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Which Way Is Down? — Vsauce luyện shadowing | TryShadowing
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Vsauce
Which Way Is Down?
Which Way Is Down?
Vsauce
·
26:10 · 2 thg 11, 2017
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Ghi âm
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Chấm điểm phát âm chưa hỗ trợ trên trình duyệt này — bạn vẫn ghi âm & nghe lại được.
hey
Vsauce
Michael
here
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0:00
hey Vsauce Michael here
0:02
down here but which way is down and
0:07
how much does down way
0:09
well down raise about a hundredth
0:12
of a gram per
0:13
cubic centimeter it is light and airy
0:16
which makes it a great source of insulation and buoyancy for
0:19
water birds but if you let go of down
0:21
it falls so that's which way down is it's the direction gravity
0:27
is pulling everything in now
0:29
for someone on the other side of the earth my
0:31
down is their up but
0:33
where are falling things going
0:36
why do things are they being pushed
0:39
or pulled or is
0:41
it because of time travel
0:43
first things first let's
0:45
turn the Sun into a
0:47
black hole we can do that using universe
0:50
sandbox 2 this simulator
0:52
will blow your mind I love it in fact I love it so much
0:56
I put a code to keep the game for free in the current curiosity
0:59
box if you're not subscribed to the box yet you are missing out okay
1:02
look for the purposes of this video we
1:04
want the solar system
1:06
and here it is notice
1:07
that everything's moving pretty
1:09
quickly around the Sun that's because we currently have the game set so that
1:12
every second that passes for
1:13
us is 14 days almost
1:16
in the game if I change this to one
1:19
second we're looking at the solar system in real time
1:23
you'll notice that it almost looks like it's frozen
1:26
even though the earth is traveling around the Sun at about thirty kilometers
1:29
per second it barely appears to be moving that is how vast space
1:33
is anyway let's go back to 14 days I like that motion now look
1:37
at the Sun it
1:38
is not currently a black hole but we can change that what
1:41
we need to do is compress
1:42
the Sun so let's lock
1:44
its mass so that it doesn't change while
1:47
we make its radius smaller
1:48
let's make its radius
1:50
as small as we can
1:52
and oh where to go what's
1:55
still there it's just become a black
1:57
hole pretty spooky but now let's
2:01
look at the rest of the solar system all
2:03
right zooming out and
2:06
huh nothing's changed I mean something's
2:10
changed it's colder and darker but
2:13
nothing's flying off into space or getting sucked in you
2:16
see by shrinking the Sun we didn't change the direction of down for the
2:19
planets they're always being pulled by gravity towards
2:22
its middle and making it smaller didn't move where the middle was but
2:25
also the strength of that force pulling
2:28
them to the middle of the Sun
2:30
stayed the same that
2:32
gives us a clue as to what down is the
2:35
clue is the other thing we didn't change
2:38
mass mass is a measure of how hard it is to accelerate
2:43
something to change its motion now
2:45
right now these two
2:47
balls have zero motion
2:48
relative to me slapping
2:50
around this hollow plastic
2:52
ball is pretty easy but
2:56
doing the same to this solid steel ball
3:00
there's a lot harder now gravity
3:03
and weight have nothing to do with this gravity
3:06
acts downward not against
3:08
my horizontal slapping of
3:11
course gravity does contribute
3:12
to friction but friction works against
3:15
me when I start moving the ball but
3:17
works with me when I stop the ball and the steel ball is harder
3:21
to stop than the plastic ball the
3:23
difference is mass the
3:25
steel ball is more massive
3:27
it's more resistant to having its motion changed
3:29
mass is an intrinsic
3:31
property it does not depend on
3:34
what's around or change from
3:35
place to place it
3:37
can sometimes be thought of as the amount of matter
3:39
something has your mass is the same regardless of where you are on the
3:44
moon on earth in the middle of intergalactic space floating
3:47
