105
Gravity
In orbit around Earth, astronauts become
weightless and float about their spacecraft
as if gravity did not exist. In fact, gravity
is still pulling the astronauts and their
spacecraft towards Earth, but as the
spacecraft travels forwards, it is also
continually “falling” as it follows
the curve of Earth. The craft and
astronauts are in a state of free-
fall, but falling without ever
reaching the ground.CENTRE OF GRAVITY
An object’s centre of gravity is the
point at which it balances. An object with
a low centre of gravity is more stable –
so a sports car is more stable than a
double-decker bus. The secret to driving a
car on two wheels is to ensure that the
centre of gravity remains above the wheels –
any further over and the car will tip over.BLACK HOLES
When a massive star dies, its core
may collapse. As it shrinks, the core
becomes ever denser and forms a
region of space called a black hole.
The force of gravity in a black hole is
so strong that anything entering it is
swallowed up, including light.
Although invisible, black holes can be
identified by the effect their gravity
has on everything around them.
Material being sucked into the hole
heats up, emitting X-rays that can
be detected by X-ray telescopes.AIR RESISTANCE
In a vacuum, gravity
causes everything to fall at
the same speed. However, if an
apple and a feather are dropped
from the same height in Earth’s
atmosphere, the apple will fall faster. As
they fall, objects are slowed down by air
resistance, created by friction between the air and
the object. The speed an object falls depends on the
balance between gravity’s pull and the air’s resistance.EINSTEIN’S THEORY
German-born scientist Albert Einstein
(1879–1955) developed a theory of
relativity to explain how gravity works
in space. He compared space and
time to a sheet of stretchy rubber,
which everything in the Universe
rests on. Massive objects like stars
make a big dip in the rubber. Less
massive objects like planets fall
into these large dips and so are
trapped orbiting stars. The dips
create the effect we call gravity.104_105_Gravity.indd 105 03/01/19 12:10 PM