We were discussing the various basic concepts of strength of materials and also the basic concepts of thermal engineering in our previous posts. After looking strength of materials and thermal engineering topics, we will go ahead to start a new subject i.e. Fluid mechanics with the help of this post.

Before going ahead, we must have to find the post indicating the basic definition and significance of fluid mechanics.
Now we will understand here the various properties of fluid with the help of this post.

### Density or Mass density

Density or mass density is basically defined as the ratio of mass of fluid to the volume of fluid. Density of fluid will be displayed by the symbol ρ.

We can write the basic formula for mass density as mentioned here.
Density of fluid = Mass of fluid /Volume of fluid
ρ = m /v

#### Unit of density

Unit of density will be Kg/m3 in S.I system.

Density of gas changes according to pressure, but density of liquid may be considered as unchangeable in general.

Water is considered as the standard liquid and therefore we usually take water as reference liquid. Density of water at 4 0C and 1 atm pressure is 1000 Kg/m3.

### Weight density or specific weight

Weight density or specific weight is basically defined as the ratio of weight of fluid to the volume of fluid. Weight density will be displayed by symbol w.

We can write the basic formula for weight density as mentioned here.

Weight density of fluid = Weight of fluid /Volume of fluid
w = W /v
w = m. g /v
w = ρ. g

Weight density of fluid = Mass density of fluid x Acceleration due to gravity

#### Unit of weight density

Unit of weight density of fluid will be N/m3

As stated above, water is considered as the standard liquid and therefore we usually take water as reference liquid. Specific weight or weight density of water at 4 0C and 1 atm pressure is 9810 N/m3.

### Specific volume

Specific volume is basically defined as the ratio of volume of fluid to the mass of fluid. Specific volume will be displayed by symbol v.

We can write the basic formula for specific volume as mentioned here.

Specific volume = Volume of fluid /Mass of fluid
v = V /m
Specific volume = 1/Density

Therefore, we can also say that specific volume could be defined as the reciprocal of mass density of the fluid.

#### Unit of specific volume

Unit of specific volume will be m3/Kg

### Specific Gravity

Specific gravity is basically defined as the ratio of mass density of liquid to the mass density of standard liquid. We already stated above that water will be considered as standard liquid.

Therefore, we will again specify the specific gravity as the ratio of mass density of liquid to the mass density of water.

We can also define the specific gravity as the ratio of weight density of liquid to the weight density of water.

Specific gravity = Mass density of liquid / Mass density of water
Specific gravity = Weight density of liquid / Weight density of water

Specific gravity will be displayed by symbol s. Specific gravity is basically one ratio of mass density of given liquid to the mass density of water and therefore specific gravity will be unit less.

We will discuss the term Viscosity of fluid in our next post.

Do you have suggestions? Please write in comment box.

### Reference:

Fluid mechanics by Y. Nakayama and R F Boucher