Physics: Newton's First Law

Newton's first law is the one about objects at rest tending to stay at rest and objects in motion tend to stay in motion.

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Transcript

00:36

really terminal velocity ,up now it's a great film Schwarzenegger. alright we're

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done.

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hey have you ever run out of gas? just been driving along belting out Taylor

00:49

Swift at the top of your lungs when all of a sudden your car dies? look at the [car going down the road]

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fuel gauge see that the needle is way on the wrong side of e, maybe you're lucky

00:57

and there's a gas station a few blocks away and maybe you're unlucky because

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you inherited grandma's 1974 El Camino, yeah that thing's like an aircraft

01:06

carrier, but wait why'd the stupid car even stop in the first place?

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well doesn't an object in motion tend to stay in motion? why did it stop? we're on

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a perfectly flat stretch of road what force acted against it? all goes back [road shown]

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to Newton's first law of motion. remember Sir Isaac Newton crazy smart British guy

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lived in the 1600s. not the best at interpersonal relations? old Ike here

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discovered three laws of motion and a lot of physics is built on those three

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laws. we'll be taking a deep dive into each of those laws in future sessions

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but today we're gonna be looking at numero uno. Newton's first law of motion

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is that you do not talk about motion. sorry that was Newton's first law of [chalk board with newton's laws]

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Fight Club. his first law of motion is that an object's motion or lack of

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motion will stay constant unless a force acts against it. so in order for an

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object's motion to be constant that means no force can be acting on it there

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right ?you no see there's this thing called net

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force, and no it's not the next hit drama on CBS this fall. net force means that

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combined sum of all forces acting on an object and if the net force is acting on

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an object balance each other out, well the motion will be constant. so let's

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take a look at the net forces acting on one specific object. and that object is

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yep your butt. don't worry we're not gonna go look at the physics of shaking [boy falls down]

02:30

your moneymaker. for that you can explore schmoops special course on twerking. but

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chances are that you're sitting down right now and you may not think that

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sitting on your tail has anything to do with physics, but Oh your chair says

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otherwise. right now as you sit there gravity is exerting a force on your body

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it's trying to pull you toward the center of the earth but your chair is [man sits in a chair]

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holding you up exerting a force perpendicular to its surface. that force

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is called the normal force. when force is applied to a solid object that object's

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shape changes. it might deform a little or it might deform a lot. even if the chair

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is made of solid granite and a cute little mouse is sleeping on it, there's

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still a tiny amount of deformation because a force even a tiny one is being [mouse sits on rock chair]

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applied to the chair .now if that force isn't balanced by a counter force

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shedder the mouse here would just, well sink into the chair. solid objects

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maintain their shape which is what makes them, you know solid. so they act against

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the deformation by pushing against whatever is causing that deformation. now

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if a brontosaurus tried to sit in that chair the chair may not be able to

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maintain it's shape. the force of gravity acting on the dinosaur might overcome

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the normal force of the chair which is why you'll never see a depiction of a

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brontosaurus taking a load off in a Natural History Museum. so in the case of [dinosaur in museum]

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you sitting on a chair the net force equals zero. the downward force of

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gravity is balanced by the upward normal force of the chair. well here on earth

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that's the case for any object that's not moving. but what about an object that

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is actually in motion and has a constant velocity? well suppose a car has

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cruise control. you're on the highway and you press a button and your car

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maintains a steady speed, and you take a nap. don't actually do that last part. as [navigation system shown]

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we've learned the hard way your car can't just cruise without exerting force

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in burning gasoline, but if the motion is constant, well why is any force needed to

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maintain speed? well it's time to talk about the F word, and no no the other

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f word. we're talking about friction friction is a force exerted by one

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object on another object when the two objects slide across each other. [friction defined]

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in some of our previous lessons we looked at imaginary scenarios in a world

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where we could pretend friction doesn't exist .but as we've learned from when we

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slide past the end of the slip and slide friction does exist. and it can sometimes

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be painful. there's no escaping friction at least not here on earth. think you can [girl grimaces after getting scrapes]

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roll yourself away from friction? nope a car can tell you that's not true. even an

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airplane flying way up in the atmosphere still encounters drag ,which is friction

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from the air. only in outer space can you truly have a frictionless experience the

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United States launched the Voyager spacecraft in 1977 to study planets in

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our solar system. and it's still traveling through space today. [satellite shown]

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unless aliens have grabbed it and eaten it or something.

