So far we have studied the behavior of planes and bodies in a medium where there is air or vacuum, ie the medium does not interfere with the behavior.

But what if we apply a force to a body lying on water or some other fluid?

We know the effect will be different. If we study the properties of a liquid in static equilibrium, these properties can be extended to other fluids.

We call hydrostatic the science that studies liquids in static equilibrium.


Fluid is a substance that has the ability to flow. When a fluid is subjected to a tangential force, it deforms continuously, that is, when placed in any container, the fluid acquires its shape.

We can consider them as liquid fluids and gases.

Particularly, when talking about liquid fluids, we must talk about their viscosity, which is the friction between their molecules during a movement. The lower the viscosity, the easier the fluid flow.


Looking at scissors, we see that the side where it cuts, the blade, is thinner than the rest of the scissors. We also know that the thinner we call the "scissors thread," the better it will cut.

This happens, because when applying a force, we cause a pressure directly proportional to this force and inversely proportional to the area of ​​application.

In the case of scissors, the smaller the "scissors wire" the more intense the pressure of a force applied to it.

The pressure unit in SI is Pascal (Pan), which is the name adopted for N / m².

Mathematically, the mean pressure equals the quotient of the forces perpendicular to the application surface and the area of ​​this surface.


p = Pressure (Pa)

F = Strength (N)

A = Area (m²)


An intensity force 30N is applied perpendicular to the surface of a block of 0.3 m² area, what is the pressure exerted by this force?


When we compare two bodies made of different materials, but with the same volume, when we say that one is heavier than the other, we are actually referring to its density. The correct statement would be that one body is denser than the other.

The density unit in SI is kg / m³.

Density is the quantity that relates the mass of a body to its volume.


d = Density (kg / m³)

m = mass (kg)

V = Volume (m³)


What is the mass of a body of volume 1m³, if this body is made of iron?

Given: iron density = 7.85g / cm³

Converting density to SI: