2. Gravity and Weight
Imagine walking on the Moon. You have to wear a heavy
spacesuit to protect you because there is no oxygen. Im
agine how hard it would be to move around with all that
heavy gear.
Actually, it would be much easier than you think! On
Earth, your weight is caused by gravity pulling your
mass toward the center of the planet. The Moon has
much less mass, or total amount of matter, than Earth,
so its gravitational pull is much weaker. On the Moon, yo
ur mass does not change, but you would weigh
one-sixth of what you weigh on Earth. The gear you
are wearing would also be much lighter.
Your muscles, however, are just as strong on the Moon
as on Earth. It is much easier to lift your body and your
spacesuit off the surface. Astronauts on the Moon
had to move very carefully to avoid stepping or
jumping too far. It’s all about mass and gravity, but
what is gravity?
3. Gravity and Distance
Gravity is a force that attracts all objects to each other. How strongly they
are attracted depends on two key factors: the masses of the objects and the
distance between them.
Two large, rocky asteroids in space attract one another because of gravity. If
they are far apart, the force is very weak. If they begin to move toward one
another, the force increases. They may pull together and form one larger
asteroid.
4. Gravity and Mass
Are you and the classmate sitting next to you pulled together by each other’
s gravitational force? Yes, you are! Gravity attracts you to your pencil and to
the bird outside your window and to any other objects that have mass, but it
is hard to notice small pulls of gravity.
Even large buildings are not massive enough to cause noticeable attraction.
Earth’s pull on these objects is much greater than the pull of these objects
on each other, because Earth is so much more massive than they are. The
more massive, the stronger the attraction. Objects like moons, planets, and
stars have a noticeable gravitational pull.
5. Gravity and the Moon
Earth’s gravity keeps the moon orbiting earth once every 27.3 days. The moon
pulls on earth, too, causing ocean tides. Water on the side of earth facing the
moon is pulled toward the moon. This causes a high tide. As the moon pulls
on earth, the water on the opposite side gets left behind. This causes a high
tide on that side, too. Low tides occur between the high tides. As earth rotate
s, each coastal location passes through high and low tides twice a day.
6. The Sun and the Planets
You probably know that Earth orbits the Sun. It is the Sun’s gravity that keeps
Earth and the other planets in the solar system orbiting around it. Can you
guess what determines the strength of the gravitational pull between each
planet and the Sun? It is the mass of each planet and its distance from the Sun
.
7. The Sun and the Planets
Think about how the Sun’s gravity would act on each planet based
on its mass and its distance from the Sun.
8. Orbiting the Sun
When planets move around the Sun, they do not travel in a circle. Their orbit is
an ellipse. An ellipse looks like a circle that has been flattened a little on two
sides. The amount of flattening is too small to be obvious in a drawing like the
one below.
The path of planets around the Sun is mostly smooth and follows the elliptical
curve. However, planets are attracted to each other, too! The nearer planets get
to each other, the more they are pulled. This causes them to travel a little out
of their paths and towards each other.
Before Neptune was ever observed, astronomers predicted that it existed and
where it was located. They calculated how the Sun and the large planets, Jupit
er and Saturn, affect the motion of Uranus. Then they found that the real orbit
of Uranus was slightly different. That meant there was another large planet to
be found. This turned out to be Neptune.
9. Gravity and You
Would you like to be taller? Try going into space for a while! Astronauts on
the International Space Station grow taller while in space. Gravity on Earth
pulls down on and compresses the vertebrae (bones) in our spine. But in
space, where the force of Earth’s gravity is much weaker, astronauts’
vertebrae decompress (loosen) and expand. Astronauts come back to Earth
taller than when they left! After being back on Earth for several months, how
ever, their spines go back to normal. They return to their original height.
Gravity causes tides and orbits, and it even affects how tall we are! That is a
very attractive force!
10. Read-Think-Write
1. Define weight in your own words.
2. Use the words “weaker” and “stronger” to complete these two sentences:
a. The more mass two objects have, the _ the gravitational attraction
between them.
b. The farther apart two objects are, the the gravitational attraction
between them.
3. Look at the two photos of tides. Based on what you know about tides and
4. the Moon’s gravity, explain why the water level changes.
5. Jupiter is farther from Earth than Mars is. Why does Jupiter have a stronger
6. gravitational pull on Earth than Mars does?
7. Read this sentence: “Planets in the solar system are affected only by the
8. Sun’s gravity.” Is this a true statement? How do you know?
FOCUS Question
How does gravity affect objects in the solar system?
