Damping and Friction

Properties and methods that enable physical objects to resist movement.

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To slow down Physics Bodies, simulate atmospheric drag, or add resistance to a hinge, there are two properties available to Physics Bodies and Physics Constraints, Linear Damping and Angular Damping. Linear Damping controls how much the Physics Body or Constraint resists translation, and Angular Damping controls how much they resist rotating.

Physics Bodies

Every Physics Body has properties for Linear Damping and Angular Damping. These properties are mainly used to simulate drag, and even a value of 1.0 affects the Physics Body.

Phys_LinearandAngularDampProps.png

For reference, in normal gravity (9.8m/s^2), a Linear Damping value of 30 is enough to stop any single body Actor from falling under the initial pull of gravity at the start of the simulation. When more than one Physics Body is interacting (through the use of Physics Constraints or if a collision is occurring), the amount of damping required to stop the Physics Body goes up. 100 is the minimum Linear Damping value to stop a Physics Body that has had force applied to it. Finally, with no Angular Damping, a Physics Body will continue to rotate until it is acted upon by an outside force. Even small values can reduce rotation rapidly, while a value of 100 will almost immediately stop any angular motion caused by an outside source.

Constraints

The Damping for Physics Constraints work a bit differently than with Physics Bodies because they only work on Limited Motions and when a motion attempts to go beyond its limit. Use the following options to set constrains.

Phys_ConstraintProps.png

Property

Description

Lock Position

Locks movement along the X, Y or Z axis.

Lock Rotation

Locks rotation along the X, Y, or Z axis.

Mode

This option is only available with PhysX.

Locks physical movement along a specified axis. This option's default state is set to inherit the degrees of freedom from the Project Settings.

Modes

The following options are only available with PhysX.

In addition to the Default state, which inherits the degrees of freedom from the project settings, the following options are available.

Mode

Description

Six DOF

Enables the user to specify along which axis to freeze rotation and movement.

YZPlane

Enables 2D movement along the Y-Z plane.

XZPlane

Enables 2D movement along the X-Z plane.

XYPlane

Enables 2D movement along the X-Y plane.

Custom Plane

Enables 2D movement along the plane of a given normal. After selecting this option, enter the plane normal's X-Y-Z coordinates as shown here. Phys_CustomPlaneNormal.png

None

Explicitly disables constraints.

Degrees of Freedom

By default, a Physics Body's Constraints Mode is set to inherit the project's degrees of freedom. To change the Default Degrees Of Freedom, open Project Properties > Engine - Physics > Simulation, and select one of the following options.

Option

Description

Full 3D

Enables full 3D movement and rotation.

YZPlane

Enables 2D movement along the Y-Z plane.

XZPlane

Enables 2D movement along the X-Z plane.

XYPlane

Enables 2D movement along the X-Y plane.

Setting the degrees of freedom in the project's default physics settings is useful for constraining all of the object's in the game world — for example, when designing a 3D environment for a 2D game.

Limits

The two Angular Swing Motions share Damping values, the Angular Twist receives its own Damping values, and all the Linear axes use the same Damping values. While they may have separate properties, the properties all have the same effect on the Motions they relate to.

Property

Description

Limit Stiffness

When a limit is exceeded, this value affects how harshly the constraint attempts to stop the offending motion. The default of 50 will barely affect the motion, a value of 5000 will appear to bounce back into limit, and 50000 will completely deflect the motion.

Limit Damping

Like the Damping on Physics Bodies, this defines how much the motion should be slowed. Unlike Damping on Physics Bodies, the damping affect occurs only when the Motion Limit is exceeded, draining momentum until the Physics Body stops.

The two values work together to create different effects. A high Stiffness and low Damping will look like a hard limit, while having them both high will appear to be move through tar, coming to a stop gently but quickly. A good deal of experimentation is necessary to find the right values for your specific needs.

Additional Information

For more information about Physics Constraints, read the following.

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