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Spanish # Particle System Basic If you download the NetLogo application, this model is included. You can also Try running it in NetLogo Web

## 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). https://dl.acm.org/doi/10.1145/357318.357320

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

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

Thanks to Daniel Kornhauser for his work on this model.

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