## 9th Class Physics Unit 7 Notes

Kinetic molecular model explains the three states of matter assuming that:

Matter is made up of particles called molecules

The molecules remain in continuous motion

Molecules attract each other.

At very high temperature, the collision between atoms and molecules tears off their electrons. Atoms become positive ions. This ionic state of matter is called plasma-the fourth state of matter.

Density is the ratio of mass to volume of a substance. Density of water is 1000 kgm-3.

Pressure is the normal force acting per unit area. Its SI unit is Nm-2 or pascal (Pa).

Atmospheric pressure acts in all directions.

The instruments that measure atmospheric pressure are called barometers.

The atmospheric pressure decreases as we go up. Thus, knowing the atmospheric pressure of a place, we can determine its altitude.

The changes in atmospheric pressure at a certain place indicate the expected changes in the weather conditions of that place.

Liquids also exert pressure given by: P = p g h

Liquids transmit pressure equally in all directions. This is called Pascal’s law.

When a body is immersed wholly or partially in a liquid, it loses its weight equal to the weight of the liquid displaced. This is known as Archimedes principle.

For an object to float, its weight must be equal or less than the upthrust of the liquid acting on it.

The property of matter by virtue of which matter resists any force which tries to change its length, shape or volume is called elasticity.

Stress is the deforming force acting per unit area.

The ratio of change of length to the original length is called tensile strain.

The ratio between stress and tensile strain is called Young’s modulus.