Atoy train rolls around a horizontal 1.0-m-diameter track. the coefficient of rolling friction is 0.10. how long does it take the train to stop if it's released with an angular speed of 40 rpm?

Deceleration due to rolling resistance (α) = coefficient*g/R = 0.1*9.81/0.5 = 1.962 rad/sec^2

Equations of motion:
ωf=ωo+αt
But ωf = 0 rad/sec; ωo= 2*pi*RPM/60 = 4.19 rad/sec

Therefore,
0=4.19-1.962t => 1.962t = 4.19 => t = 4.19/1.962 = 2.13 seconds