## Gravity Chaos

Drag or gently toss the spheres below and see what happens. Scroll down for explanations.

### Understanding gravitation through direct manipulation

Apart from mere equation solving, few people can claim to have a deep understanding on how celestial bodies move under the influence of gravitation. Metaphorical explanations such as the "ever-falling moon" are useful but they fail at providing answers to most basic questions. Why these elliptic trajectories? What is really the importance of initial speed? Why and when an orbiting object accelerates and decelerates? Why are three-body orbits so rare?

It is no surprise that gravitational force challenges our basic intuition: we never experience it in our everyday life, other than through its impoverished, homogeneous and unidirectional incarnation. If we were given hyper-dense spheres to play with when we were kids, imagining an object orbiting around another one would certainly be as natural as figuring out a bouncing ball.

Although such toys might actually be difficult to build and manipulate, the Flash application above gives you the immediate opportunity to play with a simulation through a direct manipulation interface.

### Using Gravity Chaos

Gravity Chaos is the Flash application above. Its interface should not need extensive explanation and you can just play with it aimlessly. You can also proceed in the three following steps:
• 1. Click on a medium-sized ball, then drag another ball towards its proximity. Keep dragging and teasing the other ball while trying to avoid collision. This will allow you to feel the interplay of gravitational force and inertia. Realizing that interplay is essential in understanding orbits.
• 2. Try to put a ball into orbit by tossing it near another one. Repeating this process will allow you to experience the influence of initial speed and direction on the resulting trajectory. Playing with different ball sizes will additionnaly give you an idea of the relationship between masses and orbiting speeds (all balls are equally dense).
• 3. Try to make a stable enough orbit with three balls. Making orbits that last several minutes is possible, though it requires a fair number of trials.

A few more tips:

• Once an orbit is set up, it can be traced by pressing the pencil icon.
• The increasing scrolling speed is a normal consequence of tossing new balls in always the same direction. To stop the scrolling, remove all balls from the simulation by tossing them with force outside the screen.
• The simulation unfortunately operates with limited accurracy, particularly at high orbiting speeds. One consequence is that two balls will tend to orbit closer and closer and enventually crash, even if they are not supposed to do so in perfect two-body systems.