WHAT IS IT?
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The purpose of this model is to demonstrate what a gravity field looks like in 2D if one has several gravitating objects.

HOW TO USE IT
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1) Click on the setup button to clear the Graphics window.

2) Push on the setup-masses forever button and click on the Graphics window.  You will see yellow ball-shaped turtles positioned at the points where you click; these turtles  correspond to the gravitating masses that create the vector field.  Use the slider called m to change the masses that you create.

3) Once you have placed all the masses you want click on the setup-field button.  As a result the setup-masses button will be stopped and the vector field will be computed and plotted as a 2D plot in the Graphics screen.

4) Once you have the vector field push on the setup-particles button to create a uniform grid of blue turtles.  The blue turtles will be attracted by the gravity centers.

5) After you have setup-particles push the run button.  You will see how the blue turtles move in the field of gravity.

NOTE: If you want to change the vector field simply press on setup-masses.  You can then move the existing masses with the mouse or place new masses by clicking on the screen.  Push on the setup-field button when you have all the masses in the right places.

EXTENDING THE MODEL
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Try to rewrite the code so that one or two masses move and the vector field changes with time.

Find a way to display how many particles are pulled into each mass.