One revolution of the propeller corresponds to a rotation of 2<em>π</em> radians, so that the propeller has an initial angular velocity of
10 rev/s = (10 rev/s) • (2<em>π</em> rad/rev) = 20<em>π</em> rad/s
The propeller thus has an angular velocity <em>ω</em> at time <em>t</em> of
<em>ω</em> = 20<em>π</em> rad/s - (2.0 rad/s²) <em>t</em>
so that at <em>t</em> = 40 s, its angular speed is reduced to
20<em>π</em> rad/s - (2.0 rad/s²) (40 s) = (20<em>π</em> - 80) rad/s
Convert this to a rotation rate by dividing this result by 2<em>π</em> :
(20<em>π</em> - 80) rad/s = ((20<em>π</em> - 80) rad/s) • (1/(2<em>π</em>) rev/rad) ≈ -2.73 rev/s
which would suggest that the propeller has started to turn in the opposite direction at a rate of 2.73 rev/s.