Central Thrusting Force: This tool subjects a body element to a force that causes the body to move along a cyclic path. The force is either constant or impulsive. A constant force constrains the body to a circular path, while an impulsive force constrains the body to move along a path that forms a polygon.

To place the central force element, first place a body element. Then click on the Central Force Tool button and click on the body element. Note: When you place the Central Thrusting Force Element, the properties of the body revert to the default values: a blue body of mass 3 kg labeled Body N, where N equals 1 or greater. The body has a rightward speed of 50. No forces act on the body except the central thrusting force. You can change the parameters of the body and its speed in their Properties Boxes.

By default, the shape of the path followed by a body subject to a central force is a circle. In the Properties Box (open by double-clicking on the central green dot), you can change the shape to a polygon. The default number of polygon's sides is 4, meaning that the body's path is a square, but you can set the number of sides between 2 and 12 (two sides corresponds to a body moving back and forth along a horizontal line segment).

The figure below shows bodies moving on (from left to right) a circular path, a square path, an octogonal path, and a linear path.

The Radius is either the radius of the circular path, the distance from the center of the polygonal path to the midpoint of one of the polygon's sides, or (when number of sides = 2) one-half the length of the line segment. You can also choose the direction of the body's motion around the path--clockwise or counterclockwise (default).

By default, the central force element functions in automatic mode. In this mode, the body's motion is constrained to the path. For a circular path, the central force always points to the center. For a polygonal path, the central force turns on for a time specified by the duration property as the body approaches a corner (or vertex) of the polygon (see the figure below). You can turn off the central force by tapping the space bar during Run mode.

Automatic mode is the only mode available for circular paths. For polygonal paths, you can also choose manual mode. In manual mode, activate the central thrusting force manually by tapping the space bar when the body gets close to the polygon's vertices.

Caution: Avoid running circular central forces and polygonal central forces in manual mode simultaneously. If you do so, then you will turn off the circular forces when you activate the polygonal central thrusting forces.