Bo: Hey, guys.
Bobby: Hi, Bo. Billy: Hi, Bo.
♫ (lyrics) Flipping Physics ♫
Mr. P: Ladies and gentlepeople.
The bell has rung and therefore class has begun.
Therefore, you should be seated in your seats,
ready, and excited to learn about free fall.
(book dropping)
(laughter)
Mr. P: Very nicely examples.
Alright, here we go.
This medicine ball is in free fall
from the moment it leaves my hands until.
You can't really see that.
So, uh, here.
Please take a look at this video that I made outside
where I could throw the medicine ball higher.
So what you'll see here is the medicine ball
is in free fall from the moment it leaves my hands
until right before it strikes the ground.
In other words, from right here to right here.
That's when the medicine ball is in free fall.
So Bo, please describe when we know
the medicine ball is in free fall.
Bo: An object is falling freely if the only
force acting on it is the force of gravity.
Mr. P: True.
That is what it says in the textbooks,
but what is the force of gravity?
Bo: Um, I'm not really sure.
Mr. P: Yeah, at this point that is correct.
This is a great definition for
when we know an object is in free fall.
For example, when the only force
acting on it is the force of gravity.
But unfortunately, we haven't defined force nor
have we defined the force of gravity at this point.
So until we define the force of gravity,
we need a different way of knowing
when an object is in free fall.
So, Bobby why don't you look at the video
example again and tell me what do you see
about when the object is freely falling.
Bobby: Well, it starts being in free fall as
soon as it leaves your hand and stops
right before it strikes the ground, so...
Billy: Maybe, as long as you aren't touching it,
it's in free fall.
Bobby: So, yeah.
As long as no other object is touching it,
it's an object in free fall.
Mr. P.: Exactly.
For now, one condition for an object in
free fall is that it must not
be touching any other objects.
However, there is one more condition for free fall.
There's this pesky air stuff
all around us that gets in the way.
Therefore, for an object to truly be in
free fall there needs to be no air resistance.
So, everybody please take a deep breath and enjoy.
Because today, we have all entered what I like to call,
"The vacuum that you can breathe"
Inhale and enjoy.
(deep breathing)
Billy: It seems the same to me.
Bo: I think it's just hypothetical.
Mr. P: So yes, welcome to the vacuum that you can breathe,
and we are going to be in the
vacuum that you can breathe for the rest of this class,
unless otherwise stated because every once
in a while we will step out of the vacuum
that you can breathe into the air
and talk about what happens.
However, again unless otherwise stated we
are always in the vacuum that you can breathe,
and there is no air resistance.
So that's how we identify an object that is in free fall,
for right now, but what does it mean,
Billy, for an object to be freely falling?
What do we know about the object then?
Billy: It was something having to do with
the acceleration of the object.
Bobby: Yeah, the acceleration was
-9.81 meters per second squared.
Mr. P: Close, the acceleration due to gravity
in the Y direction is equal to
negative nine point eight one meters per second squared.
And of course there is more to it then that.
So, yes, the acceleration in the Y direction
of an object that is freely falling on Earth
is equal to negative nine point eight one
meters per second squared;
however, let's first talk about
this little piece right here.
The acceleration in the Y direction for
an object in free fall is defined as
negative little g, where little g is the
acceleration due to gravity,
and here on planet Earth the acceleration due to
gravity is nine point eight one meters per second squared.
In other words, an object in free fall will have an
acceleration in the Y direction equal to negative little g,
where little g is defined as the acceleration due to gravity,
and here on planet Earth the acceleration due to gravity
is nine point eight one meters per second squared.
In other words, the acceleration in the Y direction
for an object in free fall on planet Earth
is negative nine point eight one meters per second squared.
Bobby: You keep saying, "On Earth."
What is the acceleration due to gravity somewhere else?
Mr. P: Sure, it actually depends on
what planet or moon you happen to be on.
For example,
The acceleration due to gravity on the
planet Mercury is three point
six one meters per second squared.
On Venus it's eight point eight
three meters per second squared.
On Mars, its three point seven five
meters per second squared.
On our moon it is one point six two
meters per second squared,
which is approximately 1/6th of the acceleration
due to gravity here on planet Earth.
And on the moon Io, the moon of Jupiter,
the acceleration due to gravity is one point
eight one meters per second squared.
However, you should be aware that the acceleration
due to gravity on Earth is actually not constant.
Nine point eight one meters per second squared
is the accepted value for the acceleration due to
gravity here on planet earth, but again,
remember, it is not actually constant.
Also, I embedded a widget from Wolfram Alpha
on flippingphysics.com along with this video,
so you can actually determine the acceleration
due to gravity at various places on planet earth.
Bobby: Cool.
Billy: I'm going to check that out.
Mr. P: Next, I want to go over one common
misconception about the concept of free fall.
I think it comes down to this word "fall" in free fall.
A lot of students have a tendency to think
that an object must be going
downward in order to be in free fall.
This is not true.
As you can see in the video of the medicine ball,
remember from the moment it leaves my hands
until right before it strikes the ground,
it is an object in free fall
because it's not touching anything,
and it's in the vacuum that you can breathe,
and for part of that motion it is going upward.
