bài 1

\(a,W=\dfrac{1}{2}mv^2+mgh=\dfrac{1}{2}.0,1.20^2+0,1.10.0=20J\\ b,v^2-v_0^2=2gh\Rightarrow h_{max}=\dfrac{0^2-20^2}{2.10}=20\left(m\right)\\ c,\left\{{}\begin{matrix}W_đ+W_t=20\\3W_đ=W_t\end{matrix}\right.\Rightarrow4W_đ=20\Rightarrow4mgh_1=20\Rightarrow h_1=5\left(m\right)\) 

bài 2, 

\(T=\dfrac{t}{N}=\dfrac{60}{120}=0,5\\ f=\dfrac{1}{T}=\dfrac{1}{0,5}=2\\ \omega=\dfrac{2\pi}{T}=\dfrac{2.3,14}{0,5}=12,56\\ V=r\omega=0,2.12,56=2,512\)