# Newton’s Second Law of Motion

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Welcome back. We’re now ready for Newton’s

second law. And Newton’s second law can

simply be stated– and you’ve probably seen this before as

force is equal to mass times acceleration. This is probably, if not the

most famous formula in all of time or all of physics,

it’s up there. It’s probably up there with

E equals mc squared. But that one’s a little

bit more complicated. So what does this tell us? This tells us that the force,

the net force upon an object, is equal to the object’s mass

times its acceleration. So let’s stay in the metric

system because most of what you’ll do in physic class is in

the metric system, and that tends to be because the metric

system makes more sense. So let’s say that I have

a 1 kilogram object. So its mass is 1 kilogram. And it’s being pulled down

at– let’s say its acceleration. It’s being accelerated downward

at 9.8 meters per second squared. These kind of units should be

familiar with you from all the projectile motion problems. So the force applied on that

object in order to get this type of acceleration would be–

you just multiply mass times acceleration. The force would have had to be

9.8 kilogram times the meter. kilogram. times meter over

second square. That’s the force applied

on the object. And you’re saying, sal,

this is very messy. I don’t like writing kilogram

meters per second squared. And you are in luck because

there is a unit and that unit is the Newton. 1 Newton is equal to 1 kilogram meter per second squared. So if I’m pulling down on an

object at 9.8 Newtons, that’s just this, right? This is 1 Newton. If I’m pulling down at 9.8

Newtons on an object that is 1 kilogram, its acceleration is

going to be 9.8 meters per second squared down. And notice I said the word down,

but I didn’t write it anywhere in the formula. And I guess we can imply that

both force and acceleration have direction by writing

this in the formula. That force is a vector and

acceleration is a vector. And so we could have written

9.8 Newtons– I don’t know. You’ll never see this

convention. We could say Newtons down is

equal to 1 kilogram times 9.8 meters per second down. So what can we do with

this formula? Well we can solve problems.

So let’s say that I have an object. So my object weighs–

not weighs. The mass of my object. And I’ll differentiate between

weight and mass in a second. Let’s say the mass of some

object is– I don’t know– 50 kilograms. That’s how much a

normal person might weigh or a light person. Mass weighs 50 kilograms. And

let’s say we’re in an inertial frame of reference. We’re in deep space, so we don’t

have all these other– the force of wind and

the force of gravity acting on us, et cetera. My force, let’s say I apply

it to the right. So we know that force

is a vector. Let’s say I apply a force of–

I don’t know– 100 Newtons. And let’s say I apply

it to the right. So this is the object, 50

kilograms. And I’m applying a force to the right

of 100 Newtons. So what’s going to happen

to this object? Well, let’s use the formula. Force is equal to mass

times acceleration. The force is 100 Newtons. 100 Newtons is equal

to the mass. The mass is 50 kilograms.

50 kilograms times the acceleration. So we can divide both sides by

50 and you get 100 Newtons over 50 kilograms is equal

to the acceleration. And it’s 100 Newtons

to the right. I’ll use this little

arrow here. That’s not a traditional

convention, but that’s how we know it’s to the right. So it’s 100 divided by 50. So it’s 2. We get this weird units here,

Newtons per kilogram is equal to the acceleration

to the right. This is also going to be to

the right because the direction of the force is going

to be the same as the direction of the acceleration. So what is this, 2 Newtons

per kilogram? Well, if you remember– well you

could just guess that the unit of acceleration is meters

per second squared. But let’s show that this

simplifies to that. So we said earlier that– let

me just switch colors. That a Newton is kilogram meter

per second squared. And we’re taking this Newton

over this kilogram over kilogram, right? So that will cancel out with

that and you get meters per second squared. And you wouldn’t have to do this

on a test. Essentially, if you did everything right, you

would know that the unit acceleration is meters

per second squared. So you would have the

acceleration– I’m just switching the two sides–

is equal to 2 meters per second squared. And it’ll be to the right. So that’s useful. We just figured out based on how

