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In the above diagram you see a 3d Mesh ( usually coming from 3rd party applications like Maya / 3dsmax /blender etc ) in blue and rigidody created using a physics engine like Ageia / Havok /Bullet etc in gray.

The following points hold true for a rigidbody:

• A rigidbody is something close to an outer covering/shell that represent the 3d mesh or encapsulates it.( as shown in diagram.)
• Given a really complex 3d mesh , rigidbody are very rough duplication of the 3d model/mesh. This rough outer shell/cover helps speed up mathematical calculation... HOW ? Just remember that lesser the vertices lesser the contact points on the surface hence lesser the number of calculations when 2 or more objects collide in your game.
• A rigidBody helps the physics engine set some rules and guidelines which it in turn uses later on to rapidly solve mathematical crazy stuff.
• A rigidBody can give you control and define which objects collide and which once don't ( suppose you don't enable collision between a game character and wall , the game character can just walk through the wall ;) )
• There are lot more things you can do.. but the above points pretty much introduce you to the idea of why they are needed ...

Which Physics engine would my game engine be using ?

Next: Types of rigidBody

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