Download the earth`s gravity

GRAVITY
1
ancient = old
Gravity is a force that attracts objects. It is also the force that makes objects
fall down when you drop them. Because of gravity things on earth have a certain
weight. Gravity on Earth pulls objects to the centre of the planet.
astronomer = person who
studies the objects in the
sky; the planets and the sun
Every time you throw a ball up into the air it is pulled back to Earth. This occurs
in nature all the time.
WORDS
attract = pull towards
something
depend on =to be decided
by something else
D ISCOVERY OF GRAVITY
direction =path, way, route
Ancient astronomers observed the
movements of the moon and the
planets across the sky very
carefully. In the 17th century the
English scientist and mathematician
Isaac Newton wondered why the
moon and the planets didn’t simply
fly off into outer space but always
moved in a curve around the Earth
and the sun. He found out that the same force that pulled an apple back to the
ground also kept the moon moving around the Earth.
discover = to find out
something for the first time
distance= how far two
things are apart from each
other
explain = to tell someone
something so that is clear to
understand
force = power, energy
gravity = the force that
makes something fall to the
ground or pull it to another
object
motion = movement
movement =when the
position of something
changes
observe = watch
orbit = to move around an
object in a curved path
outer space = the space
outside the Earth’s
atmosphere; where the
planets and the stars are
path = course, track that
something takes
pull = attraction, force, the
act of moving something
scales = a machine that
shows you how heavy you
are
scientist =a person who is
trained in science
space = the area beyond the
Earth where the planets and
the stars are
speed =how fast something
is
weigh =how heavy
something is
weight = how heavy
something is
He discovered that the moon’s orbit is the result of two different movements.
The first motion makes the moon fly along a straight line in space. A planet will
always follow such a line if nothing else changes its direction or speed. The
second force pulls the moon towards the Earth. If you put both movements
together you get a curved path around the Earth.
Newton also found out that every body or object has a force of gravity , and that
every body pulls other bodies towards it. He also explained that gravity depends
on the mass of an object or the amount of material that it has. Therefore the
sun, which has a very large mass, has a greater force of gravity than the Earth,
so the Earth moves around the sun. The moon goes around the Earth because the
Earth’s gravity is larger than the moon’s.
The force of gravity also depends on the distance between two objects. If they
are close together gravity between them is greater.
M ASS AND W EIGHT
Mass and weight are not the same. The mass of a body, its
volume, always stays the same. Weight is the pull of gravity on
an object. On Earth, an astronaut may weigh 70 kg. However
when the same astronaut walks on the moon he weighs only a
sixth – about 12 kg because the moon has less mass than the
Earth and therefore its gravitational pull is smaller.
If you stand on scales you can see how much your body is pulled
towards the centre of the Earth. This is your weight.
GRAVITY
accelerate= to become faster
and faster
centrifugal = a force which
makes things move away from
the centre of something when
they are moving around it
decrease = to become smaller
distance =space between two
things
equator = the line around the
centre of the Earth; every
place on the line has the
same distance to the North
and South Pole
escape = to get away from
feather =one of the light soft
things that cover a bird’s
body
float = to move slowly in the
air without falling to the
ground
force = power, energy
fuel =material like coal or oil
that is used to produce
energy or heat
gravity = the force that
makes something fall to the
ground or pull it to another
object
launch = start, take off
law =a rule that is made by
the government
marble = a small coloured
glass ball that children can
roll along the ground
orbit = to move around an
object in a curved path
path = course, track that
something takes
reduce = to become smaller
resistance = the way in which
wind, air or water can make
an object slow down
rotation = when something
turns around a central point
spacecraft = a machine that
is able to travel through space
speed =how fast something is
spin = to turn around
straight = not curved
strength = power
surface =the top layer of an
object
therefore = that is why
weigh =how heavy something
is
weightlessness = something
that seems to have no weight
2
FALLING B ODIES
Newton’s law of gravity says that objects accelerate when
they are dropped farther away from the surface. The
acceleration near the Earth’s surface is about 9.8 metres
per second. Bodies falling in air are slowed down by air
resistance. In vacuum, a feather and a marble would fall
down at the same speed but in the air the larger surface
of the feather slows it down.
S PACECRAFT AND S ATELLITES
Spacecraft and satellites travel around the Earth in a similar way. The Earth’s
gravity keeps them in orbit and their speed prevents them from falling back to
the Earth.
Astronauts who are inside a spacecraft that travels around the Earth feel
weightlessness. They float inside the spacecraft because they are travelling at
the same speed as the spacecraft.
T HE E ARTH ’ S G RAVITY
The force of gravity is not the same everywhere on Earth. It depends on
the distance from the centre of the Earth
the spin or rotation of the Earth
A house at the seaside, for example, is closer
to the Earth’s centre. Gravity is stronger here
than at a house up in the mountains.
The Earth’s spin also reduces the strength of
gravity. The centrifugal force causes a body
or an object to move in a straight line unless
something tries to change its path. At the
equator the centrifugal effect is greater than
anywhere else on our planet. An object must
travel 40,000 km during one rotation of the
planet. The distance and the centrifugal
force decrease when you move towards the
poles. Therefore you would weigh more at
the poles than at the equator.
For this reason spacecraft are launched from places that are as near to the
equator as possible. In this way it takes less fuel and power to escape the
Earth’s gravitational force and get into orbit.
Gravity also holds our atmosphere together. We wouldn’t see any clouds or rain
if there were no gravity.