But which way is down?

And how much does down weigh? Well, down weighs about a hundredth of a gram per cubic centimeter.

It is light and airy which makes it a great source of insulation and buoyancy for water birds

but if you let go of down...

It falls down.

So that's which way down is.

It's the direction gravity is pulling everything in.

Now, for someone on the other side of the Earth, my down is their up.

But where are falling things going? Why do things fall?

Are they being pushed, or pulled, or is it because of time travel?

First things first: let's turn the Sun into a black hole.

We can do that using universe sandbox 2, this simulator will blow your mind. I love it.

In fact, I love it so much I put a code to get the game for free in the current curiosity box.

If you're not subscribed to the box yet, you are missing out!

Okay, look, for the purposes of this video, we want the solar system.

And here it is. Notice that everything's moving pretty quickly around the Sun.

That's because we currently have the game set so that every second that passes for us,

is 14 days, almost, in the game.

If I change this to one second,

We're looking at the solar system in real time.

You'll notice that it almost looks like it's frozen.

even though the earth is traveling around the Sun at about 30 km/s, it barely appears to be moving.

That is how vast space is. Anyway, let's go back to 14 days

I like that motion. Now look at the Sun

It is not, currently, a black hole, but we can change that. What we need to do is compress the Sun.

So let's lock its mass so that it doesn't change while we make its radius smaller.

Let's make its radius as small as we can.

And, oh, where'd it go? Well it's still there, it's just become a black hole.

Pretty spooky, but now, let's look at the rest of the solar system.

Alright, zooming out and-

huh.

Nothing's... changed. I mean something's changed.

It's colder and darker, but nothing's flying off into space or getting sucked in.

You see, by shrinking the sun, we didn't change the direction of down for the planets.

They're always being pulled by gravity towards its middle and making it smaller didn't move where the middle was.

But also, the strength of that force pulling them to the middle of the sun stayed the same.

That gives us a clue as to what down is.

The clue is the other thing we didn't change: mass.

Mass is a measure of how hard it is to accelerate something; to change its motion.

Now right now, these two balls have zero motion relative to me.

Slapping around this hollow plastic ball

is pretty easy, but doing the same to this solid steel ball

is a lot harder.

Now gravity and weight have nothing to do with this.

Gravity acts downward, not against my horizontal slapping.

Of course, gravity does contribute to friction, but friction works against me when I start moving the ball,

but works with me when I stop the ball.

And the steel ball is harder to stop than the plastic ball.

The difference is mass. The steel ball is more massive

It's more resistant to having its motion changed.

Mass is an intrinsic property; it does not depend on what's around or change from place to place.

It can sometimes be thought of as the amount of matter something has.

Your mass is the same regardless of where you are.

On the moon, on earth, in the middle of intergalactic space floating around.

But all of this said, mass does seem to care about what's around.

Mass loves company.

Things with mass and/or energy are attracted together by a force that we call gravity.

The feeling of gravity is just you and the earth being attracted to one another.

Now every portion of an object with mass attracts other portions towards it.

The average of all this pulling is an attraction between centers of mass.

Giant things like Earth exert an obvious pull, but everything does. Even a baseball.

These two baseballs are attracted together by their own gravities.

Except their masses are so small, the force is minuscule, and it can't overcome friction or push air out of the way.

They're never gonna come together

But if you put two baseballs one meter apart in the middle of empty space where no other forces could act on them

They would literally fall together and collide.

It would take three days to happen, but it would.

Isaac Newton found that the strength of the force bringing two things together is equal to the product of their masses

Divided by the distance between their centers of mass squared times big G, the gravitational constant.

If you make one of two objects more massive, or move them closer together, the force will be

Stronger and this force of attraction is what we call weight

So mass is intrinsic whereas weight depends on what's around

Now a weird thing happens when you weigh yourself on most scales

Weight is a force but scales display pounds or kilograms

Which are units of mass what's going on?

Is that a scale is

activated by a force?

Any force.

It doesn't have to be caused by gravity. The scale then displays what amount of mass

Near the surface of the earth would be attracted to the earth with the force. It's detecting

Now since scales tend to be used on the surface of the earth by people on which the only force acting is gravity

They tend to not be very far off, but they can be easily tricked and they further lead to the confusion between mass and weight

Notice that weight is mutual. You are pulled down by earth with the same force that you pull up on earth.

According to a scale I weigh

180 pounds on earth

And the earth weighs 180 pounds on me

but because the Earth's mass is so much greater than my own and

Because the more massive something is the more it resists being moved our

Equal and opposite weight forces accelerate me a lot more than the earth

If you drop a pencil from a height of 6 feet the pencil doesn't just fall to the earth more precisely

They both come together.

