When a body is moving on a surface,the force of friction is called

$A$ block is kept on a rough horizontal surface. The acceleration of the block increases from $6 \ m/s^2$ to $11 \ m/s^2$ when the horizontal force acting on it increases from $20 \ N$ to $30 \ N$. The coefficient of kinetic friction between the block and the surface is (Acceleration due to gravity $g = 10 \ m/s^2$):

$A$ block of a certain mass is placed on a rough floor. The coefficients of static and kinetic friction between the block and the floor are $0.4$ and $0.25$ respectively. $A$ constant horizontal force $F = 20 \text{ N}$ acts on it so that the velocity of the block varies with time according to the following graph. The mass of the block is nearly (Take $g = 10 \text{ m/s}^2$): (in $\text{ kg}$)

$A$ lift is moving downwards with an acceleration equal to acceleration due to gravity. $A$ body of mass $M$ kept on the floor of the lift is pulled horizontally. If the coefficient of friction is $\mu$,what is the frictional resistance offered by the body when the lift is moving upwards with a uniform velocity?

The blocks shown in the figure move with a constant velocity of $10 \, m/s$ towards the right. All surfaces in contact are rough. The friction force applied by the ground on block $B$ is ........ $N$.

Meena applies the front brakes while riding her bicycle along a flat road. The force that slows her bicycle is provided by the