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## 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 (COMPUTEFORCES) and another one to apply the forces (APPLYFORCES). Combined, these procedures continuously move the particles over time.
Below are the steps for moving each particle:
1. Initialization
2. Force Calculation
3. Force Summation
4. Displacement
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 PARTICLESNUMBER and STEPSIZE.
Note that you can change the GRAVITYCONSTANT and the STEPSIZE 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 GRAVITYCONSTANT 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
## CREDITS AND REFERENCES
Particle Systems
Particle Systems by Allen Martin
William T. Reeves, "Particle Systems  A Technique for Modeling a Class of Fuzzy Objects", Computer Graphics 17:3 pp. 359376, 1983 (SIGGRAPH 83).
Physically based modeling Online SIGGRAPH 2001 Course Notes
Particle Systems on Wikipedia
Thanks to Daniel Kornhauser for his work on this model.
## HOW TO CITE
If you mention this model or the NetLogo software in a publication, we ask that you include the citations below.
For the model itself:
* Kornhauser, D. and Wilensky, U. (2007). NetLogo Particle System Basic model. http://ccl.northwestern.edu/netlogo/models/ParticleSystemBasic. Center for Connected Learning and ComputerBased Modeling, Northwestern University, Evanston, IL.
Please cite the NetLogo software as:
* Wilensky, U. (1999). NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and ComputerBased Modeling, Northwestern University, Evanston, IL.
## COPYRIGHT AND LICENSE
Copyright 2007 Uri Wilensky.
![CC BYNCSA 3.0](http://ccl.northwestern.edu/images/creativecommons/byncsa.png)
This work is licensed under the Creative Commons AttributionNonCommercialShareAlike 3.0 License. To view a copy of this license, visit https://creativecommons.org/licenses/byncsa/3.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.
Commercial licenses are also available. To inquire about commercial licenses, please contact Uri Wilensky at uri@northwestern.edu.
<! 2007 Cite: Kornhauser, D. > 
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