A body is travelling with $$10 \mathrm{~ms}^{-1}$$ on a rough horizontal surface. It's velocity after 2 s is $$4 \mathrm{~ms}^{-1}$$. The coefficient of kinetic friction between the block and the plane is (acceleration due to gravity $$=10 \mathrm{~ms}^{-2}$$)

A cricket ball of mass 50 g having velocity $$50 \mathrm{~cm} \mathrm{~s}^{-1}$$ to stopped in 0.5 s. The force applied to stop the ball is

Two masses $$M_1$$ and $$M_2$$ are arranged as shown in the figure. Let $$a$$ be the magnitude of the acceleration of the mass $$M_1$$. If the mass of $$M_1$$ is doubled and that of $$M_2$$ is halved, then the acceleration of the system is (Treat all surfaces as smooth; masses of pulley and rope are negligible)