♪♪
What would our lives be like without gravity?
Think about it.
I wouldn't be here standing firmly on the ground,
nothing would be pulling me towards the Earth,
and we'd all be floating.
We're at iFLY Atlanta, an indoor skydiving facility
with a state of the art vertical wind tunnel.
Here you can get the feeling of skydiving
without having to jump out of a plane.
Highly engineered fans provide the power to move the air
at speeds as high as 160 miles an hour.
In this segment, we will use these tunnels
to explore gravity.
Why we have gravity is still a mystery,
but we think it started before the universe was formed.
Data indicates that gravity was a force
even before the Big Bang billions of years ago
and after the Big Bang scientists say
that's how stars and planets were formed.
So think of Earth as one massive object
pulling things towards it.
That's Earth's gravitational force.
Everything with mass has a gravitational force.
The larger the mass the more the pull.
Three hundred years ago, Sir Isaac Newton discovered
that all objects with mass
exert a gravitational force on all other objects,
written F sub G.
He also figured out
that the amount of the gravitational force
depends on the mass of each object,
M sub 1 is one object
and M sub 2 is a second object,
and the distance between them, which is R.
It's called Newton's Universal Law of Gravitation.
So, the more massive the object
the more gravitational force
and the farther away the two objects
are from each other,
the weaker the gravitational pull.
Isaac Newton figured it to be
an inverse square law.
That means the force decreases with the square of the distance.
So, for example,
if you double the distance between two masses
the pull between them would be four times smaller.
Remember, force is measured in Newtons.
One Newton is the force needed to accelerate one kilogram
of mass at the rate of one meter per second squared
in the direction of the applied force.
So, what if you needed to know the gravitational force
in Newtons between two objects
that are separated by some distance?
If you took two spheres one meter apart,
both with a mass of 1 kilogram,
the gravitational attraction between them
is not 1 Newton.
Because of many experiments
we know that it is 6.674
times 10 the negative 11th Newtons.
It's a constant,
which means the relationship of force to mass
and distance is a known value.
And you need to include that constant in any equation you do
when you're trying to figure out
the gravitational force between masses,
which are in kilograms, that are separated
by some distance, which is in meters.
This gravitational constant,
6.674 times 10 to the negative 11th,
the unit for which is meters cubed per kilogram
times seconds squared is called the Big G.
It let's us determine the gravitational force
in Newton's between two masses
that are a certain distance apart.
So the attraction between two objects
is the gravitational force between them.
That force is in direct proportion to their masses
multiplied together and inversely proportional
to the square of the distance between them,
with Big G tying it all together.
Let's head to a stadium where Adrian is going to give you
some examples of gravitational attraction in action.
We can use gravity to figure out
how fast an object like this ball will fall and bounce.
Okay, let's back up.
If we don't count air resistance,
which is friction,
then the ball will speed up 9.8 meters per second
every second it falls.
So start at 0, after the first second,
the ball will be moving 9.8 meters per second.
After 2 seconds, it's going 9.8 meters per second faster,
19.6 meters per second.
After 3 seconds,
the ball is traveling 29.4 meters per second,
and so on.
So Earth's gravity makes objects accelerate
at 9.8 meters per second squared.
And get this, as long as there's no air resistance,
all objects will fall with the same acceleration
regardless of their mass.
For example, drop a small ball, like a golf ball,
and a larger ball, like a basketball,
at the same time.
If gravity is the only force factored in,
they will both accelerate at 9.8 meters per second squared
and hit the Earth at the same time.
Great stuff, Adrian.
So, does the same thing happen on the moon?
During a moon walk on NASA's Apollo mission in 1971,
the astronauts dropped a feather and a hammer at the same time
to test the moon's gravity.
What do you think happened?
Drop the two of 'em here
and hopefully they'll hit the ground
at the same time.
How 'bout that?
The hammer and the feather landed at the same time.
One giant leap for mankind, right?
