# Particle System Basic ### 2 collaborators Uri Wilensky (Author) Daniel Kornhauser (Author)

### Tags

computer science

Tagged by Reuven M. Lerner over 9 years ago

particle system

Tagged by Reuven M. Lerner over 9 years ago

Model group CCL | Visible to everyone | Changeable by group members (CCL)
Model was written in NetLogo 5.0.4 • Viewed 455 times • Downloaded 38 times • Run 2 times Download this modelEmbed this model

## WHAT IS IT?

Particle systems are used in computer graphics to simulate the appearance of physical phenomena that can be modeled as a collection of particles. For example, some typical particle systems include: waterfalls, fire, smoke, explosions, snow, and meteors.

This example demonstrates how to write a very simple particle system only. For example, particles are only created at setup time. See the other particle system models for elaborations on the basic particle system idea.

## HOW IT WORKS

At each iteration of the GO routine, tiny forces steer the particle through its trajectory. Particles have a velocity in the x and y axes, a step and a force accumulator. This model uses a procedure to compute the forces (COMPUTE-FORCES) and another one to apply the forces (APPLY-FORCES). Combined, these procedures continuously move the particles over time.

Below are the steps for moving each particle:

1.- Initialization
First, the force accumulators are cleared of the previously calculated forces.

2.- Force Calculation
Force calculation is trivial for this particular particle system since the sole force of the model, gravity, is represented by a constant negative number. However, force calculation in general can be harder when more complicated forces, such as springs, are involved.

3.- Force Summation
After all of the individual forces are computed, the APPLY-FORCES routine sums all of them and calculates the resulting velocity of the particle.

4.- Displacement
Finally, a new position is calculated by multiplying the velocity by STEP-SIZE and adding the displacement to the current particle location. STEP-SIZE represents the small amount of time during which the forces are applied.

Notice that in this model the particles die when they reach the world boundaries.

## HOW TO USE IT

To observe only one particle at a time:

1.- Change the PARTICLES-NUMBER and STEP-SIZE.
2.- Press SETUP
3.- Press GO (Observe how the turtles move through the world.)
5.- You can optionally change the GRAVITY-CONSTANT and observe how the behavior changes.

Note that you can change the GRAVITY-CONSTANT and the STEP-SIZE while the particle is moving. For example, if the user wants the particles to fly higher he can decrease the gravity, but if he wishes the particles to stay lower he can increase the gravity pull.

## THINGS TO NOTICE

The more particles you create, the slower the model runs.

The greater the step size, the faster the model runs. (What happens if you make the step size too large?)

## THINGS TO TRY

Change the GRAVITY-CONSTANT slider while the particle is moving. Note how it stays floating or falls quickly depending when and how you adjust the gravity constant slider.

## RELATED MODELS

Particle System Fountain
Particle System Waterfall
Particle System Flame

## CREDITS AND REFERENCES

Particle Systems by Allen Martin
http://web.cs.wpi.edu/~matt/courses/cs563/talks/psys.html

William T. Reeves, "Particle Systems - A Technique for Modeling a Class of Fuzzy Objects", Computer Graphics 17:3 pp. 359-376, 1983 (SIGGRAPH 83).
http://portal.acm.org/citation.cfm?id=357320

Physically based modeling Online SIGGRAPH 2001 Course Notes
http://www.pixar.com/companyinfo/research/pbm2001/

Particle Systems on Wikipedia
http://en.wikipedia.org/wiki/Particle_system

Thanks to Daniel Kornhauser for his work on this model.

## HOW TO CITE

If you mention this model in a publication, we ask that you include these citations for the model itself and for the NetLogo software:

• Kornhauser, D. and Wilensky, U. (2007). NetLogo Particle System Basic model. http://ccl.northwestern.edu/netlogo/models/ParticleSystemBasic. Center for Connected Learning and Computer-Based Modeling, Northwestern Institute on Complex Systems, Northwestern University, Evanston, IL.
• Wilensky, U. (1999). NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern Institute on Complex Systems, Northwestern University, Evanston, IL. Click to Run Model

```turtles-own
[
velocity-x             ; particle velocity in the x axis
velocity-y             ; particle velocity in the y axis
force-accumulator-x    ; force exerted in the x axis
force-accumulator-y    ; force exerted in the y axis
]

to setup
clear-all
ask patch 0 (max-pycor / 2) [
sprout num-particles [
set velocity-x 10 - (random-float 20) ; initial x velocity
set velocity-y 10 - (random-float 20) ; initial y velocity
pen-down
]
]
reset-ticks
end

to go
if not any? turtles [ stop ]
compute-forces ; calculate the forces and add them to the accumulator
apply-forces   ; calculate the new location and speed by multiplying the
; forces by the step-size
display
end

; calculate and sum all the forces exerted on the particles

to compute-forces
[
; clear force accumulators
set force-accumulator-x 0
set force-accumulator-y 0
; calculate forces
apply-gravity
]
end

; updates the accumulator with the gravity force

to apply-gravity  ;; turtle procedure
set force-accumulator-y force-accumulator-y - gravity-constant
end

; calculates the position of the particles at each step

to apply-forces
[
; calculate the new velocity of the particle
set velocity-x velocity-x + (force-accumulator-x * step-size)
set velocity-y velocity-y + (force-accumulator-y * step-size)
; calculate the displacement of the particle
let step-x velocity-x * step-size
let step-y velocity-y * step-size
;; if the turtle tries to leave the world let it die
if patch-at step-x step-y = nobody [ die ]
;; if the turtle does not go out of bounds
;; add the displacement to the current position
let new-x xcor + step-x
let new-y ycor + step-y
facexy new-x new-y
setxy new-x new-y
]
end

```

There are 10 versions of this model.

Uri Wilensky over 9 years ago Updated to NetLogo 5.0.4 Download this version
Uri Wilensky over 10 years ago Updated version tag Download this version
Uri Wilensky over 10 years ago Updated to version from NetLogo 5.0.3 distribution Download this version
Uri Wilensky over 12 years ago Updated from NetLogo 4.1 Download this version
Uri Wilensky over 12 years ago Updated from NetLogo 4.1 Download this version
Uri Wilensky over 12 years ago Updated from NetLogo 4.1 Download this version
Uri Wilensky over 12 years ago Updated from NetLogo 4.1 Download this version
Uri Wilensky over 12 years ago Model from NetLogo distribution Download this version
Uri Wilensky over 12 years ago Particle System Basic Download this version

## Attached files

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