Particle System 3: Velocity Modules

Overview

Once particles are emitted, their movement is governed by velocity modules. Unity provides three main velocity-related modules that give you fine-grained control over how particles travel through space.

Velocity over Lifetime

This module adds a constant or animated velocity to each particle throughout its life.

Coordinate space

• Local — Velocity is relative to the Particle System’s own transform. The effect moves with the object.
• World — Velocity is in world space. Useful for wind effects that blow in a fixed direction regardless of emitter orientation.
• Custom — Velocity is relative to a custom transform.

Use cases

• Wind — Add a small X or Z world-space velocity to smoke/leaves to simulate wind
• Gravity alternatives — Instead of using the built-in gravity modifier, use Y velocity curves for more artistic control
• Spiraling effects — Combine X and Z velocities with a sine curve on one axis

Limit Velocity over Lifetime

This module damps a particle’s velocity when it exceeds a threshold — simulating drag.

• Speed — The maximum speed. Particles exceeding this are slowed.
• Dampen — How aggressively to reduce velocity (0 = no effect, 1 = instant clamp).

Great for: sparks that slow as they fall, bubbles that reach terminal velocity.

Inherit Velocity

When a particle system is attached to a moving object, this module lets particles inherit some or all of the emitter’s velocity at the moment of emission.

• Initial — Velocity is inherited once at birth and then the particle moves independently
• Current — Velocity continuously tracks the emitter’s current speed

Example: Exhaust from a spaceship. With Initial mode, particles shoot out the back and then drift independently in space. With Current mode, they keep following the ship’s velocity — which looks wrong for most cases.

Combining velocity modules

A realistic smoke trail uses all three:

1. Velocity over Lifetime — Upward Y drift in world space
2. Limit Velocity — Terminal velocity so smoke doesn’t go too fast
3. Inherit Velocity (Initial) — Smoke inherits the emitter’s speed when first emitted

Next steps

Lesson 4 covers the Force and External Forces modules for physics-driven particle behaviour.