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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.
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,
Full Transcript
- 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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