When a bicycle is in motion,the force of friction exerted by the ground on the two wheels is such that it acts

$A$ point particle of mass $m$ moves along the uniformly rough track $PQR$ as shown in the figure. The coefficient of friction between the particle and the rough track equals $\mu$. The particle is released from rest from the point $P$ and it comes to rest at a point $R$. The energies lost by the particle over the parts $PQ$ and $QR$ of the track are equal to each other,and no energy is lost when the particle changes direction from $PQ$ to $QR$. The values of the coefficient of friction $\mu$ and the distance $x (= QR)$ are,respectively,close to

Given $m_1 = 4m_2$. How much extra distance (in $cm$) does $m_2$ travel before coming to rest?

$A$ constant force $F = m_2g/2$ is applied on the block of mass $m_1$ as shown in the figure. The string and the pulley are light and the surface of the table is smooth. The acceleration of $m_1$ is

$A$ block of mass $m$ is on an inclined plane of angle $\theta$. The coefficient of friction between the block and the plane is $\mu$ and $\tan \theta > \mu$. The block is held stationary by applying a force $P$ parallel to the plane. The direction of force pointing up the plane is taken to be positive. As $P$ is varied from $P_1 = mg(\sin \theta - \mu \cos \theta)$ to $P_2 = mg(\sin \theta + \mu \cos \theta)$,the frictional force $f$ versus $P$ graph will look like:

Given in the figure are two blocks $A$ and $B$ of weight $20\ N$ and $100\ N,$ respectively. These are being pressed against a wall by a force $F$ as shown. If the coefficient of friction between the blocks is $0.1$ and between block $B$ and the wall is $0.15$,the frictional force applied by the wall on block $B$ is ........... $N$.