WHAT HAPPENS IF THE GRAVITATION IS SUDDENLY SWITCHED OFF? Mini tutorial - step by step

The aim of this mini tutorial is to introduce the user, by a small experiment, in making own creations in the Particle World.

This small experiment will try to answer the question: What will happen with an object, for instance a ball, placed on the bottom of a (air free) container when the gravitation suddenly is switched off?

Is any of the answers below correct?

• The ball will stay on the bottom of the container, unaffected by the fact that the gravitation is switched off.
• The ball will move up to the center of the container and stay there.
• The ball will bounce up and down between the walls of the container.
• ...

Since all real objects are elastic, to some extent, the ball will be made of a set of rigid particles connected by springs. Thereafter, the ball will be placed on the bottom of a container with rigid walls, and finally the gravitation will be switched off.

### Preparation

• Start the Particle World Java applet.
• Clean the world from other creations by choosing [File/New]
• It is often very convenient to perform the constructions with some air resistance and without gravitation. Fill the container with air by choosing [View/Environment editor], enter the value 1 as Air resistance and press the Apply button. Later, the gravitation will be switched on and the air resistance removed.

### Create a ball

• (Left) Click in the Particle World window to activate it.
• Keep the A key pressed down while making twelve (left) clicks at different (arbitrary) locations in the Partice World. This will add twelve new particles to the world.
• Select all the twelve particles by pressing the (left) mouse button on the background and drag to enclose all the particles with the selection box. (Release the mouse button when the all the particles are selected)
• Add springs between all particles by choosing [Add/Add Springs] (or simply press Ctrl-S). The situation should now look similar to the figure below.

• Again, select all the particles and all the springs with the selection box as previously.
• Now change the length of all the springs to 40 by choosing [View/Spring editor], enter the value 40 as Relaxed length and press the Apply button. Note: The particles are still selected and cannot move!
• Now click on the background of the Particle World to unselect all particles and make them mobile.
• Wait for the particles to assume stationary regular positions like the figure below.

• Pause the simulation by choosing [Simulation/Pause toggle] (or simply press P once), in order to avoid unintentional movement of the particles in the next coming operations.
• Again select all the particles and springs, and delete all springs by choosing [Edit/Delete springs] (or simply press Ctrl-D).

• Click on the background to unselect all particles and then select the particles inside the "circle" by keeping the Ctrl key down when clicking on them.
• Delete the selected particles by choosing [Edit/Delete] (or simply press Delete). The situation should now look similar to the figure below.

• Select allt the particles and add new springs between them by  choosing [Add/Add Springs] (or simply press Ctrl-S).

• Select all the springs by the selection box and increase the stiffness and damping of the springs by choosing [View/Spring editor], enter the value 50 as the Spring constant and the value 5 as the Damping and press the Apply button.
• Click on the background of the Particle World to unselect all the particles. The ball is finished!

### The last things...

• Now switch on the gravitation by choosing [View/Environment editor], enter the value 10 as the Gravity Field Y and press the Apply button.
• Start the simulation again by choosing [Simulation/Pause toggle] (or simply press P) The ball should now fall down to the bottom of the container.
• To make the current poisiton at the bottom of the container as the inital position of the ball, select all the particles and choose [Edit/Redefine inital positions] (or simply press Ctrl-I).
• Remove the air by giving the Air restistance in the Environment editor the value 0 and press the Apply button.

### The experiment

• Make sure all particles are unselected by clicking on the background of the Particle World.
• Switch off the gravitation by choosing [View/Environment editor], enter the value 0 as the Gravity Field Y and press the Apply button.

•

What happend?! Explain!

To restart the simulation, choose [Simulation/Initialize] (of simply press I).

### Save your experiment

Perhaps, it is useful to save the experiment described above, for future use or to share it with somebody else. A textfile describing all the parameters can be extracted from the applet by choosing  [File/Export]. A window will appear with the text, this text has to be copied (press Ctrl-C) to some text editor (e.g. Notepad) and saved on a local disk.

To run the experiment another time, simply choose [File/Import] and paste (press Ctrl-V) the text from the text editor into the window and press the Import button.