Sound waves exist as variations of pressure in a medium such as air. They are created by the vibration of an object, which causes the air surrounding it to vibrate. The vibrating air then causes the human eardrum to vibrate, which the brain interprets as sound.

Bar magnet is a magnet having two poles north and south at the end of magnet producing constant or uniform magnetic field since the magnetic lines of forces are more at the poles produced by them so magnetic field at the centre of magnet is zero.

due to more friction between sand and feet it is difficult to walk because sand has more contact forces upon feet so it is difficult to walk.

The maximum speed for the overturn of a car moving on a circular track is given by, v=underroot(co-efficient of friction X radius X acceleration due to gravity) v=underroot(0.2 X 100 X 9.8) v=14m/s.