This page will answer questions such as
... Why does gravity vary from planet to planet?
A gravitational attractive forces acts
between all objects of any mass, no matter how close or far apart they are.
Gravity is universal and a non-contact force
that exists
wherever there is mass.
All objects have a gravitational
field around them.
Therefore, there is always a
gravitational force
of attraction between all objects all the time, and it doesn't matter
whether they are in contact or not. - even though technically
gravity is a non-contact force, it acts through all materials even if
they are touching.
The space affect by the gravity effect of the object's
mass is called the gravitational field.
You experience gravity it as you jump up
vertically against it's force and are then pulled back down to earth by the same
force.
It was Isaac Newton who first realised
that objects fell to Earth due to gravitational field attraction.
He not only recognised that gravity was a
universal law of nature, but realised you could not only apply it to
'falling objects', but also to the motion of the planets around our Sun and
our Moon orbiting the Earth.
BUT, remember, you are attracting the
Earth at the same time as it attracts you. You don't notice this because
your mass is so much smaller than the Earth. You only consciously experience
yourself being attracted to the Earth.
Gravity makes everything fall towards the
surface of an object e.g. like a planet, and it is gravity that gives everything
'weight' (explained below).
The force of gravitational attraction
between two masses increases by two factors:
(i) The bigger the two masses involved
With its bigger mass, an elephant is
more strongly attracted to the earth than you!
The greater a planet's mass, the
greater the strength of the gravitational field around it.
(ii) The closer the two objects are
together
The further you go above the Earth's
surface the weaker the gravitational attraction between you and your
planet.
Note: Although not needed for GCSE:
attractive force
F
m1 x m2/d2
F = force of attraction
between the two objects,
m = the masses of the two attracted objects,
d = distance from
centres of gravity of the two objects
So you can relate this arithmetical proportionality
equation to the two rules above)
You don't really notice gravitational
attraction between objects around you because the gravitational fields are
too weak.
However, because the huge mass of the Earth and its very strong
gravitational field, you definitely notice objects falling towards it..
BUT,
with gravity, we are always dealing with mutual attraction, so, what you
don't notice is that your body is attracting the Earth towards it - think
about it!
INDEX physics notes FORCES
section 2 on mass, weight and gravity
Keywords, phrases and learning objectives for the force of gravity
Know that a gravitational force field exists around
any object.
Know the bigger the mass of the objects the stronger
the gravitational field and the greater the force of attraction
between objects.
Know the shorter the distance between objects, the
stronger the gravitational attractive force between the objects.
(Strictly speaking the distance is taken as the
shortest distance between the two centres of mass of the two objects
mutually attracting each other)
(See Part 2.4
The centre of mass of an object)
