The SALES model (Figure IV-1) shows the operation of sales and money in a simple system. In the model, products (Q) accumulate from a process that produces J units per time. The expression for the depreciation of the products is K1*Q.

These products are sold to the market (K3*Q), and money is received in exchange. According to the economic principle of supply and demand, the price (P) goes up if the supply of products is less and down if the supply is greater. The expression shows that the price is inverse to the supply of available Q is K4/Q. People pay more when supply is scarce.

No matter how many products accumulate, the money earning of the business stay the same. Prices vary for the consumers but income to the producer is steady. The money received is stored in a bank account (M), and then paid out in proportion to the amount in the account (K3*M).

When the program is run, products accumulate and level off, prices go down, and money increases and levels. The small rectangle on the Q tank and the line from it to the Price box indicate a sensor: it is recording Q, but not using it.

Examples of SALES Models

A citrus farmer produces oranges, stores them and sells them. The price he receives fluctuates according to the supply. In this model the more money the farmer earns, the more he spends. Another example is a craftsman who produces wooden chairs at a steady rate. If he builds up his inventory, the price per chair will go down.

"What if" Experimental Problems

1. If your uncle gives you 150 chairs to start your business (instead of the 10 in the program), how much difference will it make to the money in your bank account? Run the program; then change statement to Q=100, and run the program again. Does the second graph agree with your answer?
   


2. What happens to the business if there are several very wet years and the depreciation of the chairs from mildew increases from 0.5% to 0.7%? What would you change K1 to?
   


3. What is the effect of doubling production on the accumulation of money (M) and on the amount of product per dollar the consumer can buy? What is the difference between the effect on the business and the effect on consumers?

COMPUTER MINIMODELS AND SIMULATION EXERCISES FOR SCIENCE AND SOCIAL STUDIES

Howard T. Odum* and Elisabeth C. Odum+
* Dept. of Environmental Engineering Sciences, UF
+ Santa Fe Community College, Gainesville

Center for Environmental Policy, 424 Black Hall
University of Florida, Gainesville, FL, 32611
Copyright 1994

Autorização concedida gentilmente pelos autores para publicação na Internet
Laboratório de Engenharia Ecológica e Informática Aplicada - LEIA - Unicamp
Enrique Ortega
Mileine Furlanetti de Lima Zanghetin
Campinas, SP, 20 de julho de 2007