# What kind of motion is orbital revolution it most definitely cannot be pure rotation as no particle of body is fixed?

Consider an object say O (which is a rigid body having appreciable dimension wrt its orbit) is rotating about a fixed point C (and it is not spinning about its axis through centre of mass) at centre of circular orbit then

At first guess we would assume that the body is just translating and all particles at some instant have same velocity but if we analyse motion of particles more carefully we would find that each particle is moving in a circle about the axis through C hence it probably is doing both translation and rotation simaltaneously as different particles have different radius of orbit as body has appreciable size hence different velocities therefore not in pure translation.

So what kind of motion is this exactly?

• Essentially, the "kind of motion" that a body executes depends on the axis chosen. You can always find an axis about which the body is purely rotating. That is all the particles in the body have velocity $v_i = \omega \times R_i$ about that axis. For some other axis it is rotating as well as translating. $v_i = \omega \times R_i + v_0$ (for a different $\omega$ and $R$) – Abhijeet Melkani Apr 29 '17 at 10:11