− − ( 2 r Ω Ω θ θ ˙ ˙ ′ + r Ω Ω 2 ) r ^ ^ + ( 2 r ˙ ˙ Ω Ω ) θ θ ^ ^ The radial term Ω r is the centrifugal force per unit mass due to the system's rotation at rate Ω and the radial term 2 r Ω Ω θ θ ˙ ˙ ′ is the radial component of the Coriolis force per unit mass, where r θ θ ˙ ˙ ′ is the tangential component of the particle velocity as seen in the rotating frame. The term − − ( 2 r ˙ ˙ Ω Ω ) θ θ ^ ^ is the so-called azimuthal component of the Coriolis force per unit mass. In fact, these extra terms can be used to measure Ω and provide a test to see whether or not the frame is rotating, just as explained in the example of rotating identical spheres. If the particle's motion can be described by the observer using Newton's laws of motion without these Ω-dependent terms, the observer is in an inertial frame of reference where Ω = 0.