A rigid body is indeed a hard object but it's a little more than that, as deformations to the object are not possible when set as a rigid body. The rigid body object will always maintain its shape without a mushy, gelatinous, or spring effect.
Rigid body prop examples would be a bowling ball, wall, floor, or any object that is solid and would not oscillate when a collision occurs. If it shakes or quivers, then it's a soft body, as will be discussed in due time, otherwise, it's a rigid body as a rule of thumb.
The state of the object is crucial in defining how the object will react to the simulation. What state you choose depends on what you need that particular object to do. Let's take a moment to discuss the various states and their uses. Once again Jason Lin at Reallusion was invaluable in getting this information for us to use:
Dynamic |
For use with objects whose intent is passive, such as free fall or collisions. |
Kinematic |
Use with rigid body groups that you wish to key frame or puppet (animate) and for collision props linked to an animated character or object. |
Static |
Objects that do not allow penetration or reaction from another physics driven object, such as a floor or wall that will provide containment for the simulation instead of the objects passing through. Static objects are not moved by the collision effect. |
Frozen |
Objects that need to be held in place until dislodged or touched by another physics driven object, such as a group of dominoes, a stack of boxes, barrels, or cans. |
Mass |
Weight and volume. The more mass something has, the more inertia and gravity it will experience. |
Friction |
A damping effect that can slow down or control spin in some cases. |
Damping |
Helps to reduce the effect of oscillations or jittery movement. |
Elasticity |
Bounce or lack of bounce. |
Bound Type | |
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Box |
Surrounds the object with an invisible collision box and the default settings as of this writing. |
Sphere |
Surrounds the object with an invisible collision sphere. |
Cylinder |
Surrounds the object with an invisible collision cylinder. |
Capsule |
Surrounds the object with an invisible collision capsule. |
Convex Hull |
Surrounds the object with an invisible collision hull that would be like wrapping it with a plastic wrap as it fits to the object in that manner. |
Bounding Mesh |
Surrounds the object with an invisible collision Bounding Mesh, usually a simple low poly box. |
Self Mesh |
Surrounds the object with an invisible collision mesh based on the objects form. |
Bound Axis | |
Used with cylinder and capsule bounding type to set the orientation axis. |
We need to visualize what we are doing when working with bounding types as they have a dramatic effect on a simulation.