Câu 7: 

Ta có:

\(\begin{array}{l}
\tan 60 = \dfrac{F}{P} \Rightarrow F = mg\tan 60 = 2\sqrt 3 \left( N \right)\\
 \Rightarrow m{\omega ^2}R = 2\sqrt 3 \\
 \Rightarrow 0,2.{\omega ^2}.0,5.\sin 60 = 2\sqrt 3 \\
 \Rightarrow \omega  = 2\pi \left( {rad/s} \right)\\
v = \omega R = 2\pi .0,5.\sin 60 = \dfrac{{\pi \sqrt 3 }}{2}\left( {m/s} \right)\\
T = \dfrac{P}{{\cos \alpha }} = \dfrac{{0,2.10}}{{\cos 60}} = 4N
\end{array}\)

Câu 12: 

Ta có: 

\(\begin{array}{l}
{F_{ms}} = {F_{ht}}\\
 \Rightarrow mg\mu  = m\dfrac{{{v^2}}}{R}\\
 \Rightarrow 10.0,1 = \dfrac{{{v^2}}}{{400}}\\
 \Rightarrow v = 20\left( {m/s} \right)
\end{array}\)