And as we showed you in another segment,
when you step in a scale, keep in mind your weight
shows the gravitational pull between you and the Earth.
You weigh 6 times less on the moon than you do on Earth
because the moon has 6 times less gravity.
So remember, weight is a type of force.
It's a gravitational force, which is written capital W.
And you know force equals mass in kilograms times acceleration
in meters per second squared,
so insert weight as the force
and Earth's gravity is a type of acceleration,
so add a small G next to the A for acceleration.
And in physics we typically write the A sub G
as a lowercase G.
So you have W is equal to MG.
Gravity is not the only force acting on falling objects.
In fact, air resistance is another force at play
that you might say competes with gravity.
It pushes falling objects in the opposite direction
and slows them down.
Joining us is iFLY's David Connor,
a STEM educator at iFLY.
STEM stands for Science, Technology, Engineering,
and Math.
David, we know how gravity pulls a skydiver down to Earth,
but here you have a way of exerting force
to keep them up in the air.
How exactly does all this work?
Well the way it works at iFLY is at the top of our building
we actually have four high efficiency fans
that generate air flow
that goes around the sides of our building
called the Return Air Tower.
That we then collect all of the air at the bottom
of our building in what we call the Plenum,
which is like a giant basement.
And then, we force that air through the-- in the contractor,
which forces the air to speed up
and it generates a-a wall-to-wall cushion of air
that you see people fly on every single day.
Got it. Thank you, David.
You're welcome.
We know gravity pulls a skydiver down to Earth,
but the air around the skydiver exerts a force too.
It's called drag force,
which in this case is air resistance.
That's what the air tunnel is about
here at iFLY.
Just like water pushes back against a swimmer,
air does too.
The skydiver will stop accelerating
when the drag force equals the gravitational force.
The person will then keep falling at the same velocity.
They won't increase speed.
Be careful though,
this doesn't mean the skydiver is slowing down.
It only means that the skydiver is not picking up speed
once the forces of drag and gravity balance out.
It's called Terminal Velocity.
Terminal velocity is the fastest the object,
in this case the skydiver, can fall.
Let's say we have two differently shaped objects,
both with equal mass,
and let's drop them at the same time.
One has greater surface area creating more drag.
The one with the smaller surface area
will hit the ground first
because there's more friction or air drag
on the one with the larger surface area.
That's why a parachute works because it slows you down.
So here's a question for you,
remember that hammer and feather on the moon?
The hammer is more massive than the feather.
Let's say they are in a vacuum
and we dropped them with no air resistance
to slow them down,
which one would hit the ground first?
Trick question, right?
They would hit the ground at the same time.
Their masses wouldn't matter.
On the other hand, if they fall through the air,
which one hits the ground first?
Or do they hit the ground at the same time?
The force of air resistance comes into play.
The lighter object reaches terminal velocity sooner
and then continues falling at that rate
until it hits the ground.
It takes longer for the drag force
to balance the weight of the heavier object,
so the heavier object continues accelerating
until the force of the object's weight
and the force of drag are equal.
By the time this happens,
the heavier object has reached
a higher speed,
so it will hit the ground first.
In physics, not only do we have to account for
the universal law of gravitation
and the acceleration of gravity,
we have to take into account air resistance as well.
It's all about a balance of forces.
So to bring you back to Earth,
gravity is one of four fundamental forces
in the universe.
Joining the strong and weak nuclear force
and the electromagnetic force,
gravity is in an elite group.
The gravitational force of attraction gets bigger
with increased mass
and smaller with increased distance.
All matter, including people, have a gravitational force,
even to each other,
but we don't notice it because the Earth's
gravitational force is so much greater.
Gravity is everywhere.
It keeps us grounded.
Well, most of the time.
That's it for this segment of "Physics in Motion."
And we'll see you next time.
♪♪
(announcer) For more practice problems,
lab activities, and note-taking guides
check out the "Physics in Motion" toolkit.
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