around but all of this said mass
3:50
does seem to care about what's
3:52
around mass loves company
3:56
things with mass and/or
3:59
energy are attracted together by a force
4:01
that we call gravity
4:03
the feeling of gravity
4:05
is just you and the earth being
4:07
attracted to one another now
4:09
every portion of an object with mass attracts
4:11
other portions towards it the
4:13
average of all this pulling
4:15
is an attraction between
4:16
centers of mass giant
4:19
things like Earth exert
4:20
an obvious pull but
4:23
everything does even a baseball
4:26
these two baseballs are attracted
4:29
together by their own gravity z'
4:35
except their masses are so small the forces
4:39
minuscule and it can't overcome
4:41
friction or push air out of the way they're
4:43
never gonna come together but
4:45
if you put two baseball's
4:47
one meter apart in the middle of empty space where no other forces
4:51
could act on them they
4:52
would literally fall together
4:55
and collide it would take three days to happen
4:59
but it would Isaac
5:02
Newton found that the strength of the force bringing two things together is equal
5:06
to the product of their masses divided
5:08
by the distance between their centers of
5:10
mass squared times big
5:13
G the gravitational constant
5:15
if you make one of two objects more massive
5:18
or move them closer together the
5:20
force will be stronger
5:21
and this force of attraction is
5:23
what we call weight
5:25
so mass is intrinsic
5:28
whereas weight depends on what's around
5:31
now a weird thing happens when you weigh yourself on most scales
5:35
weight is a force
5:36
but scales display pounds
5:39
or kilograms which are units
5:40
of mass what's going on is that a scale is activated
5:45
by a force any
5:46
force it doesn't have to be caused by gravity the scale
5:49
then displays what amount of mass near
5:52
the surface of the earth would
5:54
be attracted to the earth with
5:55
the force it's detecting
5:57
now since scales tend to be used on the surface of the earth by
6:01
people on which the only force acting is gravity they
6:04
tend to not be very far off but
6:06
they can be easily tricked
6:07
and they further lead to the confusion
6:10
between mass and weight
6:12
notice that weight is mutual
6:15
you are pulled down by earth with the same force that you pull up
6:19
on earth according to a scale I weigh
6:23
180 pounds on earth
6:29
and the earth weighs
6:32
180 pounds on me
6:36
but because the Earth's mass is so much greater than my own and because
6:42
the more massive something
6:43
is the more it resists
6:44
being moved our equal
6:47
and opposite weight forces accelerate
6:49
me a lot more than the earth if
6:51
you drop a pencil
6:53
from a height of 6
6:54
feet the pencil doesn't just fall
6:57
to the earth more
6:58
precisely they both come
7:00
together they're attracted to each other by equal forces but the same force moves
7:05
the pencil a lot
7:06
more than the earth when
7:08
you let go of the pencil the earth is literally
7:10
pulled up to the pencil by the pencils
7:13
own gravity a distance
7:14
equal to about 9 trillion
7:16
the width of a proton that
7:20
same force moves the pencil the remaining distance which is still pretty
7:23
much 6 feet at
7:27
the height of the International
7:29
Space Station's orbit you and Earth are attracted
7:32
about 10% less than
7:34
when you're on the surface about
7:36
8.8 times your mass but
7:38
not zero for this reason weightless
7:41
astronauts in zero-gravity are neither weightless
7:44
nor in zero gravity their
7:47
weight force fails to bring them and earth together because
7:50
they move horizontally so
7:52
quickly that they fall just
7:54
as fast as Earth's surface curves
7:56
away from them and even
7:58
though they're experiencing 90%
8:01
of the gravity you and I are feeling right now that's
8:03
why they don't just fly away there
8:05
are no forces called
8:06
g-forces to resist their weight since everything
8:10
around them is falling to its
8:13
resistance to your weight force stress
8:16
deformation that is needed
8:18
for you to feel
8:19
weight what astronauts in orbit actually lack
8:22
is apparent weight likewise
8:26
a helium balloon has
