Hi I'm Bryce and welcome to making
things.
In our last video we made an awesome water bottle rocket launcher
and this time I have a question about it:
How come when we're pumping air into the
rocket
it doesn't launch upwards until
it has enough pressure to go really high
up in the air?
As in, how come when I push air into it
it doesn't go up a little
bit and then fall over
as soon as it's above the cork?
Some water bottle rocket
launchers have a latch
that holds the rocket down in place until it has enough
pressure
and then in order to launch it
you release the latch.
But this one
doesn't have that so what's going on here?
Let's talk forces.
If you have two people applying equal forces in opposite
directions to something
let's say like a rope in a tug-of-war
then the rope won't
move.
No one is winning here.
We can say that when we add these two forces
together they equal zero.
We say that the forces are in equilibrium.
A force is any
push or pull
There can be loads of different forces being applied to an object
And that's okay we just add them all up.
But if the force in any one
direction is stronger
the sum of all forces won't be zero
and the rope will
accelerate.
It'll move
The same is true for our rocket.
Before the rocket
launches
it has the force of gravity pushing down
and the force of the
launcher pushing back up against the rocket.
We call that the normal force
which affects the force of friction
which we'll get to in a minute.
The rocket isn't going up into the air
and it isn't falling into the ground
so we know that the forces must add up to zero.
Great!
Also the cork is really wedged
into the neck of the bottle
so it's pushing out
but the bottle is pushing
right back
with its own normal force on the cork
The cork isn't exploding out of
the bottle
and the bottle isn't crushing the cork
so we know they sum to zero as
well.
Now let's look at the rocket
right after it started to move but before it
leaves the cork.
The force of gravity is the same
but now the upward force
comes from the pressurized gas inside the bottle
pushing down through the surface
of the water and onto the cork.
The force is greater than gravity in the opposite direction
so it moves upwards.
But that's not every thing.
As the mouth of the bottle slides over the cork
the surfaces slide alongside each other.
On a microscopic level these surfaces are bumpy and jagged
and push against each other as it moves.
They even form weak bonds between
their outermost molecules
holding the surfaces together.
This resists motion
applying a force in the opposite direction
to the direction that the
rocket is traveling in
but not enough to keep it from moving.
We call this friction
or more specifically
kinetic friction
But kinetic friction isn't the only friction in town.
If you walk up to a shipping
container full of bowling balls
and start shoving it's not gonna go anywhere
unless you're Superman.
It's being held in place by friction.
And since it's
not moving
we call it static friction.
And if you push even harder
it still won't move.
But remember
if it's not moving that means the forces must sum to zero.
So if you're pushing harder
that means the frictional force must be
getting stronger too
otherwise it would be moving.
But this won't last forever
if you keep applying more force
eventually it will start to move
let's say you get
a hold of Superman
and he starts shoving the container.
As he increases his force
from zero
the force of static friction also increases
in the opposite direction
until eventually you reach a point where
it starts moving.
We call that point the
maximum static force of friction
So until you push up to that maximum static force
the object won't move.
But as soon as you push a little bit harder
it'll start accelerating
and the force of friction will become its constant value
of kinetic friction.
But look!
The force of kinetic friction is less than the maximum static friction.
Now let's get back to our rocket.
As we're pumping it up
the force of static friction between the mouth of the bottle and the cork
increases as we apply more force
from the pressurized air
but once the rocket
starts to slip along the cork
the frictional force between the bottle and
the cork drops
from the high value of the maximum static friction
down to the
much lower constant kinetic friction
but the gases in the rocket are still
pushing with the same force as before
which is plenty more than
the frictional force and gravity combined
so the rocket
goes up
and up
and up.
So now that you know a little bit more about how our rocket launchers work
you can watch our previous video to see how to build one.
And don't forget to subscribe to the channel
so you can see all of our videos
as they come out.
Thanks! I'll see you next time,
