Introduction to Physics Simulation: Rigid Bodies and Cloth

Chapter 7's own tip box pointed here directly: some motion is genuinely more practical to simulate physically than to shape by hand with keyframes and F-Curves. This chapter covers exactly that — letting Blender's own physics engine calculate realistic motion automatically, rather than hand-animating every detail.

Why Simulate Instead of Hand-Keyframe?

A stack of blocks toppling over, a piece of fabric draping naturally over a chair, dozens of small objects scattering realistically after a collision — all involve so many interacting variables that hand-keyframing a convincing result would be extraordinarily tedious, if not practically impossible. Physics simulation calculates this kind of complex, interacting motion automatically, based on real physical properties rather than manually placed keyframes.

Rigid Body Physics: Objects That Collide and React Naturally

Marking an object as a Rigid Body tells Blender's own physics engine to treat it as a solid, non-deforming object that collides realistically with other Rigid Bodies — falling under gravity, bouncing off a surface, knocking into and displacing other objects, all calculated automatically once the simulation runs.

Rigid Body Settings: Mass, Friction, and Bounciness

Mass affects how an object responds when colliding with others — a heavier object displaces a lighter one more forcefully than the reverse. Friction affects how much objects resist sliding against each other. Bounciness (restitution) controls how much energy is retained after a collision — a high bounciness value produces a rubber-ball-like bounce, while a low value produces a dull, energy-absorbing thud with little rebound.

Cloth Simulation: Fabric That Responds to Gravity and Collision

Cloth simulation treats a mesh as flexible fabric, draping, folding, and settling naturally under gravity and reacting to collision with other objects — a flag rippling in wind, a tablecloth settling over a table's own surface, a character's own clothing responding believably to their movement.

Baking a Simulation: Making the Result Reliable and Scrubbable

Baking calculates and stores the simulation's own result for every frame in advance, rather than recalculating it live every time the Timeline plays or gets scrubbed. A baked simulation behaves reliably no matter how the Timeline is navigated, since every frame's own result is already stored rather than being computed fresh, forward-only, each time.

Where Simulation Ends and Hand-Animation Still Matters

Physics simulation excels at complex, physically-driven motion, but a character's own deliberate, expressive performance — a specific gesture, a meaningful pause, a stylized exaggeration — still genuinely benefits from Chapter 7's own hand-keyframed, carefully-eased animation. The two approaches are complementary tools for different kinds of motion, not competing options where one should always be preferred over the other.

Hands-On Exercises