8:28
weight I mean it's made out
8:31
of matter it clearly has mass so it's attracted
8:34
to the earth let's
8:36
try to measure its weight force
8:40
that's a-ok it has negative apparent
8:46
weight that's because it's attraction
8:48
to the earth is weaker
8:49
than the buoyant forces from the air around it that push it up now
8:53
while it moves up it pushes
8:55
air molecules down but
8:57
they transfer that force widely
8:59
not just directly down onto the scale
9:01
buoyant forces are caused by the fact that whenever you are immersed in a
9:05
fluid like water or
9:06
air molecules lowered down are at greater pressure that
9:11
are being pressed by the weight of all the molecules above
9:13
them and are closer to earth so they're pulled to it with a stronger
9:16
force now having greater pressure means
9:19
they pack a bigger punch when
9:20
they collide with things
9:21
so horizontally those collisions cancel
9:25
out but vertically the stronger
9:27
collisions from below went
9:29
out providing lift a buoyant
9:31
force now this even happens on your own body across
9:34
its surface area air lifts
9:36
you with a force of about one
9:38
Newton which is equal to the weight force of
9:41
an apple so if you weighed yourself in
9:43
a vacuum you would weigh about this
9:46
much more but that's not all Earth's
9:50
spin causes it to bulge at the equator so the closer you are to
9:54
it the further you are from Earth's center
9:56
of mass and the
9:57
less your actual gravitational
9:59
weight will be down
10:01
is always changing I mean
10:04
where is Earth's center
10:06
of mass it would always be the same as Earth's geometric
10:10
middle if Earth's composition
10:11
was uniform but earth contains
10:14
pockets of massive rock at different depths water
10:17
mountains it's got moving
10:19
changing insides and air and seasonal ice and though
10:23
they're far away gravity
10:24
extends forever from everything
10:27
so the moon the
10:28
Sun the planets all of them pull on you
10:31
negligibly but truly you weigh
10:34
about a millionth of your weight less when the moon is directly above you
10:39
this chunky shifting balance of material
10:42
on earth and ever
10:43
where else in the universe means that down
10:46
is always changing on
10:49
top of that Earth's spin
10:50
skews what you consider the direction of down away
10:53
from its center of mass because
10:55
the push you get from Earth's spin seems
10:57
to slightly lift you reducing
10:59
your apparent weight and bending
11:00
down towards the equator the
11:03
net result is an apparent weight reduction
11:05
at the equator of about half of a percent if a
11:08
scale guesses your mass must be 200 pounds
11:11
at the poles it'll
11:13
guess that you're 199
11:15
at the equator the
11:17
9.8 multiplier used so often in physics is calculated
11:20
based on how these factors affect someone
11:22
at 45 degrees latitude all
11:25
of these influences on
11:27
the direction of down result in a total vertical
11:29
deflection that's only ever
11:32
at most a few arc seconds
11:33
anywhere on earth that's
11:35
not enough to be felt but
11:37
changes in direction and strength
11:39
can be used to study the shape of the seafloor
11:41
determine what's under you or even
11:44
help you discover ancient buried
11:46
rooms point is all
11:49
of our downs aren't
11:52
a bunch of radially
11:53
symmetric lines down is
11:55
an uncombed mess now
11:58
since solids don't flow they can have shapes that don't pay much mind to
12:01
this but water can flow
12:04
so ignoring influences like wind and tides the
12:07
surface of oceans and lakes and puddles is always
12:10
perpendicular to down if
12:13
water could pass through land or if earth were submerged in water gravity
12:18
would be the same everywhere
12:19
along its bumpy surface such
12:22
a surface is called a geoid
12:24
and can be drawn at any altitude if
12:27
you wanted to build a table that completely
12:29
enclosed the earth it
12:30
would have to have rolling
12:31
undulation z' nearly 100 meters at some points in order to be level
12:36
so that a ball placed anywhere on it wouldn't
12:39
roll here is Earth's goe
12:41
exaggerated a thousand times you'd
12:44
