Download Why don`t planets crash into each other?

Da
Name __________________________________________________________
e
Big
Weekly Question
Day
1
i l y S c i e nc
Why don’t planets crash
into each other?
Idea 4
Just as we know that the sun will rise every morning, we expect
the planets and the moon to stay in their orbits. And rightly so. For
400 years, people have understood that the movements of Earth,
the moon, the planets, and all the other bodies in the universe are
determined by precise and predictable laws of force and motion.
These laws tell us that the path of an object is controlled by two
forces: gravity and inertia.
Inertia keeps an object moving in a straight line and at a
constant speed until it is pushed or pulled on by a force. Everything
with mass has inertia, and the more massive something is, the more
inertia it has. In the absence of all other forces, such as air resistance
or gravity, inertia would allow a baseball that was hit out of the park
to travel in a straight line all the way to the edge of the universe!
WEEK 4
Vocabulary
inertia
ih-NUR-shuh
a property of
matter that keeps
an object moving
in a straight line
and at a constant
speed
A. What two forces govern the movement of the planets and all other
bodies in the universe?
1.
2.
B. Cross out the incorrect word in each sentence below and write the
correct word above it to make the statement true.
1. Gravity keeps an object moving in a straight line at a constant speed.
2. The path of a baseball is unaffected by the laws of motion.
3. The more mass an object has, the less inertia it has.
PB-:";;>;><I"
I-58E'/51:/1
117
Da
Name __________________________________________________________
e
Big
Weekly Question
Day
2
i l y S c i e nc
Why don’t planets crash
into each other?
Idea 4
Planets move in well-defined and separate paths, obeying the
laws of force and motion as they orbit the sun. What keeps planets
in their orbit is the balance between the opposing forces of gravity
and inertia. The sun’s gravity pulls planets toward it, and inertia
keeps planets moving in a straight line. The combined result is an
elliptical orbit, meaning that the planets revolve around the sun in
oval-shaped paths. The sun is almost, but not quite, at the center of
this oval. Similarly, the orbit of the moon around Earth is elliptical.
Unlike a baseball hit through the air, objects in space don’t
encounter air resistance or other types of friction that could slow
their motion. So the planets just keep circling the sun. And in the
absence of all other forces, they will continue to do this forever.
WEEK 4
Vocabulary
elliptical
gravitational
pull of the sun
bi
ta
ro
un
ds
un
ee-LIP-tih-kul
having the shape of
an oval or flattened
circle
Ea
rt
o
h’s
r
Earth’s inertia
A. Complete the sentences, using words from the passage.
1. The balance between gravity and
keeps planets
in their orbits.
2. Air resistance is a kind of
that slows down an
object’s motion.
3. Gravity and inertia maintain the moon’s
around Earth.
B. Complete the analogy.
Circle is to round as oval is to
118
.
-58E'/51:/1I"
IPB-:";;>;><
Name __________________________________________________________
Weekly Question
Day
3
Why don’t planets crash
into each other?
Astronauts orbiting Earth in a spacecraft are subject to the
same laws of gravity and inertia that control the motions of the
moon and planets. When you see astronauts floating inside their
spacecraft, they may appear to be weightless, but they are actually
still in the grip of Earth’s gravity. Astronauts float because they are
in free fall. During free fall, the gravitational force that causes a
spacecraft and its contents to “fall” toward Earth is exactly matched
by the forces of inertia that keep the spacecraft traveling in a
straight line. The result is a curved
trajectory—an elliptical orbit around
Earth. Because downward fall is balanced
by sideways motion, the spacecraft
doesn’t ever actually hit Earth.
You, too, may have experienced free
fall momentarily on an amusement park
ride. During a sudden drop, there is a
brief sensation of weightlessness as
your seat takes the plunge and inertia
attempts to keep you in place. But hold
on to your belongings, because coins
and sunglasses will seem to “float” out
of your pockets!
Da
i l y S c i e nc
e
Big
Idea 4
WEEK 4
Vocabulary
free fall
FREE fall
the unchecked fall
of an object
trajectory
trah-JEK-tuh-ree
the path of an
object moving
through space
Use the vocabulary words and information from the passage to complete
the paragraph below.
.
Every object that moves through space follows a
.
Even astronauts who appear to be floating are actually in
This means that the downward force of
balanced by
PB-:";;>;><I"
I-58E'/51:/1
is exactly
.
119
Da
Name __________________________________________________________
Why don’t planets crash
into each other?
Although the balance between inertia and gravity keeps the
planets in stable, predictable trajectories around the sun, this was
not the case when the solar system first formed. At that time, small,
solid bodies that coalesced early in the solar system’s history did not
have stable orbits. These bodies crashed into each other, and through
the process of accretion, became the large planets orbiting the sun
today. Objects such as comets and asteroids may be the leftovers.
One of the most spectacular collisions between large bodies in
our solar system may have happened right in our own “neighborhood.”
Scientists hypothesize that very early in Earth’s history, an object
the size of Mars collided with Earth and punched out a huge mass
of material that became our moon. Luckily for us, a similar collision
is probably impossible today.
e
Big
Weekly Question
Day
4
i l y S c i e nc
Idea 4
WEEK 4
Vocabulary
comet
KAH-mit
an icy chunk of rock
from a region of
space beyond the
orbit of Neptune
Write true or false.
1. When the solar system first formed, objects in space moved
in predictable trajectories.
2. Comets may be remains from the formation of the solar system.
3. The moon was formed when Earth and Mars crashed into
each other.
Talk
In the past, Earth was a planet where collisions with asteroids,
meteorites, and comets happened regularly. How do you think
these collisions helped shape life on Earth today?
120
-58E'/51:/1I"
IPB-:";;>;><
Da
Name __________________________________________________________
Weekly Question
Day
5
Why don’t planets crash
into each other?
i l y S c i e nc
e
Big
Idea 4
A. Write the word from the box that completes each sentence.
elliptical
comets
free fall
inertia
WEEK 4
trajectory
1. When astronauts float in space, they are really in
.
2. The orbit of Earth around the sun is
, instead of
completely circular.
3. An object’s
is the path it takes through space.
is the property of matter that keeps it moving
4.
in a straight line.
5. Scientists believe that
may be leftovers from
the solar system’s formation.
B. Label gravity, inertia, and orbit in the diagram below.
PB-:";;>;><I"
I-58E'/51:/1
121