# 玩微信红包扫雷有猫腻

Science class online
Chemistry and physics lessons

# Mechanics

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

 Science class Chemistry Electricity Optics Mechanics Mechanics lessons Gravity - The solar system - motions of celestial bodies - Physical interactions - Effects of gravity - Weight of an object - Difference between weight and mass - Relationship between weight and mass  Energy - Energy basics - Potential energy - Kinetic energy - Mechanical energy - Translational Kinetic Energy - kinetic energy mass and speed

## Gravity

### 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 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. ______________________________________ ______________________________________
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