Science class online
Chemistry and physics lessons
atom physics lesson


Free online mechanics lesson for elementary school,  middle school and high school.
chemistry high school


Physical interactions

1) Definition of interaction

Two 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 following characteristics:

Direction of motion
Magnetic interaction between two magnets
It depends 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 repulsion
- 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 attraction.


Speed of motion
- In every situation, if magnets are closer their motion is more rapid.
If the magnets are identical, they move at the same speed.

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.
Furthermore magnetism is a remote interaction because no contact is required to exert it.

When they interact, two magnets exert mutual actions upon each other, which properties are also valid for all other interactions:
- Actions are exerted along the same axis.
- Actions are exerted in opposite directions.
- Actions 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 respulsion )
- A magnetic pole can be a south pole or a north pole but there is only one kind of mass.