Upthrust, also known as buoyant force, is an upward force exerted on an object submerged in a fluid (liquid or gas). It arises from the pressure difference between the top and bottom surfaces of the object.
Understanding the Pressure Difference
As stated in several sources (BBC Bitesize, reference provided), an object submerged in a fluid experiences greater pressure on its bottom surface than its top surface. This is because pressure in a fluid increases with depth. This pressure difference creates a net upward force—the upthrust.
How Upthrust Acts on an Object
The magnitude of the upthrust is equal to the weight of the fluid displaced by the object (Archimedes' principle, though not explicitly stated in the references). This means:
- Completely Submerged Objects: The upthrust is equal to the weight of the fluid that would occupy the volume of the completely submerged object.
- Partially Submerged Objects (Floating Objects): The upthrust is equal to the weight of the object itself. This is why floating objects are in equilibrium: upthrust balances the object's weight.
Examples of Upthrust in Action
- Ships floating on water: A large ship displaces a significant volume of water, generating enough upthrust to counteract its weight.
- Hot air balloons rising: The heated air inside the balloon is less dense than the surrounding cooler air, creating upthrust that lifts the balloon.
- Objects submerged in water: Even if an object sinks, upthrust still acts upon it, albeit it is less than the object's weight.
Factors Affecting Upthrust
The upthrust acting on an object depends on:
- Density of the fluid: Higher density fluids produce greater upthrust.
- Volume of the displaced fluid: Larger volumes displace more fluid, resulting in greater upthrust.
