Page 110 - ExtendSim User Guide
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Areas of Application
Custom blocks
This model provides lots of room to experiment with the physics of objects:
• Smallchangesininitialpositionsorvelocitiesoftheobjectscancausebigchangesinmodel behavior.
• You can either add or remove an object, resulting in a four-body or two-body problem. In each case, make sure that each object has one, and only one, connection to each of the other objects on the worksheet; otherwise the math will get confused.
• Oneinterestingproblem,whichdemonstratesthecomplexitiesofthephysicsinvolved,isto try to modify the parameters of the objects in the three body problem to produce a system of stable orbits like the sun, earth and moon. (Before you get too frustrated trying to set this up, please note that no one has yet been able to get those parameters right.)
• Notice that even light objects will have an effect on the location and velocity of heavy objects. To replicate a relationship like that of the earth and the sun, increase the mass and density of one of the objects. No matter how much you do this, however, the heavy objects will be affected by the position of the light object.
Fish Pond
As opposed to the Predator/Prey model shown earlier that uses blocks from the Value library, the Fish Pond model uses a custom-built Fish block to represent both predator and prey.
☞ The Fish Pond model is located in the folder \Examples\Continuous\Custom Block Models. It uses a Plotter I/O block from the Plotter library and the Fish block from the Custom Blocks library. The Fish block was specifically created for this model.
About the model
This is a small pond with two types of fish: a pred- ator and a prey. The goal is to balance the pond by reducing the number of predators. The model uses a Fish block from the Custom Blocks library and a Plotter I/O from the Plotter library.
This two creature ecosystem shows how a single
block design can model many types of creatures.
By entering different parameters and connecting
the blocks differently, a more complex ecosystem
can be created. The Fish block is used to model two different species: a carrion-eating fish and a natural predator. At the left of the model, carrion-eating fish eat other fish that have died in the pond. The block to the right (a piranha) eats the carrion-eaters. Run the simulation and look at the graph; the Piranha periodically decimate the carrion-eating fish population.
Each Fish block added to the model represents another species. You can add another predator to the right of the second block and, based on the default parameters, it will control the piranha population. Try adding a controlling predator to reduce the Piranha population. To do this:
Add a Fish block and position it to the right of the “Piranha” block.
Connect the “Pot. food in” connector on the new block to the “Pot. food out” connector on the block to the left.
Fish Pond model

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