Source: http://hyperphysics.phy-astr.gsu.edu/hbase/Newt.html
http://zonalandeducation.com/mstm/physics/mechanics/forces/galileo/galileoInertia.html
3 Laws of Motion
Galileo and Inertia
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Inertia is the tendency of matter to resist
changes in its velocity.
Inertia is a property of matter. It is that property of matter which opposes changes in velocity.
Simply stated, a common object will not change its velocity spontaneously.
If something is moving along at a constant speed in a straight line, it will continue to move
along at the same constant speed in the same straight line. It will not, all on its own, speed up,
slow down, or change direction. Something else must push on the object to speed it up, slow it
down, or change its direction.
Also, if something is standing still, it will, if left to itself, continue to stand still. Something else
must push on an object to get it moving. Actually, an object standing still is just a special case of
an object keeping its velocity constant. Its velocity is constantly 0 m/s.