Newton's First Law
every object continues in its state of rest, or of uniform motion in a straight line, unless compelled to change that state by forces impressed upon it
the property of things to resist changes in motion
a push or a pull
the vector sum of forces that act on an object
the state of an object or system of objects for which there are no changes in motion.
for any object or system of objects in equilibrium, the sum of the forces acting equals zero.