[tex]v_1=12\frac{m}{s},\:v_2=72\frac{km}{h}=72\frac{1000m}{3600s}=20\frac{m}{s}[/tex]
Din teorema de variatie a energiei cinetice, stii ca:
[tex]\Delta E_c=L,\text{ unde }L\text{ este lucrul mecanic efectuat}[/tex]
[tex]\Delta E_c=\frac{m\Delta v^2}{2}=\frac{m(v_2-v_1)^2}{2}[/tex]
Egaland cele 2 relatii: [tex]\frac{m(v_2-v_1)^2}{2}=L\implies m=\frac{2L}{(v_2-v_1)^2}[/tex]
Numeric, [tex]m=\frac{128000J}{64\frac{m^2}{s^2}}=\boxed{2000kg}[/tex]
