Download A force is a push or pull on an object. In this lesson, you will be

A force is a push or pull on an object. In this lesson, you will be learning about gravitational force.
Gravity is the force of attraction between the masses of two objects. Have you ever heard the saying,
What goes up must come down?
On Earth, gravity helps keep our atmosphere from drifting off into space. It causes the ocean’s tides. It
also causes things to fall to the ground.
Sir Isaac Newton observing an apple falling from a tree hypothesized that some force was causing the
apple to accelerate towards the ground. The apple initially in the tree having a velocity of zero
experiences an increase in downward velocity as it falls from the tree. This force is the force of gravity.
Objects accelerate downward due to the force of gravity. Newton’s hypothesis has been tested and
supported over and over in experiments that it is now considered to be a law. The unit for force is a
Newton. Let’s learn more about the Law of Gravity.
If a bowling ball and a feather are dropped from the top of a building from the same height at the same
time, which will hit the ground first? If air resistance is removed, both the bowling ball and feather will
hit the ground at the same time. In this example, the only force that is acting on the bowling ball and
feather is gravity. Although the bowling ball has more mass than the feather and therefore more weight
due to the force of gravity, they both are in a state of free fall, so both will accelerate at the same rate
and hit the ground at the same time.
Free fall occurs when the only force acting on an object is the force of gravity. And remember gravity
causes objects to have a downward acceleration. The earth pull on the bowling ball with more force
than the feather due to the bowling ball having more mass. Acceleration is equal to the force divided
by the mass.
Objects that are in free fall all accelerate at the same rate on Earth. This is called the acceleration due to
gravity. On Earth, g equals 9.8 meters per second squared. Objects that are in free fall are only acted
upon by the force of gravity. They do not experience air resistance. All objects in free fall have a
downward acceleration of 9.8 meters per second squared. This picture represents a ball that is in free
fall. The ball is accelerating at a rate of 9.8 meters per second squared. That means for every second the
ball travels downward it speeds up.
You can see from this diagram, that each second, the velocity of the ball increases. It starts off at 0
meters per second and for each second, the velocity increases by 9.8 meters per second. So at a time of
1 second, the ball’s velocity would be 9.8 meters per second. At 2 seconds, the ball’s velocity would be
19.6 meters per second. At three seconds 29.4 meters per second. At four seconds, 39.2 meters per
second. And at five seconds, 49.0 meters per second.
Remember that velocity equals gravity times time. Gravity is 9.8 meters per second squared. At 6
seconds, what would the ball’s velocity be? It would be 9.8 meters per second squared times 6 seconds
which equals 58.8 meters per second.
This position versus time graph represents a free-falling object. The object starts at an initial velocity of
0 meters per second. The ball initially starts falling at a slow velocity but speeds up. It is accelerating
due to the force of gravity, which remember on Earth is 9.8 meters per second squared. At the end of
the ball’s journey to the ground, the ball has a greater velocity.
The acceleration due to gravity on Earth is 9.8 meters per second squared, but what if we were to visit
another planet or the moon? What is the gravity like in those environments? Have you ever thought
about why astronauts float in space or why they need special equipment to keep them from floating off
of the moon? It’s because there is less gravity on the moon.
Compared to the gravity on Earth, there is less gravity on the moon. The acceleration due to gravity on
the moon is 1.624 meters per second squared. The gravity on Mars is 3.71 meters per second squared
and on Saturn 8.96 meters per second squared.
If you were to stand on a scale, you would not weigh the same on the moon, Mars, or Saturn as you
would on Earth. The reason is because the gravitational force is not the same. Although your mass is
the same anywhere you are at, your weight would change. Your weight is your mass times the force of
gravity.
On Earth if you weighed 100 Newtons, on the moon you would weigh 16.6 Newton. On Mars 32.7
Newtons, and on Saturn 8.96 Newtons.
Let’s take a look at weight on the Earth compared to weight on the moon. The acceleration due to
gravity on the Earth is 9.8 meters per second squared, but there is less gravity on the moon with a
gravitational acceleration of 1.624 meters per second squared. If you weighed 100 N on the Earth, to
figure out how much you would weigh on the moon, you would need to take the gravity of the moon
divided by the gravity of Earth which is .166 and multiple that by 100. That would mean on the moon,
you would weigh 16.6 Newtons.
If you were to drop a book and a piece of paper at the same time, they wouldn’t hit the ground at the
same time due to air resistance. You will be learning more about friction a force that opposes motion
and air resistance in lesson 2. In the absence of this other force, the book and piece of paper would hit
the ground at the same time because they would be in free fall if the only force acting upon the book
and piece of paper were the force of gravity.