What is inertia?

Inertia and mass: m = F ÷ a. Everything keeps doing what it is doing, staying still or moving steadily in a straight line, until a force changes it. That stubbornness is inertia, and mass, in kilograms, is how much of it something has.

Read how it works: What is inertia? · The history of inertia and mass · More boxes on Glassbox

Chapters in this interactive model

1. Inertia: how hard it is to change motion: Push two carts with the same force, flick a card from under a coin, and whip away a tablecloth.
2. When the bus brakes, you don’t: Passengers lurch, loads slide and riders pitch forward. Inertia rides along with you.
3. Spinning things have inertia too: A skater’s arms, a bicycle wheel on a string, and the heavy disc behind every car engine.
4. Weighing without weight: A balance, a spring scale, a wobbling chair on Skylab and a spring-loaded arm on the ISS.
5. Myths, and two kinds of mass that agree: Why things stop, why ice isn’t hard work, and the most precise coincidence in physics.