Answer:
The mass of the lorry 7800 kg
Explanation:
Momentum conservation law for inelastic collision:
[tex]m1*V1 + m2*V2 =( m1 + m2)*V^{'} \\ \\1200*10^{3} g * 30 m/s + m2*0 m/s =(1200*10^{3} g + m2)*4 m/s\\ \\\frac{1200*10^{3} g * 30 m/s}{4 m/s} -1200*10^{3} g=m2\\ \\m2=9000*10^{3} g-1200*10^{3} g=7800*10^{3} g=7800 kg[/tex]