Force Fields

Motion is affected not by one force but by a variety of forces such as gravity, wind and drag. Using fields, you can simulate these forces of nature to increase the realism of your dynamics animation.

If dynamics motion is to look real, it will need to take into account forces like gravity, drag and wind. It is precisely these forces that you can simulate using fields. Your dynamics animation will usually include at least one field. A field exerts a force on objects that are inside the field. In the case of a free-moving object, this force will cause the object to accelerate or decelerate depending on the direction of the field and the direction of the object.

Fields in Dynamics are easy to use, but a few basic concepts — described in the following — will help to explain when and how you should use fields.

For illustration purposes, a field will be represented in this manual as a number of lines of force that point in the direction of the force. Note that the lines of force will not be shown in the CINEMA 4D viewport.

A field can have one of a variety of shapes, such as cubic or spherical. The shape defines the volume inside which the field will exert a force on objects, i.e. the shape marks the boundaries of the field. It is also possible to give a field an unlimited shape, in which case the field will exert its force throughout 3D space without boundaries.

A force field in the viewport. The boundaries of a field are shown as a green outline

A moving object is usually subjected to not just one but a number of forces at any given time. Hence Dynamics enables you to place as many force fields as you need over the same volume of 3D space. The forces exerted on an object by overlapping fields will be added together under the laws of physics to ensure correct motion.

Two forces, F1 and F2, are acting on a point (top left). These two forces will be added under the laws of physics to produce the correct resultant force, F3.

The diagram below demonstrates how a variety of forces can affect even a simple motion.

Lets take a look at an example using a falling sphere:

In the left-hand picture the sphere is initially being held in place by a peg. F1 is the force due to gravity. Not shown is the holding force that prevents the sphere from moving. This holding force is equal and opposite to F1, hence the forces are cancelling each other out and there is no resultant motion.

In the middle picture the peg has been removed and with it the holding force. As a result, the sphere is accelerating towards the floor due to gravity. Now that the sphere is moving, a new force, drag (F2), is being applied. Drag always exerts its force in the opposite direction to the object’s motion, hence F2 acts in the opposite direction to F1. Drag is not a constant force but instead increases with increasing fall velocity. At this stage, F2 is less than F1, causing the sphere to accelerate.

In the right-hand picture the drag force is now equal and opposite to the gravity force. The forces are cancelling each other out, so the object is no longer accelerating but instead falling with constant velocity (v3).

The falling sphere example demonstrates that a motion is not always as straightforward as it may first appear. But by using fields, you can simulate these forces and let Dynamics handle the interaction and computation. All this and more can be simulated with the three types of Dynamics fields available: gravity, wind and drag.

Gravity

The gravity field created forces that affect the mass, not the shape, of objects.

Wind

Wind moves across and around an object’s shape and any surface facing it.

Drag

The drag effects moving objects in the direction opposite to the direction in which they are moving.