The force of gravity is a force of attraction
that exists between all objects with mass,
from microscopic atoms to stars and planets.
On Earth, gravity can be seen when objects
fall to the ground, pulled by an invisible force.
In space, the force of gravity keeps the Moon
in its orbit around Earth, keeps planets in orbit
round the stars, and holds huge clusters of
stars together as galaxies.GRAVITY NEWTON’S DISCOVERY
An apple falling from a tree is said
to have inspired English scientist
Isaac Newton (1642–1727) to
explore the force of gravity. He
developed a theory stating that
every mass attracts every other
mass by a force between both
masses. The more massive an
object is and the nearer it is, the
greater its gravitational attraction.104
BIRTH OF A STAR
A star is born inside a cloud of dust and gas in space
called a nebula. The dust and gas begin to clump
together, forming a core. The clump’s gravitational
attraction increases as its mass increases, dragging
in more and more matter. The centre, or core,
becomes so massive and dense that nuclear fusion
begins, and the star begins to shine.MASS AND WEIGHT
These are not the same. An object’s mass is
the amount of matter it contains. An object’s
weight is the force exerted on its mass by gravity.
This means that on the Moon, where gravity is
just one-sixth of Earth’s gravity, an astronaut
will weigh one-sixth as much as he weighs on
Earth, although his mass is the same.MOON AND TIDES
As the Moon orbits Earth, its
gravity tugs at the water in the
oceans, making a mass of water
bulge towards the Moon. The
force of Earth’s spin creates a
matching bulge on the other
side of Earth. These bulges
cause the regular rise and fall
of the water level at the sea’s
edge that we know as tides.104_105_Gravity.indd 104 03/01/19 12:10 PM