Hi, my name is Kaeleigh, and in this lesson, we will discover what gravity
really is! It will be helpful if you've already finished the modules Newton's Laws
1, 2, and 3 before starting this Gravity module. Let's get started!
There is a famous legend, or story, about one of our favorite physicists, Sir Isaac Newton.
The legend goes, one day, Sir Isaac Newton was sitting under an apple tree, thinking about the
forces of nature, when all of a sudden, an apple fell from the tree and bonked him on the head!
He realized there must be a force that makes this apple fall from the sky, but what could it be?
We already know that a falling object, like the apple from the tree, is accelerating. We know
there must be an unbalanced force pointing in the direction of motion. In the case of
an object like the apple falling towards the earth, we call this unbalanced force "gravity."
We can define gravity as a force that pulls objects towards the center of the earth,
or really, to the center of any object.
Let's explore this definition of gravity with a quick lab at home. For this experiment, you will
start with two full apples. Ask an adult for help to cut one of the apples right down the middle.
You'll need to use one of the apple halves and one full apple.
Now you're going to hold the full apple in one hand and the half-apple in the other hand,
and we're going to drop the whole apple and the half-apple and predict
which apple will hit the ground first. But first, we need to make sure that the apples
are starting their free fall from the same height, so you're going to make sure that
your arms are out straight in front of you and that your hands are level.
But wait! Before you drop the apples, grab your lab notebook or a sheet of paper, and write down
your prediction. Will the whole apple or the half-apple hit the ground first? Remember, the
only force acting on the apples is gravity. So go ahead, pause the video here. Make your prediction,
then do the experiment a few times before you draw a conclusion.
Welcome back! Well, was your prediction correct, or is your mind totally blown? You should have
found that the apples hit the ground at the same time, so even though the half-apple has less
mass than the whole apple, they fell at the same speed towards the earth because of gravity.
It's important to note that objects of different densities
will fall at different speeds, but we'll learn about that in the next video on air resistance.
For now, let's play around with some more gravity experiments.
For this experiment, you will need a styrofoam or paper cup and water, and either take this
one outside, or do it in a place where the floor can get really wet. First thing, ask an adult to
help you cut a small hole into the lower side of the cup, like it shows in this picture.
Make sure the hole is small enough that you can plug it by placing your finger over it.
Now cover the hole with your finger, and fill the cup with water.
In a second, you're gonna remove your finger, but before you do that,
what do you think is going to happen to the water when you remove your finger from the hole?
Now go ahead, remove your finger. You can pause the video here and give it a try.
What did you observe happening? Did the water flow out of the hole? This is because the force
of gravity is pulling the water towards the center of the earth, and the water will push through the
hole in order to accelerate in the direction of the unbalanced force of gravity. Now we're
going to cover the hole with your finger and fill it back up with water. This time, you're
going to keep your finger over the hole, and then you're going to let go of the entire cup of water.
But before you let go, predict what will happen. When you drop the cup full of water,
will the water come out of the hole? Will the cup hit the ground and then all of the water?
Go ahead and pause the video here, and try it for yourself.
Make sure you keep the hole of the cup covered until the moment you drop the cup.
Make your predictions first! Now pause the video, and give it a try.
What did you notice? As the cup fell towards the earth, so did the water. The water doesn't
come out of the hole in the cup; gravity was pulling on both the cup and the water,
so they fell to the ground at the same speed, even though their masses are different.
Great job with your at-home labs! I hope you noticed the findings of both of these experiments
is that objects of different masses will fall towards the earth
at the same speed because of gravity. Great work, you guys!
Make sure you work on the practice problems, and remember to always be clever!