Creates a motion field for the selected object.

You can add a Motion field to an object moving through a fluid. The effect of a Motion field on a fluid is similar to how a boat’s propeller forms eddies, vortices, and a slipstream in the water. For example, for the animation of a boat pulling a water-skier through a fluid, you can apply a Motion field to the boat and the water-skies so that they appear to push through the water while they are in motion.

A Motion field is the same as a Drag field (Dynamics menu set > Fields > Drag), with the exception that they are parented to an object, and they have preset volume bounds, Inherit Velocity, and Magnitude. For more information on Drag fields, see Fields > Drag in the Dynamics guide.

The characteristics of a Drag field are similar to an Air field with Inherit Velocity, although the Drag field has more control over how transforming the field imparts motion to its fluids. An Air field does a simple push in the direction of the field motion and does not take rotation or scaling of the field into account. A Drag field does take this into account. For example, with Inherit Velocity on the Drag field, you can create an explosion effect by keying the scale or a twister effect by keying the rotation. However, unlike an Air field, the Inherit Velocity on a Drag field will not work when a fluid is used as the source of the field.

Another key difference between Drag and Air fields is that Air fields always have a primary force or wind that is added to the Inherit Velocity effect. Drag fields only behave like a block of air (or water or lead depending on the magnitude), and Inherit Velocity allows this block of air to be transformed. Thus a fluid feels the force of a Drag field until it is moving at the same rate as the field itself. This creates a very controllable dynamic simulation.