[tex]dane:\\s_1 = s\\v_1 = 20\frac{m}{s}\\s_2 = s\\v_2 = 10\frac{m}{s}\\szukane:\\v_{sr} = ?\\\\Rozwiazanie\\\\v_{sr} = \frac{s_{c}}{t_{c}}\\\\s_{c} = s_1+s_2 = s+s = 2s\\\\t = \frac{s}{v}\\\\t_1 = \frac{s}{v_1}, \ \ t_2 = \frac{s}{v_2}\\\\t_{c} = t_1 + t_2 =\frac{s}{v_1}+\frac{s}{v_2} = \frac{s(v_1+v_2)}{v_1v_2}[/tex]
[tex]v_{sr} = \frac{2s}{\frac{s(v_1+v_2)}{v_1v_2}} = \frac{2v_1v_2}{v_1+v_2}\\\\\\ v_{sr}=\frac{2\cdot20\frac{m}{s}\cdot10\frac{m}{s}}{20\frac{m}{s}+10\frac{m}{s}}=\frac{400}{30}\frac{m}{s}=13,(3)\frac{m}{s}\approx13,3\frac{m}{s}[/tex]
