Particle lifetime and speed in Unity - Step-by-Step Execution

Concept Flow - Particle lifetime and speed

Start Particle System

↓

Set Particle Speed

↓

Set Particle Lifetime

↓

Emit Particles

↓

Particles Move

↓

Particles Expire After Lifetime

↓

End

This flow shows how particles are created with speed and lifetime, move accordingly, and then disappear after their lifetime ends.

Execution Sample

Unity

var ps = GetComponent<ParticleSystem>();
var main = ps.main;
main.startSpeed = 5f;
main.startLifetime = 3f;
ps.Emit(1);

This code sets particle speed to 5 units/sec, lifetime to 3 seconds, then emits one particle.

Execution Table

Step	Action	Particle Speed	Particle Lifetime	Particle Age	Particle Position	Particle State
1	Set startSpeed to 5	5	-	-	-	No particles yet
2	Set startLifetime to 3	5	3	-	-	No particles yet
3	Emit 1 particle	5	3	0	(0,0,0)	Alive
4	After 1 second	5	3	1	(5,0,0)	Alive
5	After 2 seconds	5	3	2	(10,0,0)	Alive
6	After 3 seconds	5	3	3	(15,0,0)	Dead (expired)
7	After 3.1 seconds	5	3	3.1	(15.5,0,0)	Dead (expired)

💡 Particle expires after 3 seconds lifetime, so it is removed at step 6.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7
Particle Speed	-	5	5	5	5	5
Particle Lifetime	-	3	3	3	3	3
Particle Age	-	0	1	2	3	3.1
Particle Position (x)	-	0	5	10	15	15.5
Particle State	No particles	Alive	Alive	Alive	Dead	Dead

Key Moments - 3 Insights

Why does the particle disappear after 3 seconds?

Because the particle's lifetime is set to 3 seconds (see execution_table step 6), it expires and is removed after that time.

How does particle speed affect its position?

Particle speed determines how far it moves each second. At speed 5, after 1 second it moves 5 units (execution_table step 4).

What happens if we emit a particle but don't set lifetime?

The particle uses the default lifetime from the ParticleSystem. Without setting it explicitly, it may live longer or shorter depending on defaults.

Visual Quiz - 3 Questions

Look at the execution table, what is the particle's position after 2 seconds?

A (5,0,0)

B (15,0,0)

C (10,0,0)

D (0,0,0)

💡 Hint

Check the 'Particle Position' column at step 5 in the execution_table.

At which step does the particle expire and become dead?

A Step 4

B Step 6

C Step 7

D Step 3

💡 Hint

Look at the 'Particle State' column in the execution_table for when it changes to 'Dead'.

If we change startSpeed to 10, what will be the particle's position after 1 second?

A (10,0,0)

B (5,0,0)

C (15,0,0)

D (0,0,0)

💡 Hint

Particle position after 1 second equals speed * time; see variable_tracker for position updates.

Concept Snapshot

Particle lifetime and speed in Unity:
- Use ParticleSystem.main.startSpeed to set speed.
- Use ParticleSystem.main.startLifetime to set how long particles live.
- Emit particles with Emit(count).
- Particles move at speed until lifetime expires, then disappear.

Full Transcript

This visual execution shows how Unity's ParticleSystem uses startSpeed and startLifetime to control particle behavior. First, speed and lifetime are set. Then a particle is emitted at position zero with age zero. Each second, the particle moves forward by speed units. After 3 seconds, the particle's age exceeds its lifetime and it is removed. The tables track speed, lifetime, age, position, and state step-by-step. Key moments clarify why particles disappear and how speed affects movement. Quizzes test understanding of position at times and expiration timing.