hard I push something, how fast it’s going to accelerate

while I push it. And you could use the

same formula to figure out other things. Let’s say I know that an object

is accelerating– let’s say my acceleration is

3 meters per second squared to the right. Let’s say to the left, just

to switch things. And let’s say that I know the

force being applied on it is– I don’t know– 30 Newtons

to the left. And I want to figure

out the mass. Well you use the same thing. You say force, 30 Newtons to

the left is equal to mass times acceleration. Times 3 meters per second

squared to the left. Divide both sides by the 3

meters per second and you get 30 Newtons over 3 meters

per second squared is equal to the mass. 30 divided by 3 is 10. You can figure out that Newtons

is kilogram meters per second squared. So you’re just left with

10 kilograms is equal to the mass. It’s very important that if you

see a problem where the answer’s given in– I don’t

know– kilometers per second squared or you know, instead of

giving it in kilograms it’s giving it in grams or decagrams,

you should convert back to kilograms or meters just

so you make sure you’re using the right units. And that tends to be frankly,

I think, the hardest thing for people. And we’ll do all of that when

we tackle harder problems. I think now is a good time back

to actually differentiate between mass and weight. And you’ve probably thought the

two were interchangeable, but they’re not. Mass is how much of an

object there is. You can almost view it as how

much of the stuff there is or you can almost it view it–

how many atoms there are. But even atoms have mass. So just how much

stuff there is. And another way to view mass is,

how much does the object resist change? And that actually falls

out of F equals ma. Because if our mass is bigger,

it’s going to take a lot more force to make it accelerate

a certain amount. If the mass is smaller it’ll

take less force. So mass can be viewed as how

much stuff there is, of an object there is. Or you can view it as how hard

is it to change what that object is doing. If it’s stationary, how hard

is it to accelerate it? If it’s moving, how hard

is it to maybe stop it? Which would essentially

be decelerating. How hard is it to accelerate

an object? Weight is actually how much

is– what is the force of earth upon an object? So you’re weight would actually

change if you go from one planet to another because

the force of gravity changes. So your weight is 1/6 on the

moon as it is on earth because the pull of gravity is 1/6. But your mass doesn’t change. There’s still the same amount

of Sal on earth as there is on the moon. So your weight really– when you

ask someone in Europe and they say hey, you know, I weigh

50 kilograms. You should say, no, you don’t weigh 50

kilograms. You weigh whatever 50 times 9.8 is. That’s like 400 something–

you weigh 490 Newtons or something. This is mass. And it’s interesting because in

the English system, and all of us Americans, we use

the English system. When we say that we weigh 10

pounds, we’re actually using the correct terminology

because pounds are a unit of force. We’re saying, if I weigh– and

I do weigh about 150 pounds. That means the earth is this

pulling on me with 150 pounds of force. And actually, turns out that

my mass is measured in the unit called a slug, which

we might discuss later. Actually, we’ll do some problems

where we do it in the metric system and the

English system. And I’ll see you in the

next presentation.

NatawatPost authorProved useful, thanks. ๐

Cody SlabPost authorIgor watched this

AbbyPost authorThank you!

David LePost authorTHIS IS VERY HELPFUL THANK YOU

sn81Post authorwhat a great video. I am a physics student and i will say that you got everything right, and that this vid will be quite helpful to many. it doesnt get any more user friendly than that! let's not glorify the english system though by saying it is correct cuz it measures pounds on a scale. the english system is obselete. can we please move on??

santagotgangbashedPost authori left my science book at school.. got a test lesson 1 tomorrow…

saved my ass ๐ thanks mate

Joseph HamarichPost authorEven if I fail this test, you have been insightful

Jason NewtonPost authormy last name is newton ๐

TheOnlyMohammedPost authorWHAT IS A VECTOR I HAVE SEARCHED THE NET AND I STILL CAN'T UNDERSTAND IT can u help explain

Chris BianchiniPost authorhe has a video about vectors… im not sure if it is important anymore but he does

Nutterbutterz95Post authorIt's basically just a magnitude (like speed) in a direction (to the left).

