Constraints

A particle can be described by 3-coordinates in space. We say the particle has three degrees of freedom. We can arrest these degrees of freedom by putting constraints. For example, if a particle is allowed to move in a tube, it be constrained to move only in one direction, its other two degrees of freedom has been arrested. Similarly, a rigid body in space has six degrees of freedom. It can be described by six coordinates. Three coordinates will describe the position of any particle (say center particle). Keeping the position of that particle fixed, the body can be rotated about three axes. Thus, its for specify the change in its position, six coordinates are required.

If body is confined to move in a plane, it has only three degrees of freedom. It can be translated in two directions, and rotated about an axis normal to plane.