his moving versus the distance moved by the center of the door. At the handle, your hand moves about twice as much as the door, but near the hinge, your hand hardly moves at all, but the door moves a lot.

5. xvaltak says:

Excellent video, thanks for the closed caption.

6. jamal.a Jamala says:

Saya suka

7. Alphamikee says:

"You know then that the net twerk is zero"
That was literally what I personally heard

8. Cirenio Pizano says:

Torque? I think its TWERKK!!! LOL

9. hala abo el hassan says:

what is the difference between torque(F *D)and moment (NAI )? phesically

10. shikirara says:

These videos are amazingly helpful while studying! A good example to use in future would be a see-saw, and I suggest that small points you mutter to be made clearer, like all the torques equal 0, I had to re-watch the video to find that. But otherwise fantastic job thank you so much!!

11. mellygirl23 says:

You're voice is so soothing at 12:30am 🙂

12. louiethemouseful says:

Miley's favorite Physics topic.

13. SycheRyder says:

>Desperately look for videos explaining the topics of interest.
>See Khan Academy video
>smile on my face

14. Elias says:

I swear he moves so dam slow . I have to smash my arrow button so I can keep the problem going.

15. Stefano Ochoa says:

i like how this is a physics video yet 75% of the comments below are completely irrelevant lmao

16. mtndewman1022 says:

5:38 lol it's a dick

17. Innocent says:

i love khan academy

18. maak says:

So what does it mean if a car has  604 lb.-ft. of torque at 3800 rpm ?

and is it better to measure a cars power in horse power or torque?

19. adrian delpino says:

this was no helpful i have a problem getting to the point you are trying to make

20. 100Hasake says:

Is this the same as a moment?

21. DABIXX TWELVE says:

Twurk It Out Sal!!XD

22. Long Nguyen says:

Why some videos are private? What so special?

23. akashino jora says:

Thanks you soooo much…..now i've understood why my uncle was able to open up the car tyre's screw with a longer pipe…….. when i had been kicking so hard with a shorter one but still wouldn't open!!!!!!!!!!

24. kousik adak says:

wrost

25. Chaoyan Yu says:

Sal, I love physics but i am just can't understand it as fast as math

26. faisal al-rashed says:

Great explanation Bro! Thank u

27. Lakshmi Narasimha Murthy Vishnubhotla says:

As usual I love science, I love Sal. Outstanding explanation!

28. Chong Joseph says:

Just a question: if the object is in a space shuttle, how does the mass at the centre act as a pivot? Or is it just a misconception?

29. james_gats says:

"Moment arm", I think "arm" is from the terminology regarding levers ("lever arm"). Moment is a combination of a physical quantity and a distance ("moment of force" is synonymous with "torque").

30. bnynn says:

I have no idea why it should take 9 minutes to explain how to multiply 2 numbers together. It didnt even go into torque at angles! Usual Khan Academy videos. Great lesson, but at the expense of wasted time.

31. Derek Clark says:

After about the 12th video of Khan Academy, I hope everyone realizes to put this guy on speed X 2. Settings-speed-2.

32. i2007t says:

I like to visit you one day and thank you for everything I learned and learning from you.Hus.Australia

33. PixelArt01 says:

Introduction to Torque gets over half a million views.. talk about an outstanding job!

34. Hisham Almadani says:

Your way of teaching is amazing, love it

35. Rameen Saeed says:

could not have explained better!!!😍😍😍

36. shreyas m says:

Great explanation !
😊😊😊😊😊😊😊

37. Piyumi Sugandhika says:

great explaination 😀 thank you

38. Mansi Rai says:

thank you so much

39. SUDHIR JAHAGIRDAR says:

Isnt torque f x d instead of f.d

40. JASS Cat says:

Hey look it 's a cross product.

41. Sean Walker says:

woah i get itttttttttttt!!!!!!!!!!!!!

42. Jas Sihra says:

So basically when we say a car has 500Nm of torque, we basically say that the engine turns faster compared to a car with 100Nm of torque >>??

43. GamingNavi says:

Thank jegus for this channel. If this didn't exist, I would've failed my physics final.

