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Mass and weight
an object’s mass is the amount of material
it contains. Mass stays the same wherever
the object is in the universe. the weight of
an object is the force of gravity pulling
on it. weight can change. Because the
moon is smaller than earth, its gravity
is weaker, about one sixth as strong
as earth’s. therefore, an astronaut
on the moon weighs only one-sixth
of his or her weight on earth, but
her mass remains the same.earth Moves around the sun,
traveling about 50 times faster than
a rifle bullet. a strong force holds
earth in this orbit. this is the force
of gravity; without it, earth would
shoot off into space like a stone
from a catapult. everything
possesses gravity; it is a force that
attracts all objects to each other.
however, the strength of the force
depends on how much mass is in an
object, so gravity is only strong in
huge objects such as planets.
although you cannot feel it, the
force of gravity is also pulling on you.
earth’s gravity holds you to its
surface, no matter where you are on
earth. this is because gravity always
pulls toward the center of
earth. sometimes you can
see or feel the effects of
gravity. For example, the
effort you feel when you
climb up a flight of stairs
is because you are
fighting against the
force of gravity.earth’s
Gravity
People on the
opposite side of
earth are upside down
in relation to you. But they
do not fall off into space. they
are held on to the surface of earth just as you are. this is
because the force of gravity pulls everything toward the center
of earth. down is always the direction of earth’s center.Falling
earth’s gravity makes falling
objects accelerate (speed
up). their speed does not
depend on how heavy they
are: a light object falls as fast
as a heavy object unless air
slows it down. the italian
scientist galileo galilei
(1564-1642) noticed this
about 400 years ago.Objects such as a loaded tray balance
if supported directly beneath their
center of gravity.Center oF gravity
it is best to carry a large, unwieldy object
such as a ladder by holding it above its
center. the weight of the ladder
balances at the center, which
is called its center of gravity
or center of mass. an object
with a large or heavy base has
a low center of gravity. this stops
it from falling over easily.The force of gravity
gets weaker as you
go farther from the
center of Earth.
On top of a high
mountain, gravity
is slightly weaker
than at sea level;
so objects weigh
fractionally less.Gravity
Find out more
astronauts
and space travel
Physics
science, history of
Universe
weights and measuresGravity pulls all objects
down toward the
center of Earth.When you drop a ball, it falls
because gravity is pulling it
toward the center of Earth.Objects fall in the
opposite direction
on the other side
of Earth.Moon and earth
gravity keeps the moon moving in its
orbit around earth. the moon’s gravity
has effects on earth, too. when the
moon is directly over the sea, its gravity
pulls the seawater toward it, which
produces a high tide; low tide follows
when earth rotates away again.Isaac NewtoN
english scientist isaac newton
(1642-1727) was the first person
to understand the force of
gravity. in 1666, after
watching an apple fall to
the ground, he wondered
whether the force of gravity
that makes things fall also
holds the moon in its orbit
around earth. this was a
daring idea, and it took newton
many years to prove it to be true.
he declared his law of gravity to
be a universal law—a law that is
true throughout the universe.A heavy rock weighs
much more than an
egg of the same size.
However, both objects
fall at the same rate
and hit the ground
at the same time.US_243_Gravity.indd 243 21/01/16 5:00 pm