# Clarification in concept of planar motion of rigid body

• manimaran1605

#### manimaran1605

When a Rigid body is said to be in planar motion, if all all the particles in the Rigid body moves in same plane and these planes are parallel to each other.

I think i misunderstood this definition especially the particles moves in same plane and these planes are parallel to each other, does it mean all motion in two dimension are Planar motion?

Often they mean a body that is 2D, and confined to a plane; call the plane XY.

See http://ocw.nthu.edu.tw/ocw/upload/75/907/ch16.pdf

If the body is 3D then the motion (forces) are constrained so that the body cannot change its orientation ... like a table that can turn, but cannot be tilted or turned over.

Nope.

Motion constrained to a curved surface is has essentially two degrees of freedom on how to move, that is at every point, there are two directions on the tangent plane you may move in, but at different points on the curved surface, those tangent planes need not be parallell to each other.

If you think a "curved surface" is difficult to visualize, think of a particle (or small cube/rigid body) moving on the surface of the box.
Essentially, disregarding motion along the edges, you have three non-parallell planes the particle might move upon.
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Planar motion, however, is effectively reducible to 2D-motion