The stream of a river is flowing with a speed of 2km/h. A swimmer can swim at a speed of 4km/h. What should be the direction of the swimmer with respect to the flow of the river to cross the river straight ?

A system of three charges are placed as shown in the figure :

If D >> d, the potential energy of the system is best given by :

Determine the charge on the capacitor in the following circuit :

A body of mass 2 kg makes an eleastic collision with a second body at rest and continues to move in the original direction but with one fourth of its original speed. What is the mass of the second body ?