 (Adrian Monte)
 Skydivers and
 wingsuit flyers
 may call what they
 do free-falling,
 but as you may have noticed
 by now, in physics,
 words mean something
 a little more specific.
 So let's explore what
 freefall is all about
 in this segment of
 "Physics in Motion".
♪♪
Freefall is what
an object does
when it is under the influence
of only one force, gravity.
We have to go way
back in history
to get a handle on
why this matters,
but we'll
keep it short.
For thousands of years,
scientists thought that
how fast something fell
depended on how
heavy it was.
Seemed only logical.
But in the 16th century,
one of the all-time science
hall of fame geniuses,
 Galileo, had an idea.
 He suspected that all objects,
 regardless of their mass,
fell at the same
rate of acceleration.
 If he were to
 take two objects,
 one light and the other heavy,
 and drop them from a height,
 they would both hit the
 ground at the same time.
 This showed that acceleration
 is independent of mass.
 Let's go try
 this ourselves.
So, between this watermelon
and this cantaloupe,
which do you think will
hit the ground first?
 Both hit at
 the same time.
 The watermelon made
 a bigger mess,
 but as far as
 acceleration goes,
we verified
Galileo's point.
It even works
on the moon.
 Take a look at this clip
 from Apollo 15.
 The gravity on the moon,
 even though it's less
 than that of Earth,
 still made the feather
 and hammer drop
 at the same rate
 of acceleration.
 Gravity, in fact,
 is one of the most important
 forces in the universe.
So back to the
16th century.
When people realized that
objects behaved this way,
it was a major turning
point in understanding
how the universe works,
and being able to predict
how things will move.
Galileo, using the instruments
available to him,
measured the acceleration that
was due to gravity on Earth,
no matter what
the object,
at about 10 meters
per second squared.
We now know it is 9.8
meters per second squared
near the surface
of the Earth.
So he wasn't far off,
which is impressive,
considering he used his
heart rate as a timer.
That is the constant
rate on Earth
that the force of gravity
accelerates everything.
All objects,
regardless of their mass,
fall at the same rate.
9.8 meters per
second squared.
But, you say,
what about air resistance?
And you're right.
But let's put other factors
aside for right now,
since we're only concerned with
how things work in freefall.
We'll get to those
in another video.
Now, let's look at a
freefall problem together.
Let's say that we
throw a ball upwards.
 What is the ball's
 maximum height,
and why is that
a freefall problem?
Because, technically,
an object is in freefall
even when moving upwards,
or instantaneously at rest
at the top of its motion.
Because, remember,
freefall means that
gravity is the only force
acting on an object.
We can use some of
the variables we know
to plug into an equation
to solve the problem.
 The origin is the place
 the ball left our hand,
and the positive
direction is up.
 Let's say I throw the ball
 at 10 meters per second
 up in the positive
 direction.
 I want to find out what my
 ball's maximum height will be
 before it starts
 falling down.
There are steps
we can follow
any time we solve problems that
involve kinematic equations.
 Step 1 is to write
 down your variables.
The ball's
initial velocity
would be positive
10 meters per second,
 because it's traveling up
 in the positive direction.
 But when the ball
 leaves my hand,
 it immediately starts
 to decelerate,
 because it is only
 under the influence
 of the force
 of gravity.
 The ball's acceleration,
 A sub G,
 is negative 9.8 meters
 per second squared,
 because when it
 is decelerating,
 its initial velocity
 and acceleration
 have opposite signs,
 we are slowing down in
 the positive direction.
 It decelerates until it
 reaches a velocity of zero,
 at its maximum height
 in its travel.
 And that's V sub TOP.
 We now have values for
 our initial velocity,
 final velocity,
 and acceleration.
 Our unknown is our
 maximum height,
 which we represent as d,
 displacement.
 The next step is find
 one of the four equations
 you can see here that has the
 three variables that we know,
 and has our unknown variable,
 all within the same equation.
 We should use
 equation number 2.
 Final velocity squared equals
 initial velocity squared
plus two times our acceleration
times displacement.
 Now, let's do step 3,
 plugging in all of the
 variables that we know,
 along with the symbol D,
 representing the thing we are
 solving for, displacement.
 Our final velocity is
 0 meters per second,
 and it is squared.
 This is equal to 10 meters
 per second squared
 plus 2 times negative 9.8
 times the displacement.
 In the final step, step 4,
 we solve for the unknown,
 displacement.
 We get rid of the exponents
 by squaring the numbers
 and units
 within parentheses.
 0 squared is 0,
 and 10 squared is 100.
 Then, subtract 100
 from both sides.
 Which leaves us
 with the equation
 negative 100 meters squared
 per second squared
 equals negative 19.6 meters
 per second squared
 times our displacement.
 We need to get
 displacement alone,
 so we divide both
 sides by 19.6.
 This gives us a displacement
 of 5.1 meters.
 That's how far my throw
 traveled in freefall
 before it reached its maximum
 height of 5.1 meters.
 Here's a bonus
 question for you.
At what velocity
will it reach my hand
when it falls back down?
It will be the same speed
that it left my hand
to begin with,
10 meters per second.
Except, this time,
the velocity will be negative
10 meters per second,
because the ball is going
in the opposite direction.
The rate gravity slows
the ball going up
is the same rate it
speeds it up going down.
So now you know that
freefall in physics
is when an object is only under
the influence of one force,
gravity.
That's all for this segment
of "Physics in Motion".
We'll see you
next time.
 For more practice problems,
 lab activities,
 and note-taking guides,
 check out the
 "Physics in Motion" Toolkit.
