A shot travelling at the rate of `100 ms^-1` is just able to pierce a plank 4 cm thick. What velcoity is required to just pierce a plank 9 mm thick?

A layer of chromium metal 0.25 mm thick is to be plated on an auto bumper with a total area of 032 m^2 from a solution cantaining CrO_4^(2-) ? What current flow is required for this electroplating if the bumper is to be plated in 60s ? The density of chromium metal is 7.20g/ cm^3

An air capacitor has an area of 1,000 cm^2 and the two plates are 4 mm apart. It is connected in series with another capacitor having a plate ares of 20 cm^2 , the plates being separated by a dielectric 0.1 mm thick having dielectric constant o 2.5 . If the potential difference across the second capacitor is 100 V, find the voltage applied across the series combination.

A wooden bookshelf has external dimensions as follows: Height =110 cm, Depth = 25 cm, Breadth = 85 cm. The thickness of the plank is 5 cm everywhere. The external faces are to be polished and the inner faces are to be painted. If the rate of polishing is 20 paise per cm^2 and the rate of painting is 10 paise per cm^2 .Find the total expenses required for polishing and painting the surface of the bookshelf.

A long straight wire carries a current of 2A. An electron travels with a velocity of 4xx10^4ms^-1 parallel to the wire 0.1mm from it, and in a direction opposite to the current. What force does the magnetic field of current exert on the moving electron. Charge on electron =1.6xx10^(-19)C.

A card sheet divided into squares each of size 1 mm^2 is being viewed at a distance of 9 cm through a magnifying glass (a converging lens of focal length 10 cm) held close to the eye. What should be the distance between the object in converging lens and the magnifying glass if the virtual image of each square in the figure is to have an area of 6.25 mm^2 . Would you be able to see the squares distinctly with your eyes very close to the magnifier ?

A rectangular glass block of thickness 10 cm and refractive index 1.5 is placed over a small coin.A beaker is filled with water of refractive index 4//3 to a height of 10 cm and is placed over a small block. If the eye is slowly moved away from the normal at a certain position,the coin is found to diappear due to total internal reflection.At what surface does it happen and why? .