A LOOK AT GRAVITY
We have already learned about motion (speed), but now we are going to see what causes
motion to change. To change the motion of an object, someone or something must push or
pull on the object. These pushes and pulls are called forces. Our look at forces will start
with the most common, yet often overlooked, force of gravity.
Main Ideas
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Gravity is a force. Gravity is not an object or a substance; it is a force that acts between
any two objects.
Near the surface of the Earth gravity is an inescapable force that pulls all objects
vertically downward. The vertical direction is different from one place to another on the
surface of the Earth, but all vertically downward directions point towards the Earthʼs
center.
Mass and weight are different quantities. Weight is a force; mass is not. The objectʼs
mass is a measure of how much material makes up the object. The weight of an object
is equal to the size of the gravitational force pulling on the objectʼs mass.
The Force of Gravity
All objects pull on each other. The amazing thing about gravity is that the objects do not
even need to be touching or connected by a rope to pull on one another. The name of this
force is called gravity. Sometimes it is called the ʻforce of gravityʼ or the ʻgravitational forceʼ.
The good news is that usually gravity is a very weak force. Can you imagine being pulled
toward every object around you all at once?
The only time the force of gravity is large enough to measure is when one of the objects
is very, very big (like our sun, Earth, or the moon). By comparison, the gravitational force
pulling you towards your classmate at a nearby desk is so tiny that we cannot measure it with
any force meter or scale.
MAKING SENSE OF FORCES … The Earthʼs Gravity
Using Gravity to Identify the Vertical Direction – the Plumb Line
If an adult asked you “Which way is up?” you would probably wonder how could a
grown-up person ask such a silly question. The problem is that there are many ways to go up
and to go down. You could walk up a hill that has a gradual slope or you could climb up a
steep cliff to get to the top. There really are many upward directions and downward directions.
But there is only one vertically up and down direction, and we use gravity to find this unique
and important direction.
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© 2006-2007 Delaware Science Coalition / Delaware Dept of Education
When an object, such as a big paper clip or a fishing sinker, is hung from a string it will
hang in a special direction. This direction is called the vertical direction. The direction of the
force of gravity is what determines the vertical direction. When a hanging object and string are
used to indicate the vertical direction they are called a plumb line. If you hold an object above
the floor and release it, the only force pulling on the object will be gravity. Gravity will pull the
object straight down.
Our planet pulls us and everything else downward. We have already learned that by
downward, we really mean vertically downward. If all of our plumb lines hung in the same
direction, we all agreed which way was vertically downwards. But would students using plumb
lines in China also agree on which way is vertically downwards in their classroom? Would their
plumb lines hang in the same direction as yours? Is vertically downwards the same direction
everywhere on Earth?
Letʼs Investigate …
 Show the direction gravity acts at different points on the Earth. Ask yourself which way is up and
which way is down at each point and then decide which way gravity acts.
 Using the picture below of the Earth, select different points and draw force arrows to represent
gravity at the different points.
Use your model or your diagram to answer these questions:
• Would vertically downward for an explorer standing on the Earthʼs North Pole point in the
same direction as vertically downward for a scientist standing on the South Pole?
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Does the force of gravity act in different directions at different points on the Earth?
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Is there anything common about the direction in which the force of gravity acts at different
points on the Earthʼs surface?
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© 2006-2007 Delaware Science Coalition / Delaware Dept of Education
MAKING SENSE OF FORCES … The Difference Between Mass
and Weight
Until now, we have focused mostly on the direction of the force of gravity. When the
force of gravity acts on an object, the force has size as well as direction. On the surface of
the Earth, the weight of an object is equal to the size of the force of gravity pulling on
the object. In fact, the weight of an object is the gravitational force acting on it! Near the
surface of our planet, the weight of an object will be due to the Earthʼs gravity. The weight of
an astronaut on our Moon would be due to the Moonʼs gravity. The Earth can exert larger
gravitational forces than the Moon can.
The stuff that you are made of (scientists called it matter) determines
your mass. How hard the force of gravity pulls your mass downward is your
weight. If you go to the moon, you bring all of your matter with you so your
mass would stay the same. The Moonʼs gravity is weaker than Earthʼs gravity,
so you would not be pulled down as hard on a scale. You would weigh less.
The Moonʼs gravity is only 1/6 (one sixth) the size of Earthʼs gravity, so you
would weigh only 1/6 of what you weigh on Earth.
Why is there so much confusion between mass and weight?
Mostly this confusion happens because we spend our entire lives on or near the Earthʼs
surface. Earthʼs gravity stays the same as you move around the surface of our planet. Even if
you take a trip in a jet, Earthʼs gravity is only slightly weaker while you are flying miles up in the
atmosphere. You would never notice the difference. Since the mass does not change and the
weight does not change, it is really hard to tell that they are different quantities.
Until you start making weekend trips to the moon, remember that
 Mass is the amount of matter (stuff) from which you are
made.
 Weight is the size of the gravitational force pulling your
mass downwards.
 Changing gravity by traveling to the moon or a different
planet will change only your weight, not your mass.
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© 2006-2007 Delaware Science Coalition / Delaware Dept of Education
GRAVITY: Checking your understanding
1. What is gravity? Do all things have a “gravity force”?
2. What factors affect the strength of the pull of gravity?
3. In which direction does gravity pull? Is it the same for all places on the Earth?
4. What is the difference between mass and weight? Why do astronauts weigh less on the
Moon?
5. If your mass on Earth is 100 kg, what would your mass be on Mercury? (gravity is 38%
as strong on Mercury as it is on Earth) Explain.
6. If your weight on Earth is 100 pounds, what would your weight be on Venus? (gravity is
91% as strong on Venus as it is on Earth) Explain.
7. Are astronauts orbiting the Earth in space “weightless?” Why or why not? (Hint: think
about whether there is gravity in space pulling on astronauts)
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© 2006-2007 Delaware Science Coalition / Delaware Dept of Education
Investigating Further …
N O G RAVITY
IN SPACE ?.......
Many people believe that there is no gravity in space. This is not true! There is proof
that gravity exists in space. We can find that proof right in our own solar system.
Can you think what would prove that gravity exists in our
solar system? You guessed it, all of the planets orbit our Sun!
That is why the planets move around the Sun in a curved path.
If the Sun wasnʼt there the Earth, planets, and the asteroids
would move in a straight line.
In the past, many people thought that the Sun orbited the
Earth because the Sun appeared to move across Earthʼs sky.
Today we know that the Earth orbits the Sun. How large is the
Sun? Did you ever try to guess how many jellybeans would fit
into a jar? If we had a Sun-sized jar and we had Earth-sized
jellybeans, it would take 1,300,000 Earth-sized jellybeans to fill it.
Due to the Sunʼs massive size, the gravitational pull between it and other objects in the
solar system is very large. The Sun can even keep Pluto in orbit, despite Pluto being on
average a distance of 3½ million miles from it (Earth is 93 million miles from the Sun). Pluto
takes about 248 Earth years to orbit the Sun moving at a speed of 10,623 mph. No gravity in
space? Of course there is!
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© 2006-2007 Delaware Science Coalition / Delaware Dept of Education