If the point p does not move in M, its velocity in F is given by v P = P ˙ ˙ = [ A ˙ ˙ ( t ) ] p . It is convenient to eliminate the coordinates p and write this as an operation on the trajectory P (t), v P = [ A ˙ ˙ ( t ) ] [ A ( t ) − − 1 ] P = [ Ω Ω ] P , where the matrix [ Ω Ω ] = [ 0 − − ω ω ω ω 0 ] , is known as the angular velocity matrix of M relative to F. The parameter ω is the time derivative of the angle θ, that is: ω ω = d θ θ d t .