In which of the following graphs,the total change in momentum is zero?

Which of the following is not a method of reducing friction?

$A$ system consists of three masses $m_1, m_2$ and $m_3$ connected by a string passing over a pulley $P$. The mass $m_1$ hangs freely and $m_2$ and $m_3$ are on a rough horizontal table (the coefficient of friction $= \mu$). The pulley is frictionless and of negligible mass. The downward acceleration of mass $m_1$ is (Assume $m_1 = m_2 = m_3 = m$)

Which one of the following is not used to reduce friction?

In the figures $(i), (ii)$ and $(iii)$ shown, the objects $A, B$ and $C$ have the same mass $m$. The string, spring, and pulley are massless. Object $C$ strikes object $B$ with velocity $u$ in each case and sticks to it. Find the ratio of the velocity of $B$ in case $(i)$ to $(ii)$ to $(iii)$.

$Assertion$ : $A$ man and a block rest on a smooth horizontal surface. The man holds a rope which is connected to the block. The man cannot move on the horizontal surface.
$Reason$ : $A$ man standing at rest on a smooth horizontal surface cannot start walking due to the absence of friction (The man is only in contact with the floor as shown).