All the tech we rely on, from cars to smartphones, was engineered using physics. You don’t need to know the science to use these things. But a well-rounded human should understand at least some of the key concepts—along with some music, art, history, and economics. Robert Heinlein said it all in Time Enough for Love:
“A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.”
So, in the interest of not being insects, here’s my top-five physics equations you should know.
1. Newton’s Second Law
I’m sure you’ve seen this one before—it’s over 300 years old, and it’s popular for science memes and T-shirts. It says the net force on an object equals its mass (m) times acceleration (a). But what does that really mean? It’s all about interactions—like when you kick a soccer ball or drop a water bottle on the floor.
Newton’s second law says we can describe these interactions with the concept of “force.” And what do forces do? The net force on an object changes the object’s motion. But wait! There’s a bunch more cool stuff in this simple-looking equation.
See those arrows over F and a? That indicates variables that are vectors, meaning they contain more than one piece of information. For example, if someone asks you to “socially distance” yourself by 1 meter, where would you end up? Who knows? You could go 1 meter to the east or west or 39 degrees from north. The distance by itself isn’t the full story; you also need to specify a direction. This is true for both the forces and the acceleration. Other quantities (like mass or temperature) don’t have direction. We call those scalar values.
Newton’s second law is super useful, but weirdly, people don’t seem to believe it. The common misconception is that a constant force makes an object move at a constant speed. What this equation says, rather, is that if you push on an object with a steady force, it will keep accelerating.