what goes up must come down a ball you
throw in the air an Apple that drops
from a tree even you when you're on a
trampoline at someone's awesome birthday
party but why my friends let me
introduce you to one of the most
wonderful forces in the universe it's
one we're all experiencing right now
without even trying
fasten your seat belts because we're
going to explore the power of gravity
if I told you that you just won the
lottery what would you do if you're like
me you jump up and down and scream and
after you are done freaking out and
jumping in the air you'd lend on your
feet right but why would you land back
on the ground instead of just floating
off into space
it's because of a little something we
call gravity gravity is what pulls
everything toward the ground including
you without the force of gravity there
would be no life on earth air water
animals everything would fly off into
space
there'd be no you know me no french
fries nada think of gravity like the
invisible super glue that holds our
massive world together you can't see it
but it's always there an English
scientist named Isaac Newton was the
first person to seriously study gravity
over 300 years ago there's a famous
story about him that you might have
heard supposedly Sir Isaac was hanging
out underneath an apple tree thinking
probably partly napping when an apple
fell from the tree and conked him on the
head
that's when Sir Isaac had an aha moment
why did the Apple fall down to the
ground and not up or sideways he
realized that a special kind of force
which we now know as gravity was acting
on all of the objects on earth pulling
them toward it once the Apple became too
heavy for its stem to hold it anymore
the gravitational pull of earth brought
the Apple down onto Newton's noggin
Newton also realized it doesn't matter
how heavy an object is either whether
you're holding an apple or a bowling
ball or a feather if you let go of it
that suckers going down we're gonna make
a whole video about this later
basically he determined that what goes
up must come down
Sir Isaac was a pretty smart dude okay
so you know that if you jump up you'll
eventually land back on the ground and
you know that an apple drop down will
land on the ground too but what if you
throw something in front of you or to
the left or the right
to see how gravity will act pick up the
tennis ball or any small round object
and hold it in your hand let's toss it
in the air and watch it fall to the
ground no surprise here okay now pick it
up and hold it over your head let go and
watch it fall once more again not a
shocker now throw it to your left fall
down
pitch it to the right and watch it go
down again no matter where you throw the
ball it's going down so we've determined
that near the surface of the earth where
we all are gravity is the cause that
produces the effect of all unsupported
objects falling down the ball will go up
or to the left or to the right for a
little bit but eventually it's going to
be pulled back down to the ground no
matter what gravity has got to hold on
well everything so eventually everything
that you can think of can be thrown up
into the air and will come back down
even if you throw it to the left or the
right or any other direction that's
gravity doing its thing but when you
really think about it which way is down
exactly I mean the earth is round and
there are all kinds of things on what we
think of as the bottom of the earth so
how does gravity keep them from falling
off I know a penguin who can answer that
question
so you know that the earth is round and
you know that gravity is the force that
pulls objects down but if the earth is
round and there's stuff at the bottom of
the earth say a penguin in Antarctica
why doesn't gravity pull the penguin
down off of the earth I mean does
gravity really pull down when we talk
about gravity and we say things like up
or down we don't mean those things in
the sense that you're used to in this
case up just means away from the earth
and down means toward it
so when you hear people say gravity
pulls things down to earth they really
mean that gravity pulls things toward
the earth now think of it this way
gravity is the force of attraction
between any two objects made of matter
right well I have news for you
you're made of matter and so is the
earth that means you and the earth have
an attraction to each other oh you guys
anyway the scientific argument for
gravity is that any object that's on or
close to Earth's surface and is made of
less matter than Earth will be pulled in
by our planet stronger gravitational
pull want to do a little demonstration
to show how Earth's gravity can pull an
object like the penguin we mentioned
before toward it no matter where on
earth that penguin is all you need is a
tennis ball and a rubber bands oh and
your index finger now let's pretend the
tennis ball is earth and the rubber band
represents the force of gravity that
makes your finger our adorable little
penguin just chillin on the surface of
the earth now stretch the rubber band
around your tennis ball earth and stick
your finger under the rubber band now
try to lightly pulled your finger away
from the ball the penguin is trying to
jump off the earth
even though penguins can't fly what are
you doing penguin but what happens not
much your finger doesn't get far before
the rubber band pulls it back toward the
ball right and the effect is the same no
matter where your finger penguin is on
your earth ball whether it's at the top
or the side or on the very bottom the
same thing happens the penguin is forced
back to earth no matter how hard it
tries to jump off so what does this mean
it means that no matter where on earth
an object is the planet's gravitational
pull will draw the object toward it and
that's how you should think about
gravity it's the force that pulls things
toward earth so basically we have
gravity to think for the fact that
penguins stick to the bottom of the
earth and I for one am grateful I like
penguins interesting huh when we say
that gravity pulls down what we really
mean is that it pulls everything towards
the center of the earth but what if I
want to take a break from the earth what
if I'm an astronaut and I want to take a
vacation on the moon how can I possibly
escape the gravity of the earth and if I
can what will happen as I get closer to
the moon I guess there's only one way to
find out
you know how many people have actually
been on the moon 12 that's all but one
day we may go back there we have a lot
more to learn about the moon and space
in general so someday maybe in your
lifetime
we'll send astronauts out there to study
and explore do you want to be one of
them if you do let me give you some
helpful tips about a little thing called
gravity because you're gonna need to
know all about what gravity is and how
it works if you're going to escape the
pull of earth and fly to the moon now
you already know that astronauts can
leave the earth but it takes a lot of
effort to boldly go where few have gone
before
but once astronauts reach the speed
called escape velocity they're able to
overcome the force of Earth's gravity
and get into orbit around our planet or
head on over to the moon so this brings
up an interesting question what happens
when an object gets away from Earth's
