Parameter Examples

Motion type & Layers

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There are three different Motion types for a body, defining how the body moves :

Motion type	Description
Static	The body cannot move, regardless of forces applied
Kinematic	The body can move with Velocity but not with Forces
Dynamic	The body can move with Forces and with Velocity

There are three different Layer types for a body, defining how the body collides with other bodies :

Layer type	Description
Static	Body is not meant to move, only react. It will physically collide with other bodies.
Moving	Body is meant to move and react. It will physically collide with other bodies.
Sensor	Body will send Trigger Events on output pin when it collides, but not physically collide with other bodies

*Note: Kinematic objects are useful for scenery or background objects that move around with constant speed but with which other objects can still interact, such as moving platforms in video-games where the player can jump on.

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Example

Example	Description
	Blue Dynamic/Moving Vx = 2.0 Red Dynamic/Moving Vx = -2.0 Velocity (V) is a constant speed so balls are constantly bouncing off each other with equal force
	Blue Dynamic/Moving Fx = 2.0 Red Dynamic/Moving Fx = -2.0 Forces (F) cancel each other out after collision and balls become immobile
	Blue Kinematic/Moving Vx = 2.0 Red Dynamic/Moving Vx = -2.0 Kinematic movement is stronger than Dynamic Note that Kinematic movement does not make Blue’s movement roll but rather translate
	Blue Dynamic/Sensor Vx = 2.0 Red Dynamic/Moving Vx = -2.0 Sensor does not physically collide. Collision is still sent to node’s output pin

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Weight

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Weight represents the mass of the object in kilograms and is used in various calculations, such as:

• Inertia: The resistance to changes in motion.
• Momentum: The product of the body’s mass and velocity.
• Force: The push or pull exerted on the object.
• Collision detection and response: The weight property is used to determine the outcome of collisions between objects.

*Note: the weight property is not the same as the density of the object, which is a measure of the mass per unit volume. In this Physics Engine, the density property is not defined.

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Example

Same weight	Different weight
Blue weight: 10 Red weight: 10	Blue weight: 10 Red weight: 100

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Motion Type

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Each body has a motion quality setting that can be set to the following:

• Discrete: (default) A test for collision detection will be run at the beginning of each simulation step and if no collision is found the body will move according to its velocity.
  This works fine in most cases for objects that are relatively big and not moving too fast.

• Linear Cast: This mode is intended for smaller and faster objects which can accidentally “tunnel” through other objects because their velocity is such that the collision is not detected in between two simulation steps. With this mode active, additional computing is done to check after a simulation step for any collisions that may have been missed. If such is the case, the body is placed back to the point where the collision occured.
  This mode may also have the disadvantage of showing movements that are non-linear in speed and should be reserved for whenever fast moving and or smaller objects do not collide properly since more computing is involved.

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Restitution

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Restitution describes the elasticity of a collision between two bodies (or with the floor) and will adjust the velocity of the bodies involved in the collision. The restitution coefficient is usually a value between 0 and 1, where:

• 0 represents a completely inelastic collision, where the bodies stick together and do not bounce off each other.
• 1 represents a completely elastic collision, where the bodies bounce off each other with no loss of energy.

The restitution coefficient can be set for each body individually and in case of collision the greatest of the two values will be used in computing the reaction to apply on the momentum of each body.

*Note: Setting a restitution value above 1 on your bodies will set them in a potentially volatile and unstable state where they gain additional energy with each collision. This can be a desired situation but is very hard to keep stable.

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Example

Same restitution	Different restitution
Blue restitution: 0.5 Red restitution: 0.5	Blue restitution: 0.2 Red restitution: 0.8

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Analysis

With the same value of restitution, the balls bounce back from each other slightly and with the same energy.

When Red has a higher restitution than Blue: Red is pushed back slightly more but Blue is pushed back significantly more.

Note also that as Red’s restitution value approaches 1 it becomes slightly jittery and bounces off the floor.

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Friction

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Red has Friction set to 50 and Blue has Friction set to 1.0.
Both have a force applied of Vx = 1.0
Friction has close to no effect on spheres as the surface of contact with the floor is extremely small and close to a point.

Stone box (closest to camera) has Friction set to 1.0 and Brick box (farthest from camera) has Friction set to 0.20 (default)
On boxes sliding down a surface, differences in friction and its impact on acceleration and speed is far more visible as the surface in contact is much greater
Note that in this example Angular Speed Limit was set to its minimum value of 0.10 to improve readability of motion.

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Linear Speed Limit

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Red has Linear Speed Limit set to 1.0. Blue is set to 500 (default).
Both have a force applied of Fx = 1.0

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Angular Speed Limit

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Red has Angluar Speed Limit set to 10.0. Blue is set to 47.12 (default).
Both have a rotation force applied of TAy = 2.0

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