Vom aplica legea conservarii energiei, sub forma [tex]\Delta E_c=-L_f[/tex]
In primul rand, avem nevoie sa calculam lucrul mecanic al fortei de frecare, care actioneaza atat la urcare, cat si la coborare:
[tex]L_f=2F_fl=2\dfrac{G}{10}l=\dfrac{2mgl}{10}=\dfrac{mgl}{5}.[/tex]
Acum scriem explicit ecuatia conservarii energiei:
[tex]E_{c_{final}}-E_{c_{initial}}=-L_f \\ \\ \Rightarrow E_{c_{final}}=E_{c_{initial}}-L_f \\ \\ \Rightarrow E_{c_{final}}=E_{c_{initial}}-\dfrac{mgl}{5}\\ \\ \Rightarrow E_{c_{final}}=20-\dfrac{2\cdot 10 \cdot 1}{5}=...[/tex]
