If a shell fired from a cannon, explodes in mid air, then

A bungee jumper is jumping with help of elastic ideal rope (Force constant $K$). Jumper steps off the bridge and falls from the rest towards the river below. He does not hit the water. The mass of jumper is $m$, natural length of rope is $l$. Gravity is $g$, assume every thing ideal. then, choose the incorrect option

A number of identical cubical blocks of edge $'a'$ and mass $m$ are lying on a horizontal table. The work done on the blocks in arranging them in a column of height $(n + 1)a$ on the table is

A neutron moving with velocity $u$ collides elastically with an atom of mass number $A$ . If the collision is head-on and the initial kinetic energy of neutron is $E$ , then the final kinetic energy of the neutron after collision is

A block of mass $1\,kg$ slides on a rough horizontal surface. If the speed of the block decreases from $10\,m/s$ to $8\,m/s$ , the thermal energy developed in this process .................. $\mathrm{J}$

A small bar starts sliding down on inclined plane forming an angle $\theta $ with the horizontal. The friction coefficient depends on the distance $x$ covered as $\mu  = kx$ , where $k$ is a constant. Find the distance covered by the bar till it stops