weigh about a hundredth of a percent less a few grams here
12:48
than you would say here
12:49
where gravity is a bit stronger
12:51
point is the strength and direction
12:54
down is variable by location and changes
12:57
over time so down
13:00
is a fluctuating vector
13:02
easy enough except why
13:05
should matter attract matter in the
13:07
first place Isaac Newton was able to describe attraction
13:12
but not explain it humanity
13:14
got closer however when Albert Einstein introduced
13:17
his general theory of relativity
13:20
Einstein thought a lot about the fact that everything
13:23
falls to the ground at
13:24
the same rate no matter
13:26
how massive something is when
13:28
dropped it will accelerate towards
13:30
the earth down gaining
13:32
about 9.8 meters per second
13:34
for every second that it falls that
13:36
means that a hammer that's
13:38
quite massive and a not so massive
13:40
feather when dropped from the same height will
13:42
hit the ground at the same time
13:47
okay what just happened was
13:50
in error in order
13:53
to fall through air a thing has to push air out of the way
13:56
but if it has a large surface area and a low weight force it
14:00
will have a lot of air to move but
14:02
will be able to move that air very quickly in
14:04
a vacuum things do fall at the same rate regardless of
14:07
mass this was famously demonstrated
14:09
by Apollo 15 commander David Scott on the moon
14:20
that's weird right I mean if a more massive object is pulled with a
14:25
greater force shouldn't it fall faster
14:28
well Newton's explanation was simple larger
14:31
masses are attracted with greater forces but
14:35
will also require more force
14:36
to be accelerated the same as a less massive thing something
14:40
a hundred times more massive might require a hundred times the force but
14:45
it will be pulled by gravity 100
14:47
times more so everything
14:49
falls to earth at
14:51
the same rate what
14:53
a fun coincidence right
14:56
maybe Einstein realized that there's another way for things to appear
15:02
to fall of their masses
15:04
imagine a feather and a hammer floating
15:07
in space in a room if the
15:09
room is suddenly accelerated
15:11
up at 9.8 m/s^2
15:14
the feather and the hammer will hit the floor at the same time
15:18
furthermore whether it's the room coming up to meet them or gravity
15:22
being suddenly switched neither
15:25
object will feel any force
15:27
pushing them there's no way to tell which of these happened
15:31
this is Einstein's famous
15:33
equivalence principle he once admitted that his greatest thought ever was
15:39
that of a man falling
15:40
off of a roof
15:42
while falling the man would not feel
15:45
any forces on him even
15:47
though he's speeding up
15:49
freefall is indistinguishable from floating
15:52
alone in space from
15:53
having no forces on you from not being moved what
15:58
if gravity isn't a force at all what
16:01
if things fall not because they're being pushed or pulled but
16:04
because they're not being
16:06
pushed or pulled to
16:08
see how this could be we
16:10
need to talk about straight
16:11
lines what I have here is a retractable
16:15
ID badge holder this
16:18
is a great way to test for straight paths because
16:21
the string is always kept taut the
16:23
card I have behind has two lines drawn
16:26
on it and if
16:27
while I pull the string out it
16:29
always stays between those two lines I will know that
16:33
I never turned while
16:35
I pulled it because any turn will translate
16:37
into a different angle between
16:38
the lines on the card and the string now
16:41
if I put two of these on a flat table
16:43
and pull them out always
16:45
ensuring that they go straight ahead they
16:47
will never meet they
16:49
will be forever parallel
16:50
but now let's put them on a sphere
16:53
a curved surface again
16:56
I pull both strings forward
16:58
making sure that they always are
17:00
pulled out straight no turning
17:02
wait they came together
17:05
well they didn't turn look maybe
17:09
there's some kind of weird force
17:10
that pulled my hands together and
17:13
just like gravity I didn't feel it but it happened
17:15
no what happened was not the result of a force it was just a
17:21
natural result of curvature
17:23
you might be thinking wait
17:26
a second are those really straight lines I mean they don't look that straight
17:29
to me Oh also
17:31
what if they've just moved along latitude
17:33
lines then they'd never come together and those
17:36
look pretty darn straight
17:37
but they're not a straight
17:40
line never turns and
17:42
although latitude lines look straight at first glance
17:45
following one requires turning
17:48
to find straight line paths on surfaces
17:51
whether they're flat like this or curved
17:54
I love the ribbon test now you can use an actual ribbon but I
17:58
have found that a strip of paper works
18:01
even better let's take a look at this path right here it's
18:04
straight at first but then it curves now
18:07
if two people are traveling along this curve and they want to stay together
18:12
the person on the inside will have to cover a shorter distance than the
18:15
person on the outside
18:16
since both sides of this strip of paper cannot change their lengths they'll help
18:20
us find a straight
18:22
if the strip of paper can
18:24
lay flat we'll know that we have found a straight line and as you
18:28
can see the strip can lay flat and follow the straight part of this
18:32
path but when it comes to the curve in order to follow the path
18:35
now the strip well
18:37
it has too much material
18:39
on the inside and
18:39
that material bunches up and leaves the
18:41
plane therefore we know that this part of the path is not
18:47
let's use the ribbon test to find straight lines on the surface of a
18:51
cone well from the looks of it aligned
18:55
directly from the base to
18:56
the tip seems like it would be straight and sure enough yeah
18:59
the ribbon lays flat on that path but
19:01
what about a ring
19:02
around the cone nope
19:05
doesn't work shorter distances
19:07
around nearer the tip of the cone mean that there's too much ribbon up
19:10
at the top so
19:12
it doesn't lay flat let's
19:13
see what else is there though besides this well
19:16
if I start here and just allow
19:18
the ribbon to lay flat
19:22
huh I get a little curvy
19:24
looking shape like this I say
19:27
curvy looking because while to someone say at the base this path might seem
19:32
to go up slow
19:33
down change direction and then fall down faster
19:36
and faster since a ribbon
19:38
on such a path is flat it's
19:40
actually for inhabitants on the cones surface
19:43
perfectly straight if we trace the ribbons path on to the cone we can
19:48
see this clearly because
19:49
a cone can be flattened a straight
19:51
line on a curved
19:52
surface is called a geodesic
19:56
here is a geodesic
19:58
on a sphere the
20:00
Equator is one here's
20:01
another a line of latitude is
20:04
not a geodesic it's not a straight line to
20:07
see why let's try to follow it with the ribbon you know
20:10
what I have to keep kind
20:12
of lifting it yeah
20:13
see distances around the sphere becomes shorter as we go up so
20:18
there's too much material
20:19
on the ribbon up here and it leaves the surface
20:21
this path contains turns and in order to turn a force
20:25
has to act on you if
20:27
no forces did this
20:28
is the path you would take notice
20:30
that the ribbon begins
20:31
moving due east but then falls
20:34
south Falls Einstein realized that curvature
20:41
could cause things to be seemingly
20:43
attracted to one another without
20:45
needing to invent the existence
20:47
of forces like gravity
20:48
but attraction only happens if things move along
20:53
the surface if they stay still they well
20:57
they don't come together so
20:59
for something at rest
21:00
how does falling begin I mean the thing has to move in this direction
21:04
but it's at rest right
21:07
well yes but it's only at rest in space
21:10
and that's not the whole story
21:13
up down forward backward
21:15
and left-right are all you need to describe where an event occur but
21:20
a complete description will also need to describe
21:22
when together these four dimensions formed
21:26
a setting in which everything
21:27
in our universe happens
21:29
space-time since we can talk about a falling pencil using just one spatial dimension
