Also, equation of the day: Newton's Second Law
where F is the net force, a is acceleration, and m is the mass of the object in question. It's such a simple-looking equation, but it contains so much physics. Want to know the path of a free-falling object subject only to the force of gravity? You use this equation. Want to know the attractive force and classical orbit of planets/atoms? You use this equation. Want to know the physics of a car skidding on pavement? You get the idea.
This equation is a staple of physics and is used extensively in intro and classical physics. Newton, you clever devil, you.