If the rigid body decreases its rate of rotation in the counterclockwise (positive) direction then the z -component of the angular acceleration is negative, \(\alpha_{z}=d^{2} \theta / d t^{2}=d \omeg...If the rigid body decreases its rate of rotation in the counterclockwise (positive) direction then the z -component of the angular acceleration is negative, αz=d2θ/dt2=dωz/dt<0 The angular acceleration vector then points in the −ˆk-direction. The z -component of the angular acceleration is negative, the disc is slowing down and so the angular acceleration vector then points in the −ˆk-direction as shown in Figure 16.7.
