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Setting the Rigid Body’s Elasticity and Friction

The values for the Surface Properties on the Rigid Body property page for each collision object are multiplied by the values of the corresponding parameters set for other collision objects. That means that the results of a collision are modulated by the elasticity and friction of all colliding rigid bodies.

To keep this relationship simple, it’s usually best to set the values for only one collision object at a time and test out the results.

Elasticity

Elasticity is the amount of kinetic energy lost from an object when it collides with another object. For example, when a billiard ball hits the table, elasticity influences how much the ball rebounds.

A value of 0 means that the object loses all its kinetic energy upon collision (no bounce), while a value of 1 means that it keeps all its kinetic energy (lots o’ bounce). You can enter values higher than is allowed by the Elasticity slider range, but you cannot enter negative values.

Remember that using values above 1 make the rigid body actually return more energy at each collision. As well, using high values for elasticity can make a simulation less stable.

Friction

Friction is the resistive force acting between rigid bodies that tends to oppose and dampen motion. For example, a billiard ball rolling along a table has a lower friction value than a rubber ball would. Likewise, a billiard ball rolling on a carpet would encounter more friction than if it was rolling on a marble floor.

Friction can be either static or dynamic. In general, static friction is greater than dynamic friction.

Dynamic friction is not available for the ODE dynamics engine.

• Static friction is the force that opposes a rigid body from changing its state from resting to motion. For instance, if you place a ball on an sloped plane, the static friction determines how easily the ball begins its initial slide or roll down the plane. Static friction has little or no effect after an object is moving.

• Dynamic (also known as kinetic) friction is how much a moving rigid body resists movement against another rigid body’s surface. This is the force that tends to slow down a rigid body in motion. This force is generally proportional to any force that has been applied to the rigid body, so you can think of it as the ratio of frictional force to the applied force on the rigid body.

The value range for either type of friction is between 0 and 1, with 0 being no friction (free movement) and 1 being full friction (no movement). You cannot enter negative values or values over 1.

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