Think of a tornado. A vector that represents a tornado could be something like 500 mph toward someone's house. Obviously a grim example, but I hope it explains.

Nutterbutterz95Post authorIt's really just for the sake of reference. I, for one, should know whether or not a ball is going toward me, or toward the ground. Speed, the scalar, is really unspecific.

Rory O'ConnellPost authorThe law he works out if F=MA.

Force = Mass multiplied by acceleration

Force is what is called a vector. A Vector is a unit of magnitude and direction. How fast something is moving and in which direction it is moving in.

As opposed to a vector, a Scalar only requires magnitude. If you just give an acceleration of 3 metres per second squared, that is a scalar.

However if you give 3 metres per second squared to the left, that is a vector.

Rory O'ConnellPost authorYou can give more specific terms of direction. For example you could say 20 degrees North of West.

I hope that clears it up for you, however if you need anymore clarification I would be happy to offer an explanation.

k1773nsPost author๐ฎ thanks that really helped me with my homework !

—k

udeadking45Post authoris there already a symbol or or equation showing the absence of all forces such as deep space? just a question.

Vladimir KrylovPost authorThis is soo much more sence then sitting in the class and listening to the teacher -.- Thanks mate!

crystalidxPost author@vlad7753 yeah i was having so much confusion! everyone in my class seem to be kk expect for me.

Bonnie LamPost authorThank you so much for your explanation as I felt so confusing to the third law before that;)

Kalp PatelPost authorms^-2!!

avantgardener3Post authorReally clear. Thanks

Hellsslave666Post authorI always assumed that the metric system is better (im german hehehe, but with mass and weight the english system makes a lot more sense. I knew kg for weight is "wrong", but i didn't knew that lb is "correct"!

Thatks a lot!

tbirdsciencePost author@MrGMauler

Objects most certainly do accelerate in space! Acceleration is defined as a change in velocity – and velocity is speed with direction. So…change the speed, you accelerate. Change the direction – you accelerate. So objects (such as the earth) that are orbiting an object (like the sun) are constantly accelerating because they are constantly changing direction.

ChevailerNoirPost authorYet another great video! Thank you!

Emma FranklyPost authorThank you!

Carl Benedict MonterroyoPost authorNIce video

Carl Benedict MonterroyoPost authornice video thank you

Cavin Dionel N. BatingaPost authorthere! now i can report this thing… thankz

Ad1gaJudy7797Post author@danglang11

changing direction is acceleration because acceleration is defined as the change in velocity and velocity is the speed AND direction of an object ๐

tejbzfanPost authorvery helpful

Gabe GallegosPost authorwhy aren't there people like you in the classroom, instead of teachers that don't even explain things that well!!

mike davinciPost authorthanx very much , and if i'm right then

F = ma

=m x v/t

F=momentum/time (momentum = mass x velocity)

fcdog555Post author@floopsie666 wrong, F=dp/dt where p=m0v/sqrt(1-v^2/c^2)

Relativity ftw.

Moonman2197Post authorโข Its all wrong. Its called laws of Cuck Norris

1) Inertia: Objects will remain at rest or in a uniform motion in a straight line, unless acted upon by Chuck Norris.

2) Force, Mass, Acceleration: The acceleration of an object depends upon Chuck Norris and the magnitude of his force.

3) Action & Reaction: For every action there is an equal or opposite reaction (except with Chuck Norris, there is no such thing as an equal reaction, Chuck Norris is always one step ahead).

keigerb19Post authorthank you very much i need this for our homework

MaddixPost authorE=MCยฒ is false ๐

SwagMcBallerPost authoryay! im learning! ๐

naresh kumarPost author@vlad7753 so right

Vladimir KrylovPost author@tuesday2998 Well, it helped me, so that's what really matters ๐

jaakbrelPost authorso when a boxer hits someone, it has to do with his mass and accaleration?

tangredssPost authorhow is newtons 2nd law of motion applied during a 100m sprint?

