Law of conservation of momentum:Aconservation lawstating that the total linearmomentumof a closed system remains constant through time, regardless of other possible changes within the system.

Molecular rangeis the maximum distance up to which amoleculecan exertforceof attraction on anothermolecule. It is of the order of 10-9 m for solids and liquids. liquid consist of a very large number ofmolecules.

Thesemoleculesattract each other bycohesive force. But theseforcehave only a shortrange.

Surface tension is tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimize surface area.

Thespecific heatis the amount ofheatper unit mass required to raise the temperature by one degree Celsius. The relationship betweenheatand temperature change is usually expressed in the form shown below where c is thespecific heat. .

Things which are bad conductors of heat are used as insulators. In same occasions it is important (1) to prevent heat loss from a hot body (2) to prevent heat from entering cold body.1. The handles of saucepans, teapots, kettles, etc, are always made from substances like wood, plastic, etc, to prevent heat from entering the hand so that it can he held comfortably2. Air is a poor conductor of heat, this is why heat from our bodies does not escape easily to the atmosphere3. Fur, wool, cotton wool, cork, feathers, sawdust, trapped air in birds’ feathers etc. act as good insulators.Fur and feathers keep most animals and birds during winterBirds flutter their feathers in cold weather to trap air between their feathersWoolen clothes keep as warm during winterIn cold countries, glass windows in houses are double paned4. Refrigerators, ice boxes, ovens, solar heater panels and connecting pipes are made of double walls with filling of cork, air, cotton wool or polyurethane in between their walls.5. Quilts are always filled with either cotton or foamThe use of a non conducting material to reduce heat from escaping is called lagging

Frequencyis the number of occurrences of a repeating event per unit oftime. It is also referred to as temporalfrequency, which emphasizes the contrast to spatialfrequencyand angularfrequency. Theperiodis the duration oftimeof one cycle in a repeating event, so theperiodis the reciprocal of thefrequency.