Static and kinetic friction, laws of friction, rolling friction

There are two inclined surfaces of equal length (\(L\)) and same angle of inclination \(45^\circ\) with the horizontal. One of them is rough and the other is perfectly smooth. A given body takes 2 times as much time to slide down on rough surface than on the smooth surface. The coefficient of kinetic friction (\(\mu_k\)) between the object and the rough surface is close to

A horizontal force 10 \(N\) is applied to a block \(A\) as shown in figure. The mass of blocks \(A\) and \(B\) are 2 kg and 3 kg, respectively. The blocks slide over a frictionless surface. The force exerted by block \(A\) on block \(B\) is :

Calculate the maximum acceleration of a moving car so that a body lying on the floor of the car remains stationary. The coefficient of static friction between the body and the floor is 0.15 (\(g=10\,\mathrm{m\,s^{-2}}\)).

A block of mass 10 kg is in contact against the inner wall of a hollow cylindrical drum of radius 1 m. The coefficient of friction between the block and the inner wall of the cylinder is 0.1. The minimum angular velocity needed for the cylinder to keep the block stationary when the cylinder is vertical and rotating about its axis, will be : (\(g = 10\) m/s\(^2\))

Which one of the following statements is incorrect ?