Which principle of hydrodynamics refers to the upward force that water exerts on an object?

The principle of hydrodynamics that refers to the upward force that water exerts on an object is buoyancy. Buoyancy is the force that allows objects to float or rise in a fluid, such as water, and is based on the principle that any object submerged in a fluid experiences an upward force equal to the weight of the fluid displaced by that object.

This phenomenon occurs due to differences in pressure within the fluid; the pressure on the bottom of the submerged object is greater than the pressure on the top, resulting in a net upward force. This can be illustrated with various examples, such as a boat floating on the water or a submerged object feeling lighter when underwater.

Viscosity refers to the measure of a fluid's resistance to flow and does not describe an upward force. Drag is a force that opposes the motion of an object through a fluid, related to resistance against moving in the fluid rather than supporting it. Resistance is a more general term that may imply various forms of opposition in physics and does not specifically pertain to the upward force provided by a fluid. Understanding buoyancy is essential in fields such as marine engineering, fluid dynamics, and various applications in swimming and aquatic activities.