Today, Newton's work is usually just called the Principia. In the Principia, Newton stated three important scientific principles that govern the motion of all objects. Knowing these principles, now called Newton's Laws of Motion, rocketeers have been able to construct modem giant rockets such as the Saturn V and those that launch the Space Shuttle into orbit. Here now, in simple form, are Newton's Laws of Motion. Objects at rest will stay at rest, and objects in motion will stay in motion in a straight line unless acted upon by an unbalanced force.
Force is equal to mass multiplied by acceleration.
For every action there is always an opposite and equal reaction. As will be explained shortly, all three laws are really simple statements of how things move. With them, we can make precise determinations of rocket performance. This law of motion is a basic statement of fact, but to know what it means it is necessary to understand the terms rest, motion and unbalanced force. Rest and motion can be thought of as being opposite to each other.