The upward force of a fluid on an object is called buoyancy. It's the force that pushes an object upward when it's submerged in a fluid (liquid or gas).

Here's a breakdown of what it means:

* Archimedes' Principle: Buoyancy is governed by Archimedes' Principle, which states that the buoyant force on an object submerged in a fluid is equal to the weight of the fluid displaced by the object.

* Density and Volume: The amount of buoyant force depends on the density of the fluid and the volume of the object submerged. A denser fluid will exert a greater buoyant force on an object. A larger submerged volume will displace more fluid and experience a larger buoyant force.

* Float or Sink: If the buoyant force is greater than the object's weight, the object will float. If the buoyant force is less than the object's weight, the object will sink.

Examples of Buoyancy:

* Ships: Ships float because they displace a large volume of water, creating a buoyant force greater than their weight.

* Hot Air Balloons: Hot air balloons rise because the hot air inside the balloon is less dense than the surrounding air, resulting in a buoyant force that lifts the balloon.

* Submarines: Submarines can control their buoyancy by adjusting the amount of water they take in or release, allowing them to dive and resurface.

Let me know if you'd like to dive into any specific aspects of buoyancy!