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### Basic Definitions in Physics

##### Density

The density is the mass of a substance per unit volume.

##### Pressure

The pressure is the force acting normally on unit area of a surface.

##### Stress

Stress is the force per unit area of cross section.

##### Strain

Strain is the extension per unit length.

##### Young Modulus

The young modulus is the ratio of stress applied to a body to strain produced.

##### Specific Heat Capacity

The Specific Heat Capacity is the heat energy required to increase the temperature of 1 kg by 1 K.

##### Molar heat capacity

The molar heat capacity is the ratio of heat supplied to unit amount of a substance to its consequent rise in temperature.

##### Specific latent heat

The specific latent heat of fusion or vaporization is the heat energy required to change the state of 1 kg of material from solid to liquid or liquid to vapor without change of temperature.

##### Activity

The activity is the number of disintegrations per second.

##### Decay constant

The decay constant is the probability of each atom of a source decaying in one second (s -1).

##### Half Life

The half life is the time taken for the number of atoms of that isotope to decrease to half the initial number.

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