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Physics Videos 34 videos

Physics: Isaac Newton
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Isaac Newton. Who was he? Why do we need to know about him? In a physics course, no less? Well, he's only the most famous physicist in history, and...

Physics: The Basics of Trigonometry
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What are the basics of trigonometry? And why are we learning about this in a physics course? Both good questions. In this video, you'll learn about...

Physics: Unit Analysis and Graphical Data Analysis
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It's time to make our liters and meters work together. Enough of the bickering, right? In this video, we'll do some unit analysis, covering SI Unit...

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Physics: Newton's First Law 961 Views


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Description:

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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English Language
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Transcript

00:02

Newton's first law or why we want to stay in bed. if I could find the right one...

00:08

a shapes motion - all right net forces -object grass motion -all [Newton's laws listed]

00:24

right unbalanced forces- isn't that

00:26

Congress ? people are all unbalanced? who would ever want that job?

00:32

anyway alright well Tiger pounds in constant motion, the real moral friction,

00:36

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

00:41

done.

00:43

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]

00:53

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

01:00

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?

01:09

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

01:14

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

01:18

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

01:23

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

01:29

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

01:33

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

01:37

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

01:41

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

01:46

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

01:51

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

01:57

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

02:02

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

02:05

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

02:13

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

02:18

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

02:22

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

02:25

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

02:35

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

02:38

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

02:42

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

02:46

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

02:50

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

02:55

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

03:00

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

03:06

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

03:10

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

03:15

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

03:19

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

03:24

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

03:28

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

03:34

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

03:37

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

03:41

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

03:46

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

03:50

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

03:54

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

03:59

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

04:03

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

04:07

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

04:11

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

04:16

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

04:19

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

04:25

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

04:30

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

04:35

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

04:41

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

04:45

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

04:49

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

04:54

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

04:58

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

05:04

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

05:09

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

05:14

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

05:18

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

05:21

unless aliens have grabbed it and eaten it or something.

05:25

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

05:30

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

05:34

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

05:37

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

05:42

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

05:47

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

05:51

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

05:57

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

06:01

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

06:06

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

06:08

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

06:13

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

06:17

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

06:20

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

06:24

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

06:29

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

06:34

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

06:36

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

06:41

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

06:45

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

06:48

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

06:51

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

06:54

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

06:58

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

07:03

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

07:08

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

07:12

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

07:15

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

07:20

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

07:24

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

07:28

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

07:33

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

07:37

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

07:41

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

07:46

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

07:51

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

07:55

the atmosphere we counter drag well drag is a specific

07:58

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

08:02

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

08:07

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

08:12

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

08:18

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

08:22

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

08:25

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

08:30

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

08:33

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

08:37

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

08:43

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

08:48

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

08:53

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

08:57

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

09:01

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

09:06

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

09:10

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

09:15

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

09:19

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

09:22

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

09:27

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

09:31

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

09:35

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

09:41

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

09:46

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

09:51

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

09:55

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

10:01

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

10:04

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

10:09

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

10:13

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

10:17

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

10:21

that's a different set of laws.

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