Zach LewisPost author@tuesday2998 thats what his laws are about? math and physics.

hexihaxiPost authorEn bra video det hรคr Jan!

Hรคlsningar Klassen.. ๐

07spider123Post authorTHANK YOU SOOOOO MUCH

Kortny FitzsimmonsPost authorWhat are three real life examples for this?

MrAircraft999Post authorthumbs up if you're learning this at school

coolbrujah2Post authorthis is a great lesson, i have a question, and please bear in mind i know little of mathmatis, ans im arrempting to improve upon that, but, would a problem arise from the left side of the equation being done in metric, and the right in well, standard?

kickodudePost authorBro I love you. That is all. No homo of course.

Iain IlottPost authorIn our physics class we use m.s(power of -2) and stead of m/s(power of2).. mathematically its the same thing, but its weird lol

jbarah05Post author@zzzMegazzz Because one is your action force and the other is reaction force which are the same thing and expressing it with -2 is telling you which side your are working with

Chirag PatelPost authorwhy do you teach online? you should be a professor!

Ravyu SivakumaranPost authori know right, we do the same thing

Ghost572Post authorI assume if you threw a ball upwards it would gain potenial energy as it goes up and then gravity pulls the ball down which is 9.8 m/s2.

Ghost572Post authorThanks for this vid man!

George WashingtonPost authorhome work done ๐

melac12Post authorCan someone please answer? If falling force is almost constant near earth, how come with the same farce object falling from higher distances hit the ground harder and with more impact, shouldnโt it be like if you want to hit something harder use more force. But in this case force is constant. I would appreciate if someone could help.

Jonathan PortierPost authormuch better than my teacher

Eithan MalerPost authorSo I ran upon a problem in physics where it said the objects mass and it's speed but not it's acceleration, how would I figure that put. If I use F=ma that would mean that 0N to keep it at that speed, and that doesn't quite make sense.

-high school physics student

Adam SharifPost author@Ethan if the speed is constant then the acceleration is zero. If the speed changes between two time intervals use your kinematics to find a.

itsmeTIBORPost authorHonestly, If we were "learning" it, we wouldn't be here. No offence Khan Academy ๐

Jamal KettrellPost author@khanacademy for the net force formula, you forgot to put the sigma sign behind the " f"

slayerA0DPost authorWhat is mass?

Ranish ChandPost authorthe amount of matter in an object

Christie WPost authoryou are j.e.s.u.s

Gaxa GlitzPost authorJust Exactly So Useful Seriously XD

Michael GuoPost authorThe first thing Sal should teach you is how to spell.

forrstfirePost authorSal should also teach you how to reply aswell

(you posted the comment 4 times)

Michael GuoPost authorHaha, my bad. XD

Jade StriderPost authorfuck college. this guy is awesome

vin muruPost authorTHANK YOU SO MUCH FOR YOUR COMMITMENT IT CHANGE LOT PEOPLE LIFES

Lola StewartPost authorThank you!

Ahmad BintouqPost authorThank u I understood in 9min what my teacher needed 2months and I didint understand

asdfPost authorA force F causes an acceleration of 0.20 m s^-2 as he works on mass m1 and an acceleration of 0.30 m s^-2 as he works on a mass m2.

The acceleration in m s^-2, which causes a force 2F mass m1 and m2 together is

A: 0.24 m s^-2

B: 0.25 m s^-2

C: 0.48 m s^-2

D: 0.50 m s^-s

abdellatef abdoPost authorThank you Khan

respect from Morocco , Afrique , Earth .

THE--FUZZZZPost authorThis guy would make such a great Jeff the Killer.

MothBirdPost authorMan this guy can explain a month of class work in 9mins so I can actualy pass my quiz

Patrick RedPost authorthen why would anyone watch the video?

quantumechanickPost authorLet's replace college with Youtube. God knows I've learned more here than during lecture!

