2.1. Angle of rotation of a solid body

With rotational movement, in contrast to translational, speed different points of the body are not the same. Therefore, the speed Any point of a rotating body cannot serve as a characteristic of the movement of the whole body.

Let t. O - the center of rotation of the body, and - fixed (or instantaneous) axis of rotation (Fig.2.2).

The position of an arbitrary m. Body will be set using the radius vector conducted from the center of O. From the figure it is clear that:

,

Where - radius vector drawn to the point of the circular arc along which m moves. For a short time, the vector rotates in the plane perpendicular on a small angle . At the same angle rotates in time. radius vector of any other point of the body, because otherwise the distance between these points should have changed. Thus, the angle of rotation characterizes the movement of the entire rotating body over a small period of time. Convenient to enter a vector elementary (small) body rotation, numerically equal and directed along the instantaneous axis so that from its end the rotation of the body is visible happening counterclockwise.