What is kinetic friction ? What is rolling friction ?

A heavy box of mass $50 \mathrm{~kg}$ is moving on a horizontal surface. If co-efficient of kinetic friction between the box and horizontal surface is $0.3$ then force of kinetic friction is :

A body of mass $40\,kg$ resting on rough horizontal surface is subjected to a force $P$ which is just enough to start the motion of the body. If $\mu_{ s }=5, \mu_{ x }=0.4$, $g =10\,m / s ^2$ and the force $P$ is continuously applied on the body, then the acceleration of the body is $.........m/s^{2}$

A bullet of mass $0.1\,kg$ moving horizontally with speed $400\,ms ^{-1}$ hits a wooden block of mass $3.9\,kg$ kept on a horizontal rough surface. The bullet gets embedded into the block and moves $20\,m$ before coming to rest. The coefficient of friction between the block and the surface is $........$ $\left(\text { Given } g=10 \,ms ^2\right. \text { ) }$

A block of mass $m$ slides along a floor while a force of magnitude $F$ is applied to it at an angle $\theta$ as shown in figure. The coefficient of kinetic friction is $\mu_{ K }$. Then, the block's acceleration $'a'$ is given by: ($g$ is acceleration due to gravity)

$STATEMENT-1$ It is easier to pull a heavy object than to push it on a level ground. and

$STATEMENT-2$ The magnitude of frictional force depends on the nature of the two surfaces in contact.