21:34
up and down we
21:36
can use a piece of paper to
21:38
model space-time for it okay
21:40
so we've got up and down but
21:42
we have to add another direction
21:44
the pencil moves in time
21:45
now if no forces
21:47
act on the pencil it won't
21:49
move through space it will only get older and as you can see if
21:53
all it does is get older it won't fall
21:56
if space-time was flat when
21:58
I let go of the pencil
22:00
it wouldn't go anywhere but
22:02
now let's allow the earth which is massive
22:04
to manipulate space-time into
22:06
say a cone now
22:09
with no forces acting on it every
22:12
part of the pencil follows a straight line but
22:14
on a cone as we saw earlier such
22:17
a path will look like this it
22:19
will fall this is because distances
22:22
around the cone are shorter
22:24
higher up time runs faster
22:26
further from a massive object but
22:28
to go straight not
22:30
turn every part of the pencil must cover an equal distance in space-time
22:34
like this only when the pencil hits the earth does
22:38
the repulsion of their mutual electrons
22:40
provide a force pushing the pencil off a geodesic
22:43
for the earth time
22:45
is a series of slices
22:47
from this evolution the
22:48
pencils force-free geodesic is why
22:51
it falls not a push or pull just
22:55
the pencils natural tendency
22:56
to follow a straight line until
22:58
something acts on it now
23:01
we only used one dimension of space and one of time because
23:04
visualizing our two universes
23:06
three of space and one of time would take us beyond the limits of
23:09
what could be shown on paper or
23:10
screens but math can
23:13
take us there general
23:15
relativity allows us to calculate
23:17
how much mass and energy curved
23:19
space-time and has been used to explain things
23:23
that Newton's older theory of falling as the result of forces couldn't
23:27
like anomalies in the orbit of mercury which
23:30
orbits nearest the Sun and is therefore most affected
23:33
by the sun's grip on space-time
23:35
many other experiments have confirmed general
23:38
relativity's picture of the universe fitting
23:40
the conclusion that there
23:42
is no there's just
23:46
space-time its curvature and a
23:50
senate as John Wheeler famously put it space-time
23:53
grips mass telling it how to move mass
23:56
grips space-time telling it how to curve
24:00
relative to the earth we
24:02
don't move very fast even
24:04
jet airplanes move negligibly
24:06
close to the speed of light so
24:08
relative to earth we
24:09
move almost exclusively through time as such
24:13
we are more affected
24:15
by the way time is curved
24:17
by mass than how space is curved
24:19
this has led many to claim that for the most part you feel as
24:23
though you're being pushed
24:24
into the ground not
24:25
because of a force called gravity but
24:28
because time is moving faster
24:30
for your head than
24:31
for your feet down
24:33
is relative and always
24:35
changing but it exists
24:37
because of and is always in the direction of slower
24:40
time Bertrand Russell called
24:43
this the law of cosmic
24:44
laziness everything is naturally
24:48
steered towards where time is
24:50
slowest we call this falling
24:53
going so you don't have to keep anything
24:58
on the down-low time
25:01
will take care of that for you and
25:04
as always thanks for watching
25:18
remember that you can support Vsauce
25:21
and Alzheimer's research by subscribing
25:23
to the Vsauce curiosity
25:25
box the current one comes with a code to get a free copy of
25:28
universe sandbox 2 which
25:29
is amazing and a whole host of other science toys and tools
25:33
picked by myself Jake and Kevin I love it all so I hope to
25:37
see you at brain candy live we are coming to many many cities very
25:41
soon hopefully one near you by
25:43
going to the show you can see Adam and I doing things that will
25:47
you may not have seen us do before we
25:49
also explore the science
25:50
and common misconceptions behind
25:53
all things err maybe
25:57
I've said too much maybe not I hope to see you there and as
26:01
always thanks for watching
Thích
Chia sẻ
Vsauce
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