gravity but close to the moon's gravity
you already know that gravity is the
force that keeps us from flying off the
surface of the earth and you know that
gravity pulls things not down but
towards the Earth's center you also know
that gravity exists between any objects
that have mass and the greater an
object's masses the greater the effect
of its gravity or pull on other objects
is but there's something more remember
Isaac Newton the apple tree guy he
determined that the amount of
gravitational force or pull between two
objects also depends on how far apart
they are so the farther away something
is from the earth the letter will feel
the pull of Earth's gravity and the
closer it gets to the moon the more it
will feel the moon's gravity pulling on
it let's do a little pretending to see
what happens to something when it moves
closer to an object that has a really
large mass and therefore a really strong
pull of gravity if you've ever made a
wish on a shooting star you've seen the
effect of Earth's gravity pulling on an
object shooting stars which are actually
meteors occur when pieces of rock break
off from a passing comet or asteroid and
get too close to the earth for example
say this globe represents the earth and
the marble represents a piece of space
rock that's flying by you can see that
there's a huge difference in size
between the two objects and if we were
to put them on a scale we'd see that
there's a big difference in their mass
to our model Earth has a larger mass if
the space rock is far away from the
earth then it can go on its merry way
since it won't be affected by the
Earth's gravity but if it gets too close
then it and the earth engage in a bit of
tug-of-war since both have gravity they
pull on one another it's not much of a
fight though the more massive heart has
a much larger gravitational pull so the
rock gets caught in Earth's gravity and
most of the time it gives us a brilliant
streak of light we call a meteor but
what does this mean for our space
travelers well when an astronaut ship
takes off for the moon and moves away
from the earth
the farther from Earth it goes the less
it feels the pull of Earth's gravity and
as it gets closer to the moon the
spaceship begins to feel the tug of the
moon's gravity more so even though the
moon has a smaller mass than the earth
and has less of a pull on the ship than
the earth does once the ship gets closer
to the moon than the earth the moon's
gravity pulls the ship toward it and
then the astronauts can make a safe
landing so we can make the argument that
two things affect the pull of gravity
first the size of the object objects
with a bigger mass have a stronger pull
of gravity and second the distance
between objects the farther apart
objects are the weaker the pull of
gravity between them and the closer
together they are the stronger the pull
of gravity all of this means that when
the day comes that you're flying a
spaceship to the moon
you just have to escape Earth's gravity
and then get close enough to the moon to
enter its gravity remember that when
you're grown up and you're welcome if
you want to thank me you can just name a
crater or something after me when you
get there so here's what we know so far
whether on earth or in space anything
that has mass will exert the force of
gravity and on earth everything falls
towards the middle of the earth at the
same rate but hold up a sec you've
probably noticed that if you drop a
piece of paper or a feather it takes
longer to reach the ground than
something like a rock or a ball so
what's up with that
it's time to investigate
I don't care who you are if you live
anywhere near this planet then you're no
stranger to gravity the force that
affects every object on earth and beyond
because of gravity whether you're an
astronaut on the International Space
Station or an ordinary clumsy earthbound
human who happens to drop her books a
lot all things caught in our planets
gravity will in some way have the
potential to fall to earth but objects
fall differently right like when I knock
a piece of paper off my desk it takes a
lot longer to fall to the ground than if
I knock a book off my desk so what's up
with that what new things seem to fall
at different speeds on earth well we
already know that all objects have mass
they have a certain amount of matter in
them and when gravity pulls on an object
it gives the object weight now a long
time ago people used to think that
heavier things fell faster than lighter
things because that's what our senses
told us I mean it certainly looks like
the book falls faster than the piece of
paper and why would we expect anything
else that would be like dropping a
hammer and a feather and expecting them
to hit the ground at the same time
except the thing I just said about the
hammer and the feather someone did
actually try that and they did both hit
the ground at the same time it just
didn't happen on earth true story when
astronaut Dave Scott was on the moon in
1971 he did an experiment where he
dropped a falcon feather and a hammer
from the same height and they hit the
ground at the same time now how can that
be if you drop a hammer and a feather on
earth the feather would take a lot
longer to reach the ground so what does
the earth have that the moon doesn't for
one thing an atmosphere the gases that
make up Earth's atmosphere push against
objects as they fall and the push of the
air against the falling object causes
friction we call that friction air
resistance so on earth the feathers flat
fluffy shape makes it run into more air
resistance than the hammer does this
makes it fall more slowly than the
hammer but since the moon has almost no
atmosphere there's almost no air
resistance so the two objects fell at
the same rate so it seems to me that
it's the resistance of air pushing
against objects that really affects how
fast objects fall and that means that
it's experiment time
but you don't have to go to the moon to
do this so don't pack your bags or
anything all you need are two pieces of
paper that are the same size crumple one
of the pieces into a tight ball and
leave the other ones smooth and flat now
drop each piece of paper from the same
height let's say a meter and then write
down in a table how long it takes each
piece to reach the ground you'll see it
takes a flat paper longer than the
crumpled paper to hit the floor this is
evidence that an object's shape affects
how fast it seems to fall to earth
because the two pieces of paper are
exactly the same they have the same mass
but air resistance makes a flat paper
seem to fall more slowly than the
crumpled paper so based on the results
of our experiment we can say at least
here on earth objects appear to fall at
different rates not because they have
different masses or weights but because
of air resistance the force that the
Earth's atmosphere has on objects as
they fall so there you have it gravity
is the force that's exerted by
everything that has mass and it's felt
by everything equally
even though falling objects on earth
might not always look like that's the
case I mean if you don't believe me
you can go to the moon and do the old
hammer and feather experiment for
yourself or you can just trust me if you
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