(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.