659acts like an invisible rod between the two constrained points. Figure 12.31 il-
lustrates this constraint.
12.4.8.3. Hinge Constraints
A hinge constraint limits rotational motion to only a single degree of freedom,
about the hinge’s axis. An unlimited hinge acts like an axle, allowing the con-
strained object to complete an unlimited number of full rotations. It’s com-
mon to defi ne limited hinges that can only move through a predefi ned range
of angles about the one allowed axis. For example, a one-way door can only
move through a 180 degree arc, because otherwise it would pass through the
adjacent wall. Likewise, a two-way door is constrained to move through a
±180 degree arc. Hinge constraints may also be given a degree of friction in
the form of a torque that resists rotation about the hinge’s axis. A limited hinge
constraint is shown in Figure 12.32.
Figure 12.32. A limited hinge constraint mimics the behavior of a door.
Figure 12.33. A prismatic constraint acts like a piston.
12.4.8.4. Prismatic Constraints
Prismatic constraints act like a piston: A constrained body’s motion is restrict-
ed to a single translational degree of freedom. A prismatic constraint may or
may not permit rotation about the translation axis of the piston. Prismatic
constraints can of course be limited or unlimited and may or may not include
friction. A prismatic constraint is illustrated in Figure 12.33.
12.4. Rigid Body Dynamics