Isaac Newton realized that we can actually predict how all the different forces acting on a body will change the motion of that body. He even went as far as to develop the math and physics we needed in order to do so! Today, we call his invention calculus. Newton was a literal game changer for physics, besides literally developing it.
Newton’s first law of motion
Newton’s first law states that an object in motion will stay in motion unless a force acts upon it. For instance, in our solar system the planets are in motion as they orbit our Sun. So long as the net force applied on each planet remains constant, the planets will continue orbiting the Sun forever! The same rule applies on Earth, but the Earth’s atmosphere introduces external forces such as friction and air resistance, which will usually be the culprit for slowing down moving bodies.
Newton’s second law of motion
Newton’s second law is the connection between force and acceleration. Remember how we said force defines the speed, velocity, and acceleration of an object? This is defined explicitly as mass times acceleration. Remember the acceleration is the measure of change in velocity, so all the information sits in this simple equation:
Force = mass x acceleration!
Newton’s third law of motion
Lastly, Newton’s third law is that for any force acted on a body, there is always an equal and opposite reaction. This means that if Earth exerts a force on me, then I must be exerting a force back on Earth. No really! That force is the same exact one we measure as “weight” on a scale. Why don’t I feel like I am exerting a force then? Consider again the second law of motion: the gravitational acceleration I am exerting onto Earth is MUCH smaller than the acceleration the Earth exerts onto me, because the Earth is so much more massive than me! See it for yourself! Try filling up a balloon with air and then releasing it {graphic with FBD illustrating this}. The air escaping the balloon will push the balloon in the opposite direction that the air is moving into!
Want to know more?
Check out Chapter 4: Making Sense of the Universe in the textbook The Cosmic Perspective by Bennet, Donahue, Voit, and Schneider, Seventh Edition, 2014. Pgs 123–125
(On) Planet Nine 