PHYSICS

We compare the density of different substances with the density of water at 4°C. Hence "the ratio of the density of a substance to the density of water at 4°C is called Relative density of the substance". 4°C should be mentioned because water has maximum density of 1g/cm^3 at 4°C only. There is no unit of relative density as it is  the ratio of two similar quantities. The value of density of any substances in g/cm^3 is equal to its relative density. For example if the density of a substance in g/cm^3 is 7.7gcm-3, then the relative density is 7.7. It means that the substance is 7.7 times heavier than water.

"For a floating body, the upthrust experienced by it is equal to its weight." Net force  on the floating  body is zero

The melting point of ice is 0°C at normal pressure but when you increase the pressure on ice it melts at the temperature below 0°C( i.e. melting point of ice decreases with pressure). When the pressure applied is released then water formed from ice starts freezing. This phenomenon is called REGELATION.

Irrespective of shape of vessels or containers, a connected liquid maintains same level in all containers. This happens to maintain same pressure at a given depth. As the pressure depends on depth of a point from the liquid surface.

MIRAGE is formed by the phenomenon of total INTERNAL reflection of light. Mirage is formed in extremely high or low temperature conditions. For example: When a sand surface (desert) or road surface is heated, the air above this surface is also heated and rises up. As a consequence, the density of air in the lower level is less than that of upper level.   Now , the light from any object starting from the upper layers (denser medium) travels to lower layers ( rarer medium), total internal reflection takes place i.e, the light is reflected back towards the denser medium. When these light rays reflected back, it seems that the light rays are refected from some bright reflecting surfaces on the ground.  Hence the  image is seen below the object.

Literally,  smoke + fog= smog. Smoke is the collection of unburnt fuel particles suspended in the air. Fog is the suspended water particles in the air. The combination of these suspended particles gives the smog.

The sound produced in a flute is due to the vibration of air column in the flute. The frequency of sound produced depends on length. The frequency is inversely proportional to the length of the air column. By closing or opening the holes , the length of the air column is changed and hence the frequency can be changed.