When an object is completely or partially immersed in water, the pressure exerted by the liquid causes the object to experiences a push force from all directions that causes it to float. This pushing force is called upthrust. It always acts in a direction perpendicular to the object. The surface area of an object is directly proportional to the upthrust experienced by it. This means that the larger the surface area, the greater will be the upthrust experienced by it. Also, the pressure exerted by the liquid is directly proportional to the depth. This is the reason why the base of the object experiences more upthrust than the top of the object. In a nutshell, upthrust is a push force that acts on an object in the direction opposite to that of gravity. It is also known as buoyant force. The magnitude of upthrust can be calculated as a product of the volume of liquid displaced, the density of the liquid displaced, and acceleration due to gravity. The Greek scientist Archimedes was the first person to discover buoyancy. Upthrust force is also known as fluid force.
Buoyancy is a property of fluids by virtue of which it is able to exert a push force on the objects suspended in it. Buoyancy is exhibited by both liquids and gases. Helium is less dense in nature than the air and hence, helium-filled balloons rise and float up in the air because they experience greater upthrust than the balloons filled with normal air.
A force of gravity that acts on a ship in order to pull it towards the downward direction is opposed by a counterforce acting on it in the upward direction, called the upthrust. Both the forces acting on the ship are equal and opposite in nature. Hence, a balanced force is said to be existing. This balance between fluid force i.e., upthrust force and the gravitational force helps the ship to float on the surface of the water.
When a rubber cork is pushed into a container filled with water, it experiences an upward force that does not allow it to sink to the base of the container. The magnitude of the buoyant force is greater than the weight of the cork. Hence, upthrust causes the cork to float on the surface of the water.
4. An object immersed in water
When an object is completely immersed in water, it feels lighter. This is because of the upthrust acting on the object in the upward direction. Therefore, the upthrust is equal to the difference between the weight of the object in the air and the weight of the object in the water.
When a straw is kept idle in a glass filled with some liquid, it tends to float and pop out of the glass. The straw experiences a push force from all directions. This force compels the straw to move upwards and float. The lower portion of the straw that is submersed properly in the fluid experiences more upthrust force than the top portion of the straw. This is because buoyancy is directly proportional to the depth.
6. Floating in Sea
The upthrust force acting on our body helps us to float in a pool. It is comparatively easier to float in seas that contain salt-rich water than to float in freshwater ponds because salt adds mass to water and makes it denser. The dead sea, located in Israel, is the saltiest water body present on earth. Hence, a person is able to float in the dead sea with great ease. The upthrust force experienced by the body is equal to the water displaced by it.