They're attracted to each other by equal forces

but the same force moves the pencil a

Lot more than the earth when you let go of the pencil the earth is literally pulled up

To the pencil by the pencils own gravity a distance equal to about 9 trillion

the width of a proton. That

same force moves the pencil the remaining distance, which is still pretty much six feet

At the height of the International Space Station's orbit you and earth are attracted about

10% less than when you're on the surface about eight point eight times your mass but not zero

for this reason weightless astronauts in zero gravity are neither weightless nor in zero gravity

their weight force fails to bring them and earth together because they move horizontally

So quickly that they fall. Just as fast as Earth's surface curves away from them and

Even though they're experiencing 90 percent of the gravity you and I are feeling right now

That's why they don't just fly away

There are no forces called g-forces to resist their weight since everything around them is falling too

It's resistance to your weight force stress

Deformation that is needed for you to feel weight what astronauts in orbit actually lack is

apparent weight

likewise a helium balloon has weight

I mean, it's made out of matter it clearly has mass so it's attracted to the earth

Let's try to measure its weight force

That's eight

Okay it has negative apparent weight

That's because its attraction to the earth is weaker than the buoyant forces from the air around it that push it up

Now while it moves up

It pushes air molecules down, but they transfer that force widely. Not just directly down onto the scale

Buoyant forces are caused by the fact that whenever you are immersed in a fluid like water or air

Molecules lower down are at greater pressure

That are being pressed by the weight of all the molecules

Above them and are closer to earth so they're pulled to it with a stronger force now having greater pressure

Means they pack a bigger punch when they collide with things.

So

horizontally those collisions cancel out

But vertically the stronger collisions from below went out providing lift a buoyant force

This even happens on your own body across its surface area air lifts you with the force of about one

Newton which is equal to the weight force of an apple so if you weighed yourself in a vacuum you would weigh about

this much more

But that's not all Earth's spin causes it to bulge at the equator so the closer you are to it

The further you are from Earth's center of mass and the less your actual gravitational weight will be down is

Always changing, I mean

where is Earth's center of mass? It would always be the same as Earth's geometric middle if Earth's

composition was uniform, but earth contains pockets of massive rock at different depths water mountains

It's got moving changing insides and air and seasonal ice and though they're far away

Gravity extends forever from everything so the moon the Sun the planets all of them pull on you

negligibly,

But truly. You weigh about a millionth of your weight less when the moon is directly above you

This chunky shifting balance of material on earth and ever where else in the universe means that down is

always

changing on top of that Earth's spin

Skews what you consider the direction of down away from its center of mass because the push you get from Earth's spin

Seems to slightly lift you reducing your apparent weight and bending down

towards the equator

The net result is an apparent weight reduction at the equator of about half of a percent if a scale guesses your mass must be

200 pounds at the poles it'll guess that you're

199 at the equator. The 9.8

Multiplier used so often in physics is calculated based on how these factors affect someone at 45 degrees latitude

all of these influences on the direction of down result in a total vertical deflection.

That's only ever at most a few arc seconds anywhere on earth

That's not enough to be felt, but changes in direction and strength can be used to study the shape of the seafloor

Determine what's under you or even help you discover ancient buried rooms?

Point is all of our downs aren't a bunch of radially symmetric lines

Down is an uncombed mess.

Now since solids don't flow they can have shapes that don't pay much mind to this but water can

Flow so ignoring influences like wind and tides the surface of oceans and lakes and puddles is always perpendicular

To down if water could pass through land, or if earth were submerged in water

Gravity would be the same everywhere along its bumpy surface

Such a surface is called a geoid and can be drawn at any altitude

If you wanted to build a table that completely enclosed the earth it would have to have rolling undulation

Z' nearly 100 meters at some points in order to be

Level so that a ball placed anywhere on it wouldn't roll

Here is Earth's G. I exaggerated a thousand times

You'd weigh about a hundredth of a percent less a few grams here?

Then you would say here where gravity is a bit stronger point is the strength and direction

Down is variable by location and changes over time

So down is a fluctuating vector easy enough except?

Why should matter attract matter in the first place?

Isaac Newton was able to describe attraction, but not explain it

humanity got closer however when Albert Einstein introduced his general theory of relativity

Einstein thought a lot about the fact that everything falls to the ground

At the same rate no matter

How massive something is when dropped it will accelerate towards the earth down gaining about 9.8

Meters per second for every second that it falls. That means that a hammer

That's quite massive, and a not so massive feather when dropped from the same height will hit the ground at the same time

Okay, what just happened...

was an "air"ror.

awkward laugh

In order to fall through air a thing has to push air out of the way

But if it has a large surface area and a low weight force it will have a lot of air to move

But won't be able to move that air very quickly. In a vacuum, things do fall at the same rate regardless

Of mass. This was famously demonstrated by Apollo 15 commander David Scott on the moon

And I'll, uh, drop the two of them here and hopefully, they'll hit the ground at the same time.

How 'bout that?

That's weird right? I mean if a more massive object is pulled with a greater force

Shouldn't it fall faster? Well Newton's explanation was simple:

Larger masses are attracted with greater forces

But will also require more force to be accelerated the same as a less massive thing

Something a hundred times more massive might require a hundred times the force, but it will be pulled by gravity

100 times more so everything falls to earth at the same rate

What a fun coincidence right?

Maybe not

Einstein realized that there's another way for things to appear to fall together

of their masses

Imagine a feather and a hammer floating in space in a room if the room is suddenly accelerated up at 9.8