Difference Between Buoyancy and Upthrust

Main Difference – Buoyancy vs Upthrust

The two terms Buoyancy and Upthrust are frequently used in fundamental physics. Even though some people use them interchangeably, they have different and particular meanings. Buoyancy is the ability of an object to float in a fluid whereas upthrust is a force exerted on an object by a fluid within which the object is submerged. Simply, buoyancy is just a tendency of an object to rise in a given fluid when it is submerged. But upthrust is the buoyant force exerted on the object by the fluid. This is the main difference between buoyancy and upthrust.

What is Buoyancy

Buoyancy is what we perceive as the tendency of an object to rise when it is submerged in a fluid. It can be defined as the ability of an object to float in a fluid (liquids or air). Buoyancy can also be explained as the ability of a fluid to make a given object float in the fluid. However, the power of the buoyancy or the ability of an object to float depends on the density of the fluid as well as the properties of the object (density of the object, shape of the object, etc.). The denser the fluid, the greater the buoyancy effect of the fluid. The lighter the object, the greater the buoyancy effect of the object. Simply, the power of the buoyancy is determined by the density difference between the fluid and object. However, the effective density of an object can be controlled in different ways. For example, the effective density of a hollow cylinder is less than that of a solid cylinder made of the same material.

There are hundreds of applications of buoyancy effect such as hydrometers, ships and, submarines. The buoyancy effect can also be used to determine the density of solids and liquids.

What is Upthrust

Upthrust, also known as the buoyant force, is the force exerted by a fluid on an object. As the term suggests, upthrust always acts upwards. Therefore, upthrust always acts against the weight of an object. The buoyancy of a given object in a particular fluid is determined by the upthrust acting on the object.

If the upthrust is equal to the weight of the object, it will float in the fluid. Flotation can be seen when the effective density of the object is less than that of the liquid. But if the effective density of the object is greater than that of the liquid, the weight of the object becomes greater than the upthrust exerted by the liquid on the object. So, the object sinks in the fluid.

The magnitude of the upthrust can be determined using the Archimedes’ principle. The Archimedes’ principle states that the upthrust is precisely equal to the weight of the fluid displaced by an object.

Upthrust is useful in many applications, especially in marine engineering and cargo transport. Vessels such as ships and boats are designed in such a way that the weight of water displaced by a vessel equals the weight of the vessel when it is partially submerged in the sea. Therefore, they float in the sea according to the Archimedes’ principle. The volume of the underwater portion of the hull gradually increases with the weight of the load. In order to avoid overloading, it is very important to monitor the power of the upthrust (stability of the vessel) while loading. Normally, the hull of a vessel is marked with load lines (also known as Plimsoll lines) which help to monitor the stability of the vessel.

Unlike ordinary vessels or ships, submarines are designed to operate underwater. A submarine has a ballast tank which is used to control its depth. When water is pumped into the ballast tank, the weight of the submarine becomes greater than that of the sea water (upthrust) displaced by the submarine. So, the submarine sinks according to the Archimedes’ principle. When water is released from the ballast tanks, the weight of the submarine becomes less than that of the sea water (upthrust) displaced by the submarine. So, the submarine moves upward. Submarines can travel at a fixed depth by balancing the upthrust and its weight.