What is friction ? What is impending motion ?

A child weighing $25$ kg slides down a rope hanging from the branch of a tall tree. If the force of friction acting against him is $2\, N$,  ........ $m/s^2$ is the acceleration of the child (Take $g = 9.8\,m/{s^2})$

The limiting friction between two bodies in contact is independent of

A bullet of mass $20\, g$ travelling horizontally with a speed of $500 \,m/s$ passes  through a wooden block of mass $10.0 \,kg$ initially at rest on a surface. The bullet  emerges with a speed of $100\, m/s$ and the block slides $20 \,cm$ on the surface  before coming to rest, the coefficient of friction between the block and the surface. $(g = 10\, m/s^2)$

A block of mass $M$ is being pulled along rough horizontal surface. The coefficient of friction between the block and the surface is $\mu $. If another block of mass $M/2$ is placed on the block and it is again pulled on the surface, the coefficient of friction between the block and the surface will be

Which one of the following is not used to reduce friction