Friction

Mass

Inertia

Gravity

all objects with mass

only objects at rest

only objects in motion

only objects whose motion is being changed

gravity

zero net force

balanced forces

unbalanced forces

slowing motion in a straight line

accelerating motion in a straight line

constant motion in a straight line

constant motion in an unpredictable direction

F=ma

Every action has an equal and opposite reaction

An object at rest stays at rest, an object in motion stays in motion. 

Friction