A thin solid disk of 1 kg is rotating along its diameter axis at the speed of 1800 rpm. By applying an external torque of 25 π Nm for 40 s, the speed…

A thin solid disk of $\displaystyle 1$ kg is rotating along its diameter axis at the speed of $\displaystyle 1800$ rpm. By applying an external torque of $\displaystyle 25 \pi \mathrm{Nm}$ for $\displaystyle 40$ s , the speed increases to $\displaystyle 2100$ rpm . The diameter of the disk is $\displaystyle \_\_\_\_$ m.