The Big Idea
Applied forces may cause objects to accelerate. All forces come in pairs because they arise in the interaction of two objects — you can’t push without being pushed back! The more force applied, the greater the acceleration that is produced. Objects with high masses are difficult to accelerate without a large force. In the absence of applied forces, objects move in a straight line at a constant speed (or remain at rest). In formal language:
Newton's First Law
Anybody continues in its state of rest, or of uniform motion in a right (straight) line, unless it is compelled to change that state by forces impressed upon it.
Newton's Second Law
The change of motion is proportional to the motive force impressed; and is made in the direction of the right (straight) line in which that force is impressed.
Newton's Third Law
To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.
(Taken from the Principia in modern English, Isaac Newton, University of California Press, 1934).
Understanding motion comes in two stages. The first stage you’ve already seen: you can figure out where something will go, and how fast it will get there, if you know its acceleration. The second stage is much more interesting: where did the acceleration come from? How can you predict the amount of acceleration? Mastering both stages is the key to understanding motion.