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.

Practice

1. What does the startLifetime property of a ParticleSystem control in Unity?

easy

A. How long each particle stays alive before disappearing

B. The speed at which particles move when emitted

C. The color of the particles over time

D. The size of the particles when they spawn

Solution

1. Step 1: Understand the meaning of startLifetime

   The startLifetime property sets the duration each particle exists after being emitted.

2. Step 2: Compare with other properties

   Speed controls movement, color controls appearance, size controls scale. Only lifetime controls duration.

3. Final Answer:

   How long each particle stays alive before disappearing -> Option A

4. Quick Check:

   Particle lifetime = duration alive [OK]

Hint: Lifetime means how long particles live before vanishing [OK]

Common Mistakes:

• Confusing lifetime with speed
• Thinking lifetime affects color or size
• Mixing up startLifetime with emission rate

2. Which of the following is the correct way to set the particle speed to 5 in a Unity ParticleSystem script?

easy

A. particleSystem.startSpeed = 5f;

B. particleSystem.speed = 5;

C. particleSystem.setSpeed(5);

D. particleSystem.main.startSpeed = 5f;

Solution

1. Step 1: Recall ParticleSystem API structure

   In Unity, startSpeed is inside the main module, accessed as particleSystem.main.startSpeed.

2. Step 2: Check syntax correctness

   Options A and B are incorrect because startSpeed is not a direct property of ParticleSystem. particleSystem.setSpeed(5); uses a non-existent method.

3. Final Answer:

   particleSystem.main.startSpeed = 5f; -> Option D

4. Quick Check:

   Use main module to set startSpeed [OK]

Hint: Use particleSystem.main.startSpeed to set speed [OK]

Common Mistakes:

• Trying to set startSpeed directly on particleSystem
• Using incorrect method names
• Forgetting to use the main module

3. Consider this Unity C# code snippet:

var ps = GetComponent<ParticleSystem>();
var main = ps.main;
main.startLifetime = 2f;
main.startSpeed = 3f;
Debug.Log(main.startLifetime + ", " + main.startSpeed);

What will be printed in the console?

medium

A. 2.0, 3.0

B. 2f, 3f

C. 2, 3

D. Error: Cannot assign to startLifetime

Solution

1. Step 1: Understand property types and output

   The startLifetime and startSpeed are floats. Assigning 2f and 3f sets them to 2.0 and 3.0 internally.

2. Step 2: Check Debug.Log output format

   Logging floats with string concatenation prints "2, 3" because ToString() on whole number floats omits the decimal.

3. Final Answer:

   2, 3 -> Option C

4. Quick Check:

   Whole number floats print without decimals [OK]

Hint: Whole number floats print without decimal in Debug.Log [OK]

Common Mistakes:

• Expecting decimal output like 2.0, 3.0
• Thinking 'f' suffix prints in output
• Assuming assignment causes error

4. This code tries to set particle lifetime but causes an error:

var ps = GetComponent<ParticleSystem>();
ps.startLifetime = 4f;

What is the main problem?

medium

A. startLifetime must be set via the main module, not directly

B. startLifetime is a read-only property

C. ParticleSystem component is missing

D. startLifetime requires integer value, not float

Solution

1. Step 1: Understand ParticleSystem property access

   In Unity, startLifetime is inside the main module, so it cannot be set directly on ps.

2. Step 2: Identify correct way to set lifetime

   Correct syntax is var main = ps.main; main.startLifetime = 4f;. Direct assignment causes error.

3. Final Answer:

   startLifetime must be set via the main module, not directly -> Option A

4. Quick Check:

   Use main module to set startLifetime [OK]

Hint: Always set startLifetime via particleSystem.main [OK]

Common Mistakes:

• Trying to set startLifetime directly on ParticleSystem
• Assuming startLifetime is read-only
• Confusing missing component with syntax error

5. You want to create a particle effect where particles live for 3 seconds and move faster over time, starting at speed 2 and increasing to 6. Which approach correctly sets this behavior in Unity?

hard

A. Set startLifetime = 3f and set startSpeed = 6f only

B. Set startLifetime = 3f and use velocityOverLifetime module to increase speed from 2 to 6

C. Set startLifetime = 6f and startSpeed = 2f

D. Set startLifetime = 3f and change speed in Update() manually

Solution

1. Step 1: Set particle lifetime correctly

   Particles should live 3 seconds, so startLifetime = 3f is correct.

2. Step 2: Use velocityOverLifetime for speed change

   To increase speed over time from 2 to 6, use the velocityOverLifetime module with a curve, not just startSpeed.

3. Step 3: Evaluate other options

   Set startLifetime = 3f and set startSpeed = 6f only sets constant speed 6, not increasing. Set startLifetime = 6f and startSpeed = 2f has wrong lifetime. Set startLifetime = 3f and change speed in Update() manually is inefficient and unnecessary.

4. Final Answer:

   Set startLifetime = 3f and use velocityOverLifetime module to increase speed from 2 to 6 -> Option B

5. Quick Check:

   Use velocityOverLifetime for speed changes over time [OK]

Hint: Use velocityOverLifetime to change speed during particle life [OK]

Common Mistakes:

• Setting only startSpeed for changing speed over time
• Confusing lifetime and speed values
• Trying to update speed manually every frame