44. Tanim Nasiri says:

Good basic explanation. Wish i could find videos that actually match the rigor of my physics class :/

45. NNhhL says:

if you push an object at a point other than centre of mass it will start rotating right? so what if im in space and someone pushes my legs and as I start rotating, i change my position thus my cm?

46. şule Usta says:

Altyazili olanlar cok daha kullanisliydi acikcasi

47. Gaurav Patel says:

Nobody Can understand by any vedio because of "Internet"

48. Anisa Ali says:

I think torque is cross product not dot!!!

49. Murughia Pandian says:

thank you for your explanations.now i got one doubt that whenever while finding the torque why should we take the force which is perpendicular to the axis of rotation? is there any reason ? explain me if someone knows….

50. Monkey D. Luffy says:

I love you…so much

51. Pranav Sheik says:

torque=r×f not f•d

52. Lucas Olivares says:

Physicist use the term "torque" while engineers refer to the term "moment." So instead of saying lever arm, they carried that aspect over creating "moment arm."

53. Black Ocean says:

Yabançı Umut Öncül

54. Knightwalker00our God says:

it helped a lot

55. arfat zitu says:

what is the difference between torque and moment sir?

56. John kasanga says:

How can i download this

57. Ekam 06 says:

thxx bro u were helpful😆😆

58. Ekam 06 says:

thxx bro u were helpful😆😆

59. Jeriiah Angelo says:

Torque to me.

60. Dipti says:

Thumbs up ! 🙂

61. Ed Pagan says:

what is the name of the program your writing on?

62. Shoulder Escape says:

You've taught me more than my construction teacher did in three 30 minute lectures

63. John Mandrake says:

this was helpful thank you

64. john smith says:

those explanations were great though nice work sal

65. sumit kumar says:

My new PHYSICS SOLVING APP.More then 150+ formulas,Solves for any variable you want,Covers up all physics.download now.https://play.google.com/store/apps/details?id=com.physics.lenovo.myapplication

66. CATch-up says:

One thing I hate is when you say ' i dont know' in most of your video

67. R BK says:

It's not just about force and displacement being perpendicular to each other… it's the dot product

68. TheresA Seng says:

How do we know if it is clockwise or counterclockwise if it does not specify?

69. nurfitriaini yusran says:

very helpful! thanks a lot

70. shubh Bisht says:

where is angular momentum

71. SHARWAN JHA says:

be my physics teacher plZzzzzzzzzzz

72. Adi Tejas says:

Thank you for the explanation !!

73. Physics- Jwala Singh says:

https://www.youtube.com/watch?v=xxPWshcQy-8

74. NICAKAWIRIDER says:

U forgot weight of ruler

75. Dharmendra Upadhyay says:

Good explanation but not good language pls specify the frame of reference ur talking and write equations in hard-core vector form

76. The Science Cube says:

Superb!

77. Pro_Di_Jai 321 says:

why is clockwise torque regarded as negative

78. Akash R N says:

Love this video. Helped me a lot

79. Leila Elrgdawy says:

Incredible what I can learn in 9 minutes that a trained adult cannot explain in 50. Hmph

80. Abeer Khan says:

"What twerk is.." 😂😂

81. Yan Ming Zhou says:

1m shows that lots of people had a bad physics teacher

82. Max R says:

When even your boy Khan can't save you physics grade

83. Maha Tariq says:

👌

84. sobit khatri says:

why torque = force * distance from the point of rotation

85. David Spira says:

i hope you have the guts to remake these videos on a decent resolution one day

86. vadik-grama Grama says:

noob delete cs go

87. Aisha Nazer says:

I have a doubt.can anyone pls clear it?

Are moment of force and torque the same?

88. Boda Beast says:

please , I have been extremely keen to know , in the equation where: driving force*speed = power , when the speed is zero , like if an engine is going to move a car that is at rest , what exactly goes on , thanks

89. Stratic says:

is torque the same as work. it is a vector but has the same dimensional formula

90. ajit singh says:

Difference between torque and moment??????????????😶

91. Leo Liu says:

The equation of torque is tau=r x F, but the formula that you give me is tau=F x d, which is equal to the -(r x F).

92. Chayse Wright says:

Me: Realizing that I am watching this on youtube and not getting any arbitrary energy points…

93. ashok bishnoi says:

concept in useful

94. chris kozub says:

Should be cross product though not dot product bruh