Introduction to Newton's Laws of Motion

In his Principia Mathematica, Newton formulated three laws that became the basis for mechanics, the science of how and why objects move:

1. Every body continues in its state of rest, or uniform motion in a straight line, unless compelled to change that state by forces impressed upon it.
   
   
2. The change of motion of an object is proportional to the force impressed upon it, and is made in the direction of the straight line in which the force is impressed.
   
   
3. 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.

Over time, scientists have debated whether all three "laws" are necessary. There is quite a lot of redundancy between them, as we will see. However, each one has withstood the test of time as essential in its own right. But, what are these "laws" and what do they mean?