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, whether on Earth or in space. Knowing these principles, now called Newton's Laws of Motion, rocketeers have been able to construct the modern giant rockets of the 20th century such as the Saturn 5 and the Space Shuttle. 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 times 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. But with them, precise determinations of rocket performance can be made. This law of motion is just an obvious 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.