A string of length $L$ and force constant $k$ is stretched to obtain extension $l$. It is further stretched to obtain extension $l_1$. The work done in second stretching is

A block of mass 10 kg slides down a rough slope which is inclined at an angle of $45^{\circ}$ to the horizontal. The coefficient of sliding friction is 0.30 . When the block has slide 5 m , the work done on the block by the force of friction is nearly

A body is projected with a speed $u \mathrm{~ms}^{-1}$ at an angle $\beta$ with the horizontal. The kinetic energy at the highest point is $(3 / 4)$ th of the initial kinetic energy. The value of $\beta$ is

A force of $ F=0.5 \mathrm{~N} $ is applied on lower block as shown in figure. The work done by lower block on upper block for a displacement of 3 m of the upper block with respect to ground is (Take, $ g=10 \mathrm{~m} / \mathrm{s}^{2} $ )