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  • Hey Vsauce! Michael here. Down here.

  • 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?