In a motorcycle stunt called the "well of death",the track is a vertical cylindrical surface of $18\, m$ diameter. What should be the minimum speed of the motorcyclist in $m/s$ to prevent him from sliding down? The coefficient of friction is $0.8$ and take $g = 10\, m/s^2$.

$A$ long horizontal rod has a bead which can slide along its length,and is initially placed at a distance $L$ from one end $A$ of the rod. The rod is set in angular motion about $A$ with constant angular acceleration $\alpha$. If the coefficient of friction between the rod and the bead is $\mu$,and gravity is neglected,then the time after which the bead starts slipping is

$A$ child starts running from rest along a circular track of radius $r$ with constant tangential acceleration $a$. After time $t$,the child feels that the shoes have started slipping on the ground. What is the coefficient of friction $\mu$ between the shoes and the ground? $[g = \text{acceleration due to gravity}]$

$A$ car of mass $800 \, kg$ moves on a circular track of radius $40 \, m$. If the coefficient of friction is $0.5$,then the maximum velocity with which the car can move is ......... $m/s$.

$A$ body of mass $200 \text{ g}$ is tied to a spring of spring constant $12.5 \text{ N/m}$,while the other end of the spring is fixed at point '$O$'. If the body moves about '$O$' in a circular path on a smooth horizontal surface with a constant angular speed of $5 \text{ rad/s}$,then the ratio of the extension in the spring to its natural length will be:

$A$ car when passing through a convex bridge exerts a force on it which is equal to: