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You can use formulas to control position or velocity of a body. By attaching the Anchor tool and using formula language, you can control the motion of a body independently from the rest of the simulation model.

Please see “10.6. Specifying Body Path by Position” and “10.7. Specifying Body Path by Velocity” for more information.

3.3. Body Appearance

The Appearance window controls the appearance of an object.

Figure 3-13

Appearance window for a body

To display the Appearance window:

1. Select the body whose appearance you wish to change. 2. Choose Appearance from the Window menu.

The Appearance window for that object appears (Figure 3-13).

Using the Selection Pop-up

Menu Alternately, if the Appearance window is already visible, you can simply select the object whose appearance you would like to modify from the selection pop-up menu at the top of the utility window (Figure 3-13). The menu will show the list of ID numbers and the names of all objects in the document. You can change and assign meaningful names by typing into the name field (located directly below the selection pop-up) to help searching through the list.

Changing Color and Fill

Pattern To change the color and fill pattern for an object’s interior and outline, click on the pop-up menus next to Fill and Frame in the body Appearance window.

Fill The center of a body can be transparent, a solid color, or a pattern of any two colors, including black and white.

Click on the two pop-up menus next to Fill in the Appearance window to change the fill color and pattern.

Frame You can also change both the width and the color of the outline of an object. The fill pattern may not be apparent for thin outlines.

Track Center of Mass, Track

Connect, Track Outline These three options determine which parts of a body will be traced on the screen when tracking is turned on in the World menu. Track Center of Mass will leave a point at the body's center. You can turn on Show Center of Mass to render the track more pronounced.

Track Connect will leave connecting lines between the body's center of mass at subsequent positions.

Track Outline will leave a trace of the body's outline.

Figure 3-14

Example of Track Center of Mass, Track Connect, and Track Outline

Show You can hide a body by clicking once in the field titled Show to remove the checkmark. Hidden bodies behave exactly like displayed bodies. All bodies are initially shown.

Show Name The name of a body is automatically set to its type (circle, rectangle, square, or polygon). Remember that curved body objects are named polygons by default. You can change this name by typing directly into the name field of the Appearance window.

Track of Center of Mass

Track of Outline

Choose Show Name to display the name of the body. Figure 3-15 shows a rectangle with its name displayed.

Figure 3-15

Body with name displayed

Show Center of Mass Select the Show Center of Mass box to display a body's center of mass. The center of mass indicator appears as a black-and-white disk. If you have Track Outline turned on, the indicator will also leave its track.

Figure 3-16

Center of mass symbol

Show Charge If Show Charge is selected, then positively charged bodies will have large positive (+) signs in them, while negatively charged bodies will have large negative (–) signs in them.

Show Circle Orientation Initially, each circle has a line fixed in it that passes through its geometric center and is parallel to the World frame’s x-axis. The orientation of a circle is defined to be the angle between this line and the x-axis of the World frame. Select Circle Orientation to display the line that indicates the current orientation of the circle.

To close the Appearance window, click its close box.

3.4. Body Geometry

Working Model 2D allows you to easily modify geometric parameters of bodies such as:

• Width and height of a rectangle • Radius of a circle

• Position of vertices of a polygon

• Position of control points of a curved body

To modify the geometry of bodies, you can either use the Coordinates bar or the Geometry window. The Coordinates bar provides you with quick-and- easy access for the geometry parameters, whereas the Geometry window gives complete control, including importing or exporting the geometry data of polygons to and from other applications. The Geometry window can also be used to modify vertex locations for polygons and control points for curved bodies.

This section shows how to use the Geometry window. For the use of the Coordinates bar, Please refer to “3.1. Creating Bodies”.

To display the Geometry window:

1. Select the body whose geometry you wish to change. 2. Choose Geometry from the Window menu.

Using the Selection Pop-up

Menu Alternately, if the Geometry window is already visible, you can simply select the desired object from the selection pop-up menu at the top of the window (Figure 3-17). The menu will show the list of ID numbers and the names of all objects in the document. You can change and assign meaningful names by typing into the name field in the Appearance window (see “3.3. Body Appearance”). Assigning custom names will help you search through the list of objects.

Figure 3-17

Geometry window for a rectangle

Area Bodies in Working Model 2D are defined with an area rather than with a

volume. The only way you can change the area of a body is by resizing the object with the mouse, or by changing values in the Geometry window.

Center Of Mass Offset By default, all bodies are created with the Center of Mass (COM) at the geometric center of the object. The center of mass can be moved by modifying the x- and y-offset fields, as shown in Figure 3-17.

These values are given with respect to the frame of reference for the object (see “Frame of Reference (FOR)” on page 67 for details). In Auto mode (as indicated by the radio button on Figure 3-17), the COM is automatically recomputed whenever the polygon is reshaped so that the COM coincides with the geometric center.

Using meters, you can take kinematic measurements of a body (such as position, velocity, and acceleration) in terms of its COM or of FOR. See “7.1. Meters” for details.

Also available in the formula language are explicit references to COM and FOR. For example, body[n].cofm.p refers to the COM, whereas body[n].p refers to FOR. See Appendix B, “Formula Language

Reference” for more details.

Radius Radius is the choice available in the Geometry window when a circular body is selected. Circular bodies may be accurately sized by setting their radii. The Radius can also be edited in the Coordinates bar. Please see “Creating Circles” on page 61.

Height and Width Height and width are choices available in the Geometry window when a rectangle is selected.

These parameters can also be edited in the Coordinates bar. Please see “Creating Rectangles and Squares” on page 59.

Polygon Vertices and Curved Body Control Points

The Geometry window gives you complete control of polygon vertices and curved body control points. By default, the coordinates are given with respect to the world (i.e. global coordinates). You can add vertices/control points, delete vertices/control points, and reshape polygons/curved bodies using the Geometry window.

Converting Between Polygons and Curved Bodies

The Geometry window allows you to convert polygons into curved bodies and vice versa though the “Curved body” checkbox. For polygons, this box is unchecked and the point coordinates refer to the polygons vertices. Clicking to check the box converts the polygon into a curved body with control points at the former vertex coordinates of the polygon.