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#### 浮力

Buoyancy

We know that objects fall towards the Earth

because gravity acts on them, but if gravity

acts on everything, why doesn’t everything sink?

Imagine a submarine that is stationary underwater.

We know gravity is acting on it, but the sub isn’t sinking.

So there must be a force opposing gravity.

We call this force buoyancy.

In this video, we will examine buoyancy and why some things

float while others don’t.

First, let’s review density before we move on to buoyancy.

Different materials have different densities.

It’s a measure of how much mass there is at a given volume,

and it’s determined by the following formula.

Density equals mass divided by volume.

Density is an intrinsic property,

which means it depends on the material

but not the shape or size of the object.

So while a gold bar may weigh more than a gold coin,

they have the same density.

And while a kilogram of rocks and a kilogram of feathers

have the same weight, they have very different volumes

and therefore very different densities.

Generally, rocks and metals are more dense than water and sink

while Styrofoam and wood are less

dense than water and float.

So we see that density affects buoyancy,

but it can’t possibly be everything.

Many modern ships are made of tons of metal

that are more dense than water, but the ships still float.

The boats are floating because they are not just

a solid block of metal.

Their hull is made of metal, but the inside

is full of air and room for people and cargo.

The ship with all the space inside

displaces a greater volume of water

than if it were squished into a block, in which case

it would actually sink.

Therefore, we now know that the volume of the fluid displaced

by the object and the density of the fluid

play a role in defining buoyancy.

With this in mind, let’s take a closer look

at how things actually float.

Here we are placing three identical wooden blocks

and three liquids with different density.

The liquids are honey, water, and rubbing alcohol.

Notice how the more dense the liquid is,

the higher the block floats.

Hence we see the importance of the liquid density

on the buoyancy.

Also, note that the higher the block floats,

the less liquid volume is being displaced.

This demonstrates how the displaced volume plays

a part on buoyancy as well.

So up to now, we know that buoyancy opposes gravity,

buoyancy depends on the density of the fluid,

and buoyancy depends on the submerged volume

of the floating object.

Archimedes, an ancient Greek scientist,

found that the buoyancy force is proportional to the density

of the fluid and the volume of the fluid displaced

by the object.

If this buoyancy force is greater

than the gravitational force acting on the object,

it floats.

The exact equation found for a buoyancy force is given by fb
Fb等于D乘上g乘上VFb是浮力
equals D times g times V. Where fb is the buoyancy force,
D是流体密度，g是重力加速度
D is fluid density, g is gravitational acceleration,

And V is displaced volume.

Let’s use Archimedes equation to figure out

how much weight a cargo ship can carry.

Our cargo ship is 250 meters long, 30 meters wide,

and goes 10 meters below the water’s surface.

Multiplying these together, we know

the volume displaced by the cargo ship

is 75,000 cubic meters.

Multiply them by the density of water

and gravitational acceleration, we

get that the buoyancy force acting on the ship

is 735 million newtons, or about 165 million pounds.

That means that the ship can hold 165 million pounds

of weight before it sank.

That’s the equivalent of 8,000 elephants or even 170

jumbo jets.

We can also use buoyancy to explain

some interesting experiments that you can try at home.

For example, one of these is fresh

and one was accidentally left out of the refrigerator.

How can we use buoyancy to figure out which is which?

Well, one floats and one sinks.

What’s going on?

This is a bit tricky.

While eggshells look solid, they’re actually porous.

That means that the shell is covered in small holes.

Over time, as the egg starts to rock,

the egg’s liquid leaves through these holes

and it’s replaced by air.

When this happens, the volume of the egg

stays the same while the egg’s mass is decreasing.

Therefore, the density decreases.

After enough time, the egg’s density

becomes lower than the water’s density,

and the egg starts to float

Here’s another experiment that you can try at home.

What happens when you drop raisins into soda?

Sometimes they’re on the bottom of the glass.

For other times they’re at the top.

What is going on?

Soda has carbon dioxide gas, which escapes as bubbles.

Raisins are more dense than water and initially sink.

Since raisins have wrinkled skin,

the bubbles can get trapped on the surface

as they try to escape.

These bubbles are buoyant in water

because they’re low density and cause raisins to float.

After the raisins reach the surface,

the bubbles pop and the raisins sink again.

This process repeats, and the raisins dance.

And that’s buoyancy.