With what velocity an observer should move relative to a stationary source so that a sound of triple the frequency of source is heard by an observer?

Two long straight parallel wires are separated by a distance '2d'. Each wire carries a current 'I' in the same direction. The magnetic field at a point 'P' midway between them is

If 'T' is the half life of a radioactive substance then its instantaneous rate of change of activity is proportional to

An insulated container contains a diatomic gas of molar mass ' m '. The container is moving with velocity ' $V$ ', if it is stopped suddenly, the change in temperature is ( $R=$ gas constant)