Rigid Body Physics Engine

The physics simulations shown on myPhysicsLab fall into two categories

1. Specialized - designed for one particular mechanism
2. General - uses the 2D rigid body physics engine which can simulate just about any configuration

An example in the "specialized" category is the cart + pendulum. That simulation works well, but it can only ever simulate that particular mechanism. You can vary the length of the pendulum, the mass of the objects, or spring stiffness. But you can't change how the parts are connected, or apply new forces, or collide with another object.

In contrast, the Rigid Body Physics Engine can model a wide variety of mechanisms. For example, here is the cart + pendulum with physics engine. Notice that you can drag the cart or pendulum by pressing the mouse button to activate a spring force -- this is not possible in the specialized version of the simulation.

It is remarkable that the Rigid Body Physics Engine is able to model so many different mechanisms with good accuracy. Here are some more examples:

• Specialized Double Pendulum versus Double Pendulum using Physics Engine. Notice that in the physics engine version you can apply spring forces or collide the free block into the double pendulum.
• Compare Double Pendulum shows side-by-side a specialized double pendulum and a physics engine version. Note that they stay in sync for a long time. But you can interact with the physics engine version, applying a spring force with your mouse.
• RigidBody Roller Coaster shows that the physics engine can handle situations similar to the specialized roller coaster simulations.

About the Physics Engine

The physics engine is composed of three classes that can be used separately (though we almost always use the last one).

1. The RigidBodySim class handles moving rigid bodies by applying forces to them such as springs, gravity, damping. See Rigid Body Forces for the math.
2. The ImpulseSim class extends RigidBodySim, adding collision handling. See Rigid Body Collisions for the math. See CollisionAdvance which manages the collision handling process.
3. The ContactSim class extends ImpulseSim, adding contact forces that allow objects to push against each other. See Rigid Body Contact Forces for the math.

More information about how the rigid body physics engine works is described in several places:

• 2D Physics Engine Overview
• myPhysicsLab Documentation has detailed information about the software.
• Collision Handling Methods discusses various schemes for handling collisions in rigid body simulations.
• Reaction Forces Pendulum is an example of finding contact forces for a specific simple mechanism (the pendulum) instead of using the general (and complex) physics engine.

© Erik Neumann, 2004-2021
revised Dec 19 2023 9:56

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