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Newton's third law actions and reactions....

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Right now, you're being pushed around oh sure you might think you're just [Woman martial artist appears]

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sitting there but oh no someone is giving you a shove every step you take

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you're getting pushed back well I for one have had just about enough of this

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who's the big bully? anyway it's our own planet actually

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Earth and it's time to give it what it deserves that was a real mistake so how [Woman performs karate chop]

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exactly does our planet get away with pushing us around it's all because of

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Newton's third law of motion which states that every action has an equal

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and opposite reaction it sounds all deep and philosophical and if it wasn't about

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physics you'd probably see it on Instagram or something some

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black-and-white picture of the ocean with that quote superimposed but as [Instagram post appears on mobile phone]

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we've seen over and over even though it seems like a simple statement actually

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understanding what it means can be a little tricky if you've been following

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along so far then you'll definitely recognize this thing if you haven't been

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following along this is a free body diagram it lets us visualize all the [Free body diagram appears]

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forces that are acting on an object this particular object is a box stuffed full

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of karate trophies and it's being pushed down the sidewalk to our friend Gary's [Man pushing box of trophies]

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house because Gary doesn't believe that a certain someone really is an awesome

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karate master see that applied force there F sub a

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that's an anonymous person who's doing the pushing normally we wouldn't worry

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about the source of that applied force but when we're dealing with the third

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law we recognize that the box is pushing back just as hard as the applied force

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is pushing forward so how is the box moving then and why isn't the person

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pushing going backwards the answer there is another object involved in the

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scenario and it's a big one when we think about Newton's third law we see

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that it pairs things up we can restate the third law and like this if object a

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exerts force on object B then object B will simultaneously exert an equal force

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on object a in the opposite direction that might help make things a little

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clearer and this set up is what we call third law pairs objects A and B are a [Object A and B appear]

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pair because they're both exerting forces on each other

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the third law is all about how objects or masses interact with each other

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so to understand motion we can look at one third law pair at a time so let's go

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back to the box with the trophy's that they only give out to the best martial [Woman pushing box of trophies]

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artists and I'm just going to admit that yes I'm the one pushing the box because

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it's important for Gary to recognize greatness we have one third law pair

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right there the box and me pushing the box but while my arms push against the

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box my feet are pushing against the ground [Arrows pointing at feet]

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and the ground well that's the whole planet just like the boxes pushing back

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on me as hard as I'm pushing on the box the earth is pushing back up on my feet

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just as hard as my feet are pushing down I'm able to generate more force with my

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feet than the box is able to generate back at me which means that the box

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moves forward the ground and my feet are another third law pair and really the

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box and the ground are yet another pair we may think that since we have forces [Woman with box and forces appear]

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that are equal and opposite they should cancel each other out and there

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shouldn't be any motion but they don't cancel each other out because they're

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not being applied to the same object if the box is at rest and I apply a force

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of 50 Newtons to the trophy box that's enough to overcome it and

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initial inertia and friction so it starts moving when the box is applying

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50 Newton's of force back on me that's not enough force to overcome my own

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inertia and friction so the forces are equal and opposite but they're working

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on different objects which is why the motion resulting from those forces is [Woman discussing forces]

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not the same we can express this law in a formula this equation says that the

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force object A exerts on object B is equal to the negative of the force

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object B exerts on object A since force is a vector one direction is positive

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and the other is negative you might also see it written like this it's the same [Force equations appear]

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thing so if you're pushing a lawn mower across your yard and you exert 75

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newtons of force on the garden implement then it's exerting negative 75 Newtons

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on to you and since we have force that means we can invite mass and

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acceleration to the party per the second law of motion that's the thing about

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Newton's laws well they each address separate concepts we can use them in

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conjunction with each other to understand all kinds of motion one other [Newton appears]

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thing to keep in mind as we think about third law pairs is that the forces

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between the two objects will always be of the same type that means we're not

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going to use contact force on that lawnmower and have it turn around and [Man pushing lawnmower]

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use magnetic force on us, for one thing we're not made of metal so that wouldn't

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even work but for another the third law is big on that whole equal and opposite

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thing there is no way for contact force and magnetism to be equal now I'm going

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to make a guess that you're sitting down right now okay sure you might be [Boy laying down looking at screen]

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watching this in bed or while you're standing on the bus or you're being from

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another planet and the rays of our yellow Sun have allowed you to fly but

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we're still banking that your butt is on your seat and we've seen enough force

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diagrams to know that we've got gravity pulling us down and normal force pulling [Woman watching shmoop video while sitting down]

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us up totally in line with the third law right well actually no remember a third

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law pair isn't two different forces it's two different objects that are exerting

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force on each other we can't have one single object be it's own

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third law partner that's against the law against Newton's law at least let's say

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we've got a hunk of cheese on a table why cheese because we're hungry that's [Martial Artist standing in kitchen with wedge of cheese on a plate]

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why here's our free body diagram for it nothing too complicated here

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but FBD's are showing the forces acting on one single object this is our object

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A basically what's our object B let's think about gravity first gravity is

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acting on our cheese as it always does pulling it towards the centre of the

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earth which means that the cheese and the earth are a third law pair here's