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in fact it's traveled outside of our solar system it's still moving because

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while there are no forces stopping it. not even friction. if we're stuck on the

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planet though there's no getting around the gravity thing. sure we can reduce

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friction by making surface as smooth as possible, and we add slippery stuff to

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reduce friction - yeah just go wash your hands to feel how soap reduces friction. [woman grins in a bathroom]

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or do seriously go watch out where those paws of yours have been?

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reducing friction is also why you get oil changes for your car. motor oil

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reduces friction in the engine which makes it work more efficiently, but we

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measure the friction between two objects with the coefficient of friction. if

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you're pulling a German Shepard toward the bathtub you've got a high

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coefficient of friction .if you're dragging a hot knife across the stick of [boy drags dog]

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warm butter you've got a low coefficient of friction. and the markings for a

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lovely piece of toast. we'll get more into this coefficient thing in future

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lessons but if we're using cruise control on our car that means the engine

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is producing enough force to balance out the force of friction. so we move at a

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constant speed until some moron in the fast lane decides to start exiting, and

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if we have to hit the brakes so we don't get killed. [woman drives race car]

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well friction creates heat energy which you know if you've ever rub your hands

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together to keep warm and that heat from friction is why cavemen didn't have to

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eat mammoth sashimi. as we've said friction is a force that

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acts in the opposite direction of movement. we can see that in this diagram

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right here. all right well to achieve constant [chart shown]

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unchanging motion we have to apply force in the direction of motion that's equal

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to the counter force of friction. like cruise control -for like a plane flying

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in the sky so high. think of how many counter balancing forces a 747 flying at

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a steady speed has to deal with. well it's got its jet engines providing

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enough force in the forward direction and balance the friction acting on it in

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the backwards direction. when those net forces add up to zero the plane will be

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at a cruising speed and the pilot can finish his you know word search or [men fly airplane]

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whatever. but gravity is still involved too so there has to be enough lift to

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balance that force out as well otherwise the plane would you know do the opposite

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of lift. but while we're thinking about a large object plummeting to the ground

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let's talk about terminal velocity. not starring Arnold Schwarzenegger.

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next time you go skydiving try switching out your parachute for your spare tire.

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then jump out of an airplane and see if your trip down is in a bit more exciting [animation parachutes]

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than it would be with a boring old parachute .when we're traveling through

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the atmosphere we counter drag well drag is a specific

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kind of friction that occurs when an object is moving through a fluid and gas

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air counts as a fluid in this case. the amount of drag on an object depends on a

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few different factors like the density of the fluid ,and the shape of the object.

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and the speed of an object affects the drag as well. the faster it's moving the [jam sandwich shown]

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greater the drag. now in some previous lessons we looked at falling objects and

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said that gravity would provide a constant acceleration of 9.8 meters per

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second squared .but that's not actually the case when you're dealing with drag,

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so let's go back to skydiving. like I said the faster we move through the air

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the more drag acts on us. well eventually we'll reach a point

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where the force of gravity and the force of drag balance each other out, and we'll

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stop accelerating, and just followed a nice easy rate of about 200 kilometers

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an hour. well the point we stopped accelerating is called terminal velocity

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or the end of velocity. and yeah if we don't open our chutes soon it'll [galaxy shown]

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definitely be terminal. well terminal velocity is the highest velocity an

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object can reach in a fall. well when we do open our parachute the terminal

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velocity will drop all the way down to 28 kilometers an hour .the size shape and

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mass of the parachute create much more drag in the air and that force acts in

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counter to the force of gravity slowing us down and letting, us you know live.

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well using a spare tire instead of a parachute? well let's just say that [animation using a parachute]

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falling ...falling that way would be a drag. because well there wouldn't be very much

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drag. so in order to get this rust-bucket moving we have to apply enough force to

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overcome its inertia. Newton's first law is sometimes referred to as the law of

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inertia. and inertia is a property of mass. specifically inertial mass is the

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measure of an object's resistance to acceleration or resistance to a change

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in motion. the more mass something has the more resistance it has to have to [car goes down the road]

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its motion being changed. which makes sense it's easier to pull a

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shopping cart than it is to push a 1974 El Camino because the car has a whole

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lot more mass than the shopping cart. and we're gonna have to put the concept of

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inertia to the tester is going to push the car to the gas station, but we may

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not be able to overcome its resistance to being moved. maybe we can just leave

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it here. it's a about time and get a new car anyway. of course getting a new car

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would involve overcoming dads and neural resistance to open his wallet book. [animation in office] -

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that's a different set of laws.