Gravity
G
ravity is the force that ensures that what goes up must come down. It pulls us
towards the Earth, pressing our feet onto the ground (which results in the
friction that gives us traction). Gravity also dominates the universe, holding the Moon
in orbit around the Earth, and the Earth in orbit around the Sun. It is a force that acts
between any pair of objects, whether they are in contact or not. Gravity is therefore a
non-contact force.
Gravity everywhere
No matter how large or how
small, all objects attract each other.
This force of attraction is called
gravity. Believe it or not, gravity is a
very, very small force. Even though
all objects are attracted to each
other, the effect is only felt when at
least one of the objects is massive —
as massive as a planet, moon or star.
The bigger the mass of an object,
the greater the force it pulls with.
The force of gravity between you
and your desk is very small because
both you and the desk have very
small masses. You can’t see the
effect of gravity in this case. The
force that attracts you to the Earth
and the Earth to you is much
bigger. You can see the effect of
this force, especially if you fall! The
pull of the Earth’s gravity is
towards the centre of the Earth.
Newton’s apple story takes a fall
Isaac Newton (1643–1727) was an English mathematician,
physicist, astronomer and philosopher. You might know him as the
guy who sat under the apple tree and, after being struck on the head
by a falling apple, discovered gravity. While this is a commonly
believed story, scientists aren’t convinced it happened that way.
First, how can anyone discover gravity? It has always been here!
Second, there is debate about whether Newton ever was struck on the
head with the apple. Many scientists and historians believe that
Newton was looking out of the window when he saw the apple fall.
At this point he was struck with a realisation — apples (and everything
else) always fall down, not up or sideways. He wondered about the
force that caused this to happen. He wondered what would happen
if the tree were much taller. In fact, he was able to deduce, after much
time and many calculations, that the
force that caused the apple to fall was the
same force (gravity) that kept the Moon
in orbit around the Earth. From these
ideas, Newton wrote his Law of Universal
Gravitation which describes how gravity
acts in all places, not just on Earth.
Newton was able to explain many observations,
including falling apples, tides and orbiting
planets with a single law of gravity.
Moon
Earth
If it weren’t for gravity, the Moon would fly
right past us. The gravitational attraction
between the Earth and the Moon keeps the
Moon in orbit around the Earth.
Weight and mass
You might be surprised to know that mass and weight are two different
things. Mass measures how much of a substance there is. No matter
where you go — the Earth, Moon or Mars — your mass does not change.
Mass is usually measured in kilograms, although other units like tonnes
and grams can be used.
Weight is a measure of the size of the gravity force pulling you down.
Weight is a force so, like other forces, it is measured in newtons. Your weight
depends on your mass and the mass of the object pulling you down.
Wherever you go in the universe, your mass is always the same. But
your weight is different, depending on what objects you are close to.
Close to the Earth, your weight is greater than if you were on the Moon.
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Science Alive for VELS Level 5
Go to worksheet 9.4:
Bungee cord safety test
Bungee forces
The staff at bungee jumping
venues must understand the
effect of gravity and how
forces work. For each
jump, they select
appropriate equipment
for the size of the
jumper. The mass of
the jumper
determines which
cord will be used.
The cords are
different
thicknesses for
different masses.
Gravity is the force
that pulls a bungee
jumper towards the
ground. The size of
the gravity force
depends on the mass
of the jumper. The
size of the gravity
force acting on a
person is called the
person’s weight.
While in free fall, gravity is
the main force acting on the
jumper; however, as the rope
starts to stretch, it will pull
upwards. The upward force
increases as the rope
stretches, eventually becoming
greater than gravity and
slowing the jumper to a stop.
But the rope force and gravity
are not balanced, so the
jumper bounces up, reducing
the rope force. Eventually, the
jumper stops when the rope
force is equal to gravity.
REMEMBER
1. What are the units for
measurement of (a) mass
and (b) weight?
2. In which direction does the
pull of gravity from the Earth
act?
3. How would your mass
change if you were to visit
Mars?
THINK
4. ‘Gravity exists between two
5.
6.
The faster the jumper
falls, the more air
resistance he or she will
feel. Air resistance is a
force that acts on an
object moving against
air. The air resistance
on a bungee jumper is
much smaller than the
pull of gravity and the
pull of the rope.
That’s because the Moon is much smaller than the Earth, so the
gravitational attraction between you and the Moon (your weight) is
less than that between you and the Earth.
7.
8.
BRAINSTORM
9. Make a list of sports where
gravity:
(a) makes the sport more
difficult
(b) makes the sport possible
(c) has no effect.
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9. Forces and motion
✓ learning
Separation
Gravity depends on separation as well as mass. As the distance between two
objects increases, the force of gravity becomes weaker. As the distance
between the centres of two objects doubles, the force of gravity is quartered;
if the separation is tripled, the force is reduced to a ninth of what it was.
For example, consider a system involving only you and the Earth. If you
climb a mountain on Earth, you don’t notice any significant change in the
size of the gravitational pull. That’s because the increase in altitude is very
small compared with how far we are from the centre of the Earth.
objects only if at least one of
them is massive.’ Is this
statement true or false?
Explain your answer.
What is the largest force
acting on a bungee
jumper:
(a) when the rope is slack?
(b) while slowing down?
The weight of a 3 kg brick
is 30 newtons. What would
be the weight of a 6 kg
brick?
An astronaut weighs herself
on Earth. Her weight is
600 newtons. What is her
weight at twice this distance
from the centre of the Earth?
Why do you feel just as
heavy at the top of a
mountain as at the bottom,
even though your weight
gets smaller the further
you go above the surface
of the Earth?
I CAN:
explain what gravity is
explain how the mass of objects
affects the size of the gravity force
between them
explain the difference between
mass and weight.