\(r=0.5 / 2 m=0.25 m\)
\(\omega_{2}=0, \theta=20 \times 2 \pi r a d i a n, \alpha=?\)
\(\omega_{1}=\frac{u}{r_{e}}=\frac{20}{0.25}=80 \mathrm{rad} / \mathrm{s}\)
From \(\omega_{2}^{2}-\omega_{1}^{2}=2 \alpha \theta\)
\(0-(80)^{2}=2 \alpha(20 \times 2 \pi)\)
\(\alpha=-\frac{80 \times 80}{80 \pi}=-25.5 \mathrm{rad} / \mathrm{s}^{2}\)