Chọn gốc thế năng tại A
Ta có : \(W_A=W_B\)
\(\Rightarrow W_{đ_A}+W_{t_A}=W_{đ_B}+W_{t_B}\)
\(\Rightarrow\frac{1}{2}mv_A^2=mgh_B\)
\(\Rightarrow\frac{1}{2}v_A^2=gh_B\)
( Ta có: \(h_B=HA=OA-OH\)
\(=l-l.cos\alpha=l\left(1-cos\alpha\right)\) )
\(\Rightarrow\frac{1}{2}v_A^2=gl\left(1-cos\alpha\right)\)
\(\Rightarrow\frac{1}{2}v_A^2=9,8.1\left(1-cos45\right)\)
\(\Rightarrow v_A\approx2,4\) \((m/s)\)
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