Question

Block $A$ of mass $1\, kg$ is placed on the rough surface of block $B$ of mass $3\, kg$. Block $B$ is placed on smooth horizontal surface. Blocks are given the velocities as shown. The net work done by the frictional force is ______.[in (-) ve $J$]

Answer 1

$(1+3) v=(1)(8)+(3)(4)=20 v=5 \,m / \sec$
For block $A, W_{f}=\frac{1}{2}(1)\left(5^{2}-8^{2}\right)=-\frac{39}{2} J$
For block $B, W_{f}=\frac{1}{2}(3)\left(5^{2}-4^{2}\right)=+\frac{27}{2} J$
Net work done by friction $=-6\, J$