Math and physics › Physics of motion
Why fast bodies fall through walls
Know first: Motion: velocity, acceleration and the time step, Collisions: how the engine knows that objects touch
A bullet flies through a wall. A ball ends up behind the fence after a hard hit. A crate lies under the floor after the game stutters. All three have one cause: physics sees the world in snapshots, and a fast body can jump over an obstacle that sits between two of them. This page shows how to estimate the risk with simple arithmetic, and why a body can fall through even when physics catches it inside the wall. Then it covers the fixes: thick walls, substepping, continuous collision detection, a ray in place of a bullet body and speculative contacts. Every fix has a price, and the page names each one.
Why this matters
Falling through walls is one of the most common complaints about game physics. No single checkbox solves it: every fix costs processor time, accuracy or realism of the motion. Once you understand the mechanism, you can pick the cheapest fix that works: a thick wall in one place, a ray in another, continuous collision detection in a third.