Inertia

Inertia is the tendency of an object to continue to stay at rest or in motion at a constant speed along a straight line.

Forces and Mass

A force is a push or a pull and is a vector (like velocity, displacement, and acceleration).

Mass is a property of matter that determines how difficult it is for an object to speed up or slow down and is a scalar.

An object at a state of rest or in a state of motion at a constant speed along a straight line unless compelled to change that state by a net force.

IN OTHER WORDS...

F=0

iff the object is at rest or in motion at a constant velocity (a=0)

When a net external force ΣF acts on an object of mass m, the acceleration that results is directly proportional to the net force and has a magnitude that is inversely proportional to the mass.

IN OTHER WORDS...

ΣF=ma

Newton's 3rd Law

For every action, there is an equal and opposite reaction.

In other words, a force in a action-reaction pair is equal to the force pushing against it.

F₁₂=F₂₁

F=G*m₁m₂

            r²

G=6.673*10^-11 N*m²/kg²

m=mass of object

r=distance between both objects