Therefore, an object that is
moving upward can be an object in free fall.
Please remember that.
O.K., Bo, considering that we know an
object in free fall here on planet Earth has
an acceleration in the Y direction of
nine point eight one meters per second squared,
what type of motion is an object in free fall?
Bo: What type of motion is free fall?
Well, we know the acceleration in the Y direction
is negative nine point eight one meters per second squared.
Oh, I get it.
The acceleration equals a number,
it's constant, so it's a UAM.
Mr. P: Correct, Bo.
When an object is in free fall the acceleration
in the Y direction equals a number.
In other words, the acceleration is constant.
In other words, it is in uniformly,
accelerated motion in the Y direction,
so you can use the UAM equations.
In fact, you already know one of the five variables.
The acceleration in the Y direction equals
negative nine point eight one meters per second squared.
Bobby, yes, what is your question?
Bobby: What does the negative mean in the
acceleration in the Y direction equals
negative nine point eight one meters per second squared?
Mr. P: Ah, yes, the negative.
An important question.
So, the negative simply means that the
acceleration is in the negative Y direction.
In other words, the acceleration is down.
In other words, you can write the acceleration
in the Y direction as negative
nine point eight one meters per second squared,
or nine point eight one meters per second squared down.
They are equivalent.
They mean the same thing.
I have a question.
Class, is the acceleration due to gravity
here on planet Earth positive or negative?
Billy: Negative. Bobby: Positive. Bo: Negative.
Mr. P: Be careful and listen to the question.
Is the acceleration due to gravity, little g,
positive or negative?
Bobby: Guys, it's written right there on the board.
It says, "Little g equals nine point
"eight one meters per second squared."
So g is positive.
Mr. P: Exactly right, Bobby, and please be careful
everybody because this something
that students often confuse.
The acceleration in the Y direction for an
object in free fall is equal to negative little g
where little g is the acceleration due to gravity,
and the acceleration of gravity on planet
Earth is a positive nine point
eight one meters per second squared.
Therefore, an object in free fall has an
acceleration in the Y direction equal to
negative nine point eight one meters per second squared;
however, again, the acceleration due to
gravity is positive nine point eight one meters
per second squared here on planet Earth.
So, one more time class, is the
acceleration due to gravity positive or negative?
Class: Positive.
Mr. P: Yes, please remember that little g...
Hey, um, so when I was looking over this video,
it occurred to me that I had both
not constant and uniformly accelerated motion
written on the board with respect
to the acceleration due to gravity or free fall acceleration
and that those two things don't go very well together,
and this might have confused people a little bit,
so I'm just going to take a moment to clear that up.
It turns out that you need to look at it
from both a global and local perspective.
When you look at it from a global perspective,
the acceleration due to gravity is not
going to be the same at all locations on the Earth,
it actually varies from location to location;
however, when you look at it at a local level,
at a specific location, the acceleration due
to gravity is constant, and therefore,
you can use the uniformly, accelerated motion equations.
Billy: Thanks, I was going to ask that question,
but I forgot.
Bo: Sure, sure you were.
Bobby: Actually, as long as we're here,
I have a question.
Won't a heavier object accelerate faster?
Mr. P: Good question, Bobby.
It turns out that the acceleration due to
gravity is independent of the mass of the object,
there's a really good video from NASA from 1971,
the Apollo 15 mission, Astronaut David Scott
standing on the moon and holding a hammer and a feather,
and he drops both from the same height and the same time,
and you can actually watch
them both accelerate at the same rate.
Showing that the acceleration due to gravity
is independent of the mass of the object.
It's pretty cool.
Bobby: Mass is irrelevant.
Everything accelerates at the same rate.
Got it.
Mr. P: Alright, going to bring you back
to where you were in the video.
Here you go.
Yes, please remember that little g, the acceleration
due to gravity is a positive number.
Clearly the next few lessons will be
about free fall and example problems.
Let's do a quick review.
An object is in free fall when the
only force acting on it is the force of gravity.
Sadly, we haven't defined the force of gravity just yet,
so for now, an object is in free fall
when it is not touching any other objects
and there is no air resistance
or you're in the vacuum that you can breathe,
which is what we are going to be from here on out.
We'll always be in the vacuum that you can breathe,
unless otherwise stated.
When an object is in free fall we know the
acceleration in the Y direction is
equal to negative little g,
where little g is the acceleration due to gravity,
and here on planet earth that is is equal to
nine point eight one meters per second squared.
Now, that number is not quite constant here on planet Earth.
It changes slightly.
So nine point eight one meters per second squared
is the accepted value,
which means the acceleration in the Y direction
for an object in free fall on
planet Earth is equal to negative nine point
eight one meters per second squared.
Now, for a particular problem,
that number for the acceleration
is going to be a constant number,
therefore, you can use the uniformly,
accelerated motion equations.
And, please remember, g is a positive number.
The acceleration due to gravity is always positive.
Ladies and gentle people,
I hope you enjoyed learning with me today.
I enjoyed learning with you.
(sigh)