A protractor track is a tool for measuring angular arcs. It has a vertex, a base (solid) and measuring (dotted) arm, an arc arrow and an angle readout that displays in degrees or radians.
A full 360-degree ruler (right image above) is also drawn by default. To hide the ruler, uncheck the Ruler box on the toolbar. The ruler diameter is determined by the base length, and larger diameters show finer angle resolution.
To create a protractor, click the arrow on the Measuring Tools button 
, then choose New|Protractor from the popup menu. Multiple protractors can be created as needed. New protractors are placed at the center of the current viewport.
Click and drag the vertex or circular end of either arm to adjust the arms. The arc arrow shows the direction (cw or ccw) of the arc and the angle readout and toolbar angle field display its value (positive if ccw, negative if cw).
Set the arc to a specific angle by clicking either the angle readout or toolbar field and entering the desired angle.
Drag the center of either arm to move the entire protractor without changing the angle or orientation.
Drag the arc arrow to rotate the entire protractor about its vertex without changing the angular arc. For finer control you can drag away from the vertex--a dashed line continues to connect the drag point to the arc arrow as shown.
By default, the protractor has a fixed position--that is, its vertex and arm positions are the same in all frames. Uncheck the Fixed Position checkbox in its track menu to allow these properties to vary independently from frame to frame.
It is sometimes useful to measure the distances and angles determined by independent point mass tracks. This can be accomplished by creating a protractor and attaching its ends and/or vertex to the point masses. To attach points, choose the Attach Points... item in the protractor's track menu, then select the desired point mass tracks from the dropdown lists in the Attach Points dialog. You can attach to any point mass track including a center of mass, particle model or data track.
Attaching the points automatically unfixes the protractor.
Locking the protractor prevents it from making any changes to the scale. Lock the protractor by turning on the Locked property in its track menu.