Video narration
Why does a box resist your push, then suddenly slide? Friction acts against motion, or the tendency to move. If surfaces stay stuck, static friction stops slipping; when they slide, sliding friction resists them. A rolling wheel experiences rolling friction. Air or water resisting motion is fluid friction. Here's the key: friction isn't one fixed amount. Its type depends on how surfaces move. The coefficient of friction, μ, compares friction force, F, with N, the surface's supporting push, called the normal force. So μ equals F divided by N. For static friction, that ratio describes its maximum, not every moment. Static friction adjusts to the push until it reaches that limit. So why the pause, then slide? Static friction holds the box still; once it moves, sliding friction takes over, often at a lower force.