Câu 18: C

Ta có: 

\(\begin{array}{l}
R = \dfrac{d}{2} = \dfrac{{20}}{2} = 10cm = 0,1m\\
{\omega _1} = 300vong/ph = 10\pi \left( {rad/s} \right)\\
a = \dfrac{{{\omega _1} - 0}}{{{t_1}}} = \dfrac{{10\pi }}{{30}} = \dfrac{\pi }{3}\left( {rad/{s^2}} \right)\\
{\omega _2} = a{t_2} = \dfrac{\pi }{3}.10 = \dfrac{{10\pi }}{3}\left( {rad/s} \right)\\
{v_2} = {\omega _2}R = \dfrac{{10\pi }}{3}.0,1 = 1,04\left( {m/s} \right)
\end{array}\)

Câu 19: A

Ta có: 

\(\begin{array}{l}
x = 10 + 9t - \dfrac{1}{2}{t^2} \Rightarrow v = 9 - t\\
 \Rightarrow v = 9 - 3 = 6\left( {m/s} \right)
\end{array}\)

Câu 20: D

Ta có: 

\(x = 10 + 9t - \dfrac{1}{2}{t^2} \Rightarrow a =  - 1\left( {m/{s^2}} \right)\)