P2SbestPost authorgreat video,very helpful

Ken SmithPost authorcommon sense allows me to drag water on a wood pallet, with the force i drag it…the part you need to worry about is safty for others….math of such things are common sense….common sense and safty is actually not common though…and i dont care how smart anyone is…the common sense part is all that really matters, if everyone applied this we would have "communism" i like my role and care not the pay

Daskie69Post authorI have an idiotic question but someone please refresh my mind

If you apply a constant force of 10N to a mass of 1kg, it will accelerate 10m/s^2 until the application of the force ceases, in which case the velocity of the mass will be whatever it was when the force stopped being applied.ย

Now to the question. If you bench press for instance (a small weight that you can do pretty much whatever you want to) you're applying a force to the weighted bar, right? However, the weight is not necessarily accelerating. Say you intentionally press the bar from your chest all the way up with a constant velocity (ofc t(0) won't be the same as after 2 seconds but anyway) of 0,2 m/s, what is the catch in that situation? A force must be applied constantly or the weight would drop on your chest, why isn't it accelerating?

thanks a lot for answers!

Daniel DempsterPost authorplz get a new mouse and write better, PLZPLZPLZ I <3 UR VIDS

Mr Pregnant - Atelston Fitzgerald Holder 1stPost authorโNewtonโs Third Law of Motion,โ a law in physics that has stood the test of time. With exception ofcourse to the quantum world of improbability and ambiguity. Newtonโs first law of motion states that the uniform state of an object remains in motion unless disrupted by an external force. Second law states the relationship between object, mass and acceleration, (F is F = ma). The third law states; for every force in nature thereโs an equal opposite reaction. A propagating law in physics that you can analogically and metaphorically cross-platform into mutually exclusive fields; "figuratively," and would definitely stand the test of time.

Ali AlmansourPost authorIm 160,000

Ali AlmansourPost authorIm 160,000

The Slice724Post authorIs it net force or not?

Byd Retros - PS3Post authorthanks alot for your help ๐

Bobby MillionairePost authorApplying force or newtons to a mass – does this change the weight of the object? Thank you for the video.

Mihika SathePost authorWo!!!w gr8 tomorrow's my science exam it's truely helpful!!!!!thx soo much

Abhishek AbrolPost authorTomorrow my test and realy helpful this video

x-eed 2Post authorthks

BoltofvoltPost author4:00 is the 100N being applied to the 50kg box from the right or is the 50kg box moving to the right at 100N? sorry i am kinda confuse.

SaucePost authorGet a Mac

Ramvel SelvarajanPost authorin this newton's second law examples why you should not considered a negative value for force and acceleration?

Jabari MorrisPost authorfuck this video jp it really helped me

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yooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooou

FrazuelPost authorLol everyone else is watching this for tests im 11 just watching it cuz i like stuff like this XD

Hei SenpaiPost authoreveryone here from years ago are watching this for their exam/test im just watching it for my science homework xDDD

Dylan McCallisterPost authorWho is here for a sanity check?

I use the tsiolkovsky equation all the time but I still need a sanity check here and there.

wyskunPost authorI have a very important 2 questions for me:)

1. In space, there is a space shuttle that does't move. Mass of this shuttle is always constant. This space shuttle has a rocket engine which produce a constant force of 100N. When the engine is started and the ship has a constant thrust of 100N, when the ship starts to accelerate, will the acceleration be constant and remain constant (let's assume 10m / s), or maybe the acceleration will start to decrease as the speed increases? It is related to the rule Ek = 1 / 2mv ^ 2 ???

2. Same space shuttle doesnt move. Shuttle start to accelerate from 0km/h to 10km/h in 5 seconds and it takes "X" energy for example. How much energy it will needed to accelerate also 10km/h more in also in 5 seconds when it allready have velocity of 100km/h ? Also "X" energy? Or much more "X" of energy? And why is that?

Firas AbdullahPost authorOn my science book it says a=F

โโ

M