A large slab of mass $5 \mathrm{kg}$ lies on a smooth horizontal surface, with a block of mass $4 \mathrm{kg}$ lying on the top of it. The coefficient of friction between the block and the slab is $0.25.$ If the block is pulled horizontally by a force of $F=6 \mathrm{N},$ the work done by the force of friction on the slab, between the instants $t=2 \mathrm{s}$ and $t=3 \mathrm{s}$, is $\left(g=10 \mathrm{m} {\mathrm{s}}^{-2}\right)$

A uniform chain $AB$ of mass $m$ and length $l$ is placed with one end $A$ at the highest point of a hemisphere of radius $R$. Referring to the top of the hemisphere as the datum level, the potential energy of the chain is (given that $\left(l<\frac{\pi R}{2}\right)$

Two discs, each having mass $m$, are attached rigidly to the ends of a vertical spring. One of the discs rests on a horizontal surface and the other produces a compression $x_0$ on the spring when it is in equilibrium. How much further must the spring be compressed so that when the force causing compression is removed, the extension of the spring will be able to lift the lower disc off the table?

Two ends $A$ and $B$ of a smooth chain of mass $m$ and length $l$ are situated as shown in the figure. If an external agent pulls $A$ till it comes to the same level of $B$, work done by external agent is

The blocks $A$ and $B$ shown in figure have masses $M_A=5 \mathrm{kg}$ and $M_B=4 \mathrm{kg}$. The system is released from rest. The speed of $B$ after $A$ has travelled a distance $1 \mathrm{m}$ along the incline is 

A collar $B$ of mass $2 \mathrm{kg}$ is constrained to move along a horizontal smooth and fixed circular track of radius $5 \mathrm{m}$. The spring lying in the plane of the circular track and having spring constant $200 \mathrm{N} {\mathrm{m}}^{-1}$ is undeformed when the collar is at $A$. If the collar starts from rest at $B$, the normal reaction exerted by the track on the collar when it passes through $A$ is