Download Gravity Basics The Power of Attraction

Gravity Basics
Gravitation (or gravity) is a force of attraction. It is produced by all pieces of matter in the universe and pulls on all
pieces of matter in the universe, regardless of matter type. Galaxies produce gravity. The Sun and the planets produce
gravity. Even the keyboard I’m typing on produces gravity (although only a very small amount). In short, every object is
surrounded by what we call a gravitational field that pulls any other object inside that field towards it.
Because it comes from all matter, gravity is what we call a universal force. You may tend to think of it as simply the force
which pulls us down to the Earth but, in truth, the effects that gravity has are virtually endless and can be found just
about everywhere.
Although we think of gravity as rather powerful, it is, ironically enough, the weakest of all
the fundamental interactions. Although it is a weak force, it can act over very large
distances. Whereas a force like the strong nuclear force (in the nucleus of an atom) can’t
even reach from one atom to another, gravity’s attractive powers can stretch across
almost infinite amounts of space.
What all this means is that although gravity is too weak to produce noticeable effects
between small objects like a table and a chair (which is why you don’t see things like these
clinging together), the combined gravitational forces between two very massive objects,
like the Earth and the Sun, can be incredibly strong indeed.
The Power of Attraction
The strength of the gravitational attraction between two objects depends on two factors: how big the objects are (how
much mass they have) and how far apart they are. According to Isaac Newton’s law of universal gravitation, gravity’s
strength is directly proportional to the product of two objects’ masses and inversely proportional to the square of the
distance between them. To make this easier to understand lets break it down a little.
First, the force of gravity is directly proportional to the product of the masses. (Remember that a product is one number
multiplied by another.) This means that the attraction between two objects gets stronger the larger the product of their
masses is and weaker the smaller it is. Put even more simply, the bigger something is, the harder it pulls on other
objects.
Second, the force of gravity is inversely proportional to the square of the distance between two objects. For those of you
who haven’t taken algebra, the square of a number is that number multiplied by itself. The square of one is one, the
square of two is four, the square of three is nine and so on. This means that if you double the distance between two
objects their attraction will be only 1/4 its original strength and if you triple the distance, their attraction will be only 1/9
its original strength. This is called an inverse square law.
It is important to remember, however, that when we refer to the distance between two objects we are actually talking
about the distance between their centers, not their surfaces. If that were true, gravity would pull 1/4 as hard on you two
meters above that ground as it does one meter above the ground! To summarize, the strength of the force of gravity
between two objects gets larger the larger the objects are but smaller the farther apart they are.
Orbits Explained
An orbit is a path in space described by a satellite revolving around a larger body, where
the motion of the satellite is dominated by the mutual gravitational attraction between it
and the larger body. In simpler words, an orbit is the path that something like a planet
takes around the Sun, and this path is determined by the attraction between the Sun and
the planet.
Since the Sun is such a massive object, Earth and all the other planets in the solar system
are attracted to it. You may be wondering why the planets don’t fall into the Sun directly.
The only factor preventing this from happening is the speed at which the planets are
traveling.
Formation of Planets
The most widely accepted model for the origin of the Solar System is called the nebula theory. Most
generally phrased, the theory states that the Solar System condensed from a large, lumpy cloud of cold gas
and dust. This idea was first in the late 18th century by two Europeans, Immanuel Kant and Pierre Laplace.
Extensive observations since then have confirmed that the nebula theory is the best explanation for the origin of the
Solar System. All theories are subject to refinement as new data is gathered.
According to the nebula hypothesis, the Solar System began as a nebula, an area in the Milky Way Galaxy that was a
swirling concentration of cold gas and dust. Due to some disruption, possibly from a nearby supernova, this cloud of gas
and dust began to condense, or pull together under the force of its own gravity. Condensation was slow at first, but
increased in speed as more material was drawn toward the center of the nebula. This made gravity stronger, making
condensation faster.
The Tidal Effect
For a long time, people have wondered what causes tides, which are the rise and fall of the sea level. For hundreds of
years, it was believed that the gods were responsible, but now we know the real reason for this phenomenon. It is
caused by the Moon and Sun’s gravitational pull.
Remember that the strength of a gravitational pull varies with distance. The further away something is from a massive
object, the weaker the pull of gravity, and vice versa. When the moon orbits the Earth, it exerts a certain, if small, pull on
the Earth. On the side of the Earth closer to the Moon (at a sublunar point), water heaps up because of the increased
pull of gravity. The same thing occurs with the Sun. Water will heap up directly beneath the Sun (at a subsolar point).
High tides, when the water level rises, are caused by the “heaping” behavior resulting from the flow of water toward the
sublunar and subsolar points. In turn, low tides are the compensation for the high tides. Because the Earth rotates daily,
high and low tides alternate. This is called the tidal effect, although it doesn’t just occur here on Earth! Even planets like
Mercury, which is close to the Sun, experience a type of “bulging” in the surface of the planet which is caused by the
closer distance to the Sun.
Sources:
http://www.msnucleus.org/membership/html/k-6/uc/earth/5/uce5_1a.html
http://library.thinkquest.org/27585/
1. Write an acrostic poem where each line features a
Example for CVMS:
fact about gravity from the article.
GRAVITY
Carmel Valley Middle School
G
Very busy, active place
R
Many students working hard
A
Science rules and you are great!
V
I
T
Y
2. Draw a picture in the space above to illustrate your poem.
3. Is gravity a strong force or a weak force? Use a quote from the text to support your
answer.
4. How does the mass of an object affect its gravitational pull?
5. As distance between two objects increases, what happens to the gravitational pull
between them?
6. What is an orbit? What role does gravity play in a planet’s orbit around a star?
7. According to the article, how are planets formed?
8. What is the “tidal effect”?
9. Write down 3 things you learned about gravity from this article.