A person measures the depth of a well by measuring the time interval between dropping a stone and receiving the sound of impact with the bottom of the well. The error in his measurement of time is $\delta T=0.01 \mathrm{s}$ and he measures the depth of the well to be $L=20 \mathrm{m}.$ Take the acceleration due to gravity $g=10 \mathrm{m} {\mathrm{s}}^{-2}$  and the velocity of sound is $300 \mathrm{m} {\mathrm{s}}^{-1}.$ Then the fractional error in the measurement, $\frac{\delta L}{L},$ is closest to

Two blocks $A$ and $B$ are placed one over the other on a smooth horizontal surface. The maximum horizontal force that can be applied on lower block $B,$ so that $A$ and $B$ move without separation is $49 \mathrm{N}.$ The coefficient of friction between $A$ and $B$ is

An aeroplane is flying in a horizontal direction with a velocity $u$ and at a height of $2000 \mathrm{m}$. When it is vertically below a point $A$ on the ground a food packet is released from it. The packet strikes the ground at point $B$.

If $AB=3 \mathrm{km}$ and $g=10 \mathrm{m} {\mathrm{s}}^{-2}$, then the value of $u$ is