Cogito, ergo swim?
Today on Michael’s Toys, we’re going to
be building a cartesian diver named after
Rene Descartes.
It’s a very cool thing that you can build
at home yourself.
You will need enough water to fill a squeezable
plastic bottle that has a nice tight-fitting
lid, four sugar cubes, four paper clips, at
least a couple toothpicks, a teaspoon of cinnamon,
two different colors of vinyl electrical tape,
some transparent tape, an assortment of ketchup
packets, a small metal bowl, and a 
 pencil that you have kept
in a dish of salt at least overnight.
A full list of all of these materials can
be found down in the description below.
Step one is to uncap our plastic bottle.
This will contain the ocean that our cartesian diver explores.
Our diver will be a ketchup packet.
Now you could use a packet of any other kind
of sauce, mayonnaise perhaps, mustard, but
I find that ketchup works the best.
Ketchup, it's perfect.
So what we’re gonna do is squeeze up the
little packet of ketchup and insert it into
the bottle.
Plop!
Perfect.
Now that our diver is in there let’s give
it an ocean.
I will put the bottle in this little bowl
just in case I spill which I’m sure I will.
Let’s see if I have enough water.
Perfect.
Alright.
Now make sure the cap goes on very very tightly.
Perfect.
Okay.
Now here we go.
Let’s dry this off and get ready to dive.
So our ketchup packet is floating in the bottle.
Floating up towards the top.
But for how long?
Watch this.
Watch what happens when I squeeze the bottle.
Not so floaty now are you?
But if I stop squeezing, it floats back up
to the top.
I can not only make it sink but I can make
it stay right where I want it to stay just
by applying the right amount of pressure to
the bottle.
I can send it to the bottom with a strong
squeeze
and free it to the top by simply loosening my grip.
It's enough to make anyone go Oh boy!...ancy.
When an object is immersed in a fluid whether
it be a gas or a liquid, pressure is exerted
by that fluid on the object because the object
is being bombarded by the molecules that makeup
that fluid.
Now getting bombarded by a single molecule
isn’t much but try getting bombarded by
trillions.
That could cause some pretty large pressure.
But here’s the thing, the actual magnitude
of the force that comes from being bombarded
by a fluid’s molecule increases the deeper
down that molecules is because of the weight
of all the fluid above it.
Which means when you’re submerged in air
or water the pressure at the bottom of you
is greater than the pressure from above so
any collisions that occur top down like this
pushing you down are weaker in total than
all the ones pushing you up.
Now the difference is a net force up and that
force is called a buoyant force.
But how strong is the buoyant force on an object in a fluid?
Well take a look at this ketchup packet.  Alright it is surrounded by water.
But if it wasn't in this bottle what would be in its place?
Well just water.  Water would be filling that volume.
The water of volume the exact same size and shape as that ketchup packet would be there and what would that volume of water be doing?
Would it be sinking or floating? No.
So there ya go.  The weight of that parcel of water equal in volume to this packet
would be equal and opposite the buoyant force pushing in the other direction.
So it would stay where it is and not sink or float.
This observation is known as Archimedes' principle.
The buoyant force on an object is equal in weight to the volume of water it displaces by being in that fluid.
In the case of this ketchup packet the packet has a smaller weight
than an equal volume of water so the buoyant force wins out
and the packet floats.
But not for long because inside almost all sauce packets
there's a little bit of air and air is much easier to compress
than sauce or water so when I squeeze the bottle
The water pressure inside the bottle goes up and all the air in that packet compresses. It gets smaller.
Which means the volume of the packet gets smaller.
But its weight stays the same so all of a sudden we're talking about a smaller volume
with the same weight.  That's more dense.
Denser things sink.
When I release my squeeze the buoyant force becomes once again stronger than the weight force
because the volume of the packet increases.
Because the air that is right now compressed
expands back.  The ketchup packet takes up more space.
Enough space that an equal volume of water
would weigh more.
Therefore the buoyant force moves it up.
So go ahead and try to build one of these at home and try out different kinds of sauces.
I have found that packets of ketchup work out the best.
I find that soy sauce packets, I've got one right here
They just sink right away.  They are too dense.
Hot sauce packets from Taco Bell have the opposite problem
They float too well. Put one in, it'll
float and no matter how hard I squeeze I can't get it to fall. Maybe you're strong enough.
Try it out.  It's called an experiment.
Thank you science. I find that not all ketchup brands
work equally well though. I have
some oh yeah so these Heinz packets, they also float too well.
If your packet sinks right away
it might just be on the edge and if you add some salt to your water
you may be able to get that packet to float because adding salt increases the buoyant force of the fluid in your bottle.
But I gotta tell you I've made dozens of these in my life and I always find that ketchup is the safest bet.
Watch this.
Are you guys watching?
wait watch...hold on watch this.
And as always, thanks for watching.