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the diagram for this pair an object always creates gravity so just like [Diagram for third law pair appears]

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Earth is pulling on the cheese the cheese is pulling on the earth too... To put

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this in terms of our equation the downward force on the cheese F sub EC

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equals the negative of the upward force on the earth F sub CE but that doesn't

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explain the pairing of the normal force time to get diagramming again our third [Diagram appears]

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law pairing for the normal force is the chunk of cheddar on the table the table

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is pushing up with F sub TC and the cheese is pushing down F sub CT like we

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just said the types of forces have to be the same in a third law pair since F sub

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TC is the normal force from the chair F sub CT has to be the normal force from

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the cheese yep that's right the downward force in this diagram isn't the force of [Downward force diagram of cheese]

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gravity it's the normal force boy physics can make even a piece of cheese

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sitting on furniture into a complicated situation let's look at another scenario

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here at our dojo our sensei has us chop wood as part of our training it's [Man chopping wood]

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probably just a coincidence that he has a fire pit in his backyard that he uses

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every night what third law pairs are involved in this whole wood chopping

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thing we can start with the most obvious that's the ax hitting the log those two

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things are definitely exerting force on one another we'll call the force from [Man holding ax]

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the ax to the log F sub AL and the force in the other direction from the

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log to the axe F sub L A...but we're not done with the ax just yet

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after all something is making it swing since our friend Darryl here is the one

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making the ax move we'll call the force he exerts on the

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axe F sub DA and the force the ax exerts on Darryl will be F sub AD now we

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could keep going we could pair up the normal force of the stump holding up the

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log and the log pushing down on the stump and we could include gravity [Man with ax and another man holding logs]

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pulling on Darryl and Darryl pulling right back on the earth really we could

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find a dozen different pairs in this scenario but the main action we have

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here is Darryl swinging the ax which then hits the log so we'll stop with our

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two pairs another part of our training our sensei likes us to do is where we're [Martial artists pulling a carriage]

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basically services sled dogs it's meant to build up our core strength and to

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teach us humility it's also meant to just get him around town you know this

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sense they might just be a real jerk in any case what are the forces we have in

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play here well we got students pulling the cart actually though we're not [Students pulling the sensei in a cart]

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pulling directly onto the cart we're pulling on this metal shaft which

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is attached to the cart so the shaft exerts a force on the cart and the cart

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exerts a force on the shaft giving us F sub SC equaling the negative of F sub CS

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when we've got the students exerting a force on the metal shaft and it's

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exerting force right back on us since we're already using S for shafts

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we'll use K for karate students to be consistent with our positive and

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negative vectors since the shaft pulling on the cart was

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a positive in the last equation the students pulling on the middle shaft [Students pulling middle shaft with positive symbol]

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will also be positive here so F sub KS equals F sub SK there's one last thing

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though our feet interacting with the ground to make this motion possible in

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the first place again we could get very specific and detailed if we want to

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going into friction and normal force and gravity but for now we're just going to

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consider this a basic contact force between our feet and the ground we'll

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make it into this formula F sub FG for feet to ground equals the negative of F

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sub GF after chopping wood and pulling our

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master around the last part of our training takes place on the tennis court [Students playing tennis]

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basically our sensei slam tennis balls at us as hard as you can and we use our

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skills to try and dodge them I think it's time for me to find a new dojo

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this guy's a lunatic but we can figure out the physics before we leave our

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sensei has a mass of 60 kilograms the racket is 1 kilo and the tennis ball is [Sensei with a racket and tennis ball]

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0.1 kilograms the tennis ball exerts a force of 130 Newton's on the racket how

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fast does the ball accelerate if we don't worry about gravity...Wait,

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acceleration this is looking like a second law problem but actually it's a [Woman dodges tennis ball]

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bit of both we'll go in numerical order in fact the second law tells us that

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force equals mass times acceleration we know the mass of the ball and we know a

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force but is it the right force yes and no we know the force that the ball

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exerts on the racquet but we need the force the racket exerts on the tennis

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ball ah but with the power of third law magic we can just flip this around F sub

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BR equals negative of F sub RB since we were given our force from ball to racket

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as a positive value the opposite force will be negative so the force the racket

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exerts on the ball is negative 130 Newtons now we can just plug in our [Force equation appears]

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numbers and get the solution negative 130 Newtons equals 0.1 kilograms times

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acceleration divide both sides by 0.1 kilograms to solve for acceleration and

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we get an answer of negative 1,300 meters per second squared

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holy cow sensei what are you doing teaching karate you should be playing at [Sensei strikes tennis ball at student]

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Wimbledon now that acceleration is pretty huge but

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it's only going to last a fraction of a second once the ball is no longer in

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contact with the racket the acceleration ends because there's no more force being

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applied oh and that the negative sign there is just telling us that the

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direction of motion is in the opposite direction of the force the ball exerted

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on the racket so it turns out there's no way to avoid getting pushed around by [Woman martial artists performs a kick]

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six no matter what we do we can't fight it because it'll fight us back just as

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hard and as much as we'd like to kick that crap out of physics sometimes we

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know that it's better to learn physics than to fight it just kidding!

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sneak attack [Woman kicks physics book]

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