|1) Definition of
objects are interacting if they exert a mutual action upon each others.
2) Magnetic interactions
Before to study gravity we can first try to understand another kind of
interaction: magnetic interaction.
It will be used to determine general interactions properties.
If two magnets are placed close to each other then they move with the
on the chosen poles.
- If the
same poles ( two south poles or two north poles ) are placed
in front of each other, then magnets move away from each other: it's a
- If the opposites poles ( a south pole and a north pole ) are placed
in front of each other, then the magnets gets closer: it's an
- In every situation, if magnets are closer their motion is more rapid.
- If the magnets are identical,
they move at the same
Magnetism can legitimately be called an interaction because if an
action is exerted by a magnet on another one, this other magnet also
exerts an action.
magnetism is a remote interaction because no contact is required to
interact, two magnets exert mutual actions upon each other, which
properties are also valid for all other interactions:
are exerted along the same axis.
are exerted in opposite directions.
are exerted with an identical strenght.
3) Gravitational interaction
Gravity is another interaction but contrary to magnetism it doesn't
requires any magnetic poles for being exerted: the only condition is
that objects own a mass.
Most of objects around us ( including the smallest ones ) have a mass
which make them take part to gravitationnal interaction.
Gravity is ruled by general properties of interactions but it
also has its own properties
- Like magnetism, this is a remote interaction which is stronger when
objects in interaction are closer.
- Contrary to magnetism, gravity is only attracting ( there is no
- A magnetic pole can be a south pole or a north pole but there is only
one kind of mass.