15.1 : Planar Rigid-Body Motion

Understanding the movement of a rigid body in planar motion involves recognizing that every particle within this body is traversing a path that maintains a consistent distance from a specific plane. This concept is fundamental in the study of physics and mechanical engineering, and it allows us to comprehend better how objects move in space.

Planar motion is typically divided into three distinct categories. The first is rectilinear translation, demonstrated by a subway train that moves along its track in a straight line, never deviating from its course. In this mode of motion, the paths of any two points on the body are always parallel lines. For example, every point on the subway train moves in the same direction and speed, similar to how soldiers march in a parade.

The second type is curvilinear translation, illustrated by a roller coaster smoothly moving along its winding tracks while maintaining its orientation. Unlike rectilinear translation, the motion paths in curvilinear translation are curved lines but remain equidistant. It is like a snake slithering through grass, where its head and tail follow the same path but not in a straight line.

The third category is rotation about a fixed axis, where all rigid body particles, except those on the rotation axis, follow circular paths. A spinning top exhibits this motion, rotating around its axis while staying in one place.

Lastly, we have the general planar motion, a combination of translational and rotational motion. An example is a frisbee flying through the air. As it soars, it spins while propelling forward, keeping its rotational axis perpendicular to the plane of rotation. This is a complex motion that combines both linear and rotational elements, demonstrating the beauty and intricacy of physics in our everyday lives.