When we enter a pool, we feel lighter than when we are out of it.

This is due to an upward vertical force exerted by the water we call Buoyancy, and we represent it by .

Thrust represents the resulting force exerted by the fluid on a body. As it has the opposite direction to the force Weight, causes the effect of lightness in the case of the pool.

The unit of measure of thrust in SI is Newton (N).

Archimedes Principle

It was the Greek philosopher, mathematician, physicist, engineer, inventor and astronomer Archimedes (287 BC - 212 BC) who figured out how to calculate thrust.

Archimedes found that the whole body immersed in a fluid in equilibrium, within a gravitational field, is under the action of a vertical force, opposite to this field, applied by the fluid, whose intensity equals the intensity of the weight of the fluid. It is occupied by the body.

Like this:


= Thrust (N)

= Fluid density (kg / m³)

= Volume of displaced fluid (m³)

g = Acceleration of gravity (m / s²)


In a container there is a liquid of density 2,56g / cm³. Inside the liquid is a body of 1000cm³ volume, which is totally immersed. What is the thrust suffered by this body? Given g = 10m / s²

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The thrust value does not depend on the density of the body that is immersed in the fluid, but we can use it to know if the body floats, sinks, or remains in balance with the fluid:


  • body density> fluid density: the body sinks
  • body density = fluid density: the body is in equilibrium with the fluid
  • body density <fluid density: body floats on fluid surface

Apparent weight

Knowing the Archimedes principle we can establish the concept of apparent weight, which is responsible, in the given example of the pool, for feeling lighter when submerging.

Apparent weight is the effective weight, that is, what we really feel. In the case of a fluid: