What is buoyancy?

Buoyancy is the name given to strength exercised by a fluid on an object fully or partially immersed in it. Also known as Archimedes' Principle, the thrust always has a vertical direction and an upward direction.

As soon as any object is immersed in a fluid, it starts to displace it. The thrust corresponds to the Weight of the volume of liquid displaced by the object, so we can mathematically define the buoyancy as follows:

E = P_DES

E = m_DISTRICT g

Knowing that the density is the result of the ratio between the mass and volume of a substance, we can write that:

d = m/V → m = d. V

Rewriting the thrust equation, we have:

E = d_L. V_DES. g

For this equation, we have to:

d_L= density of the liquid;

V_DES= volume of liquid displaced;

g = acceleration of gravity.

NOTE: The volume of liquid displaced corresponds exactly to the immersed volume of the object in the liquid.

How does Buoyancy arise?

The buoyancy arises because of the difference in pressure existing between the bottom and top of an object immersed in a fluid. THE

pressure at the bottom, because of the greater depth, it is greater than the pressure at the top, which results in the appearance of an upward vertical force, the buoyancy.

Who explained the thrust?

This strength was explained by Archimedes, a Greek mathematician and engineer who was commissioned by a king to show whether his crown was solid gold or whether there were impurities mixed in with it. While taking a bath in a bathtub, he noticed that the water came out of the bathtub when he entered it and that his body was subjected to a kind of force. From there, he was able to develop his idea about buoyancy and state that:

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“Every body immersed in a fluid is acted upon by a vertical and upward force that corresponds to the volume of liquid displaced by the body.”

fluctuation cases

1) Body sinks:

If an object immersed in a fluid sinks, we can say that its density is greater than that of the liquid and that its weight is greater than the buoyancy.

2) Body in balance

If the body and fluid densities are equal, the body will remain in balance in the fluid and we can say that the buoyancy equals the body weight.

3) Floating body:

An object floats on the surface of a liquid whenever its density is less than the density of the liquid. In this case, the thrust acting on the body is greater than its weight.

Application example

The submarines used by the Armed Forces are a classic example of the application of thrust. While the submarine remains floating on the surface of the water, we can say that the thrust is equal to weight and the vessel remains in balance. To submerge, the submarine fills some reservoirs with water, thus its weight becomes greater than thrust and it sinks.

By Joab Silas
Graduated in Physics

Would you like to reference this text in a school or academic work? Look:

JUNIOR, Joab Silas da Silva. "What is buoyancy?"; Brazil School. Available in: https://brasilescola.uol.com.br/o-que-e/fisica/o-que-e-empuxo.htm. Accessed on June 28, 2021.

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