Name: buoyant force in sinking and floating
Uploaded: 2025-12-22T10:52:41.000Z
Duration: 1 min 12 s
Channel: JoVE

K12 Standards

Alabama Course of Study Science

9-12

Physics

motion-and-stability-forces-and-interactions

Motion and Stability: Forces and Interactions

9-12.P.1.4

Buoyancy

In a fluid, the pressure increases with depth due to an increase in the weight of the fluid pushing it down from above.Consider a fluid in a container. With multiple holes on its walls at different heights, fluid from each hole will follow a different trajectory.The greater the pressure, the farther the fluid falls from the container.Thus, an object in a fluid experiences a greater pressure at its bottom than at its top.This pressure difference produces a net upward force on the object known as the buoyant force.If the buoyant force on a completely submerged object is greater than its weight, it rises to the surface and floats.When the object protrudes above the surface of the fluid, its buoyant force decreases because the air at the top exerts a lower pressure than the water at the bottom. Since its buoyant force is equal to its weight, it floats.If the buoyant force is less than its weight, the object sinks.If the buoyant force equals the object&#8217;s weight, the object remains suspended at that depth in the fluid.

buoyancy

When an object is placed in a fluid, it either floats or sinks. All objects in a fluid experience a buoyant force. For example, a metal ball sinks, while a rubber ball floats. Similarly, a submarine can sink and float by adjusting its buoyancy.&#160; The concept of buoyancy raises several interesting questions. For instance, where does this buoyant force come from? How much buoyant force is required to make an object sink or float? Do objects that sink get any support at all from the fluid?&#160;
To get an insight into these questions, we need to understand how fluid pressure varies with depth. The weight of a fluid increases with an increase in depth; thus, the pressure also increases. This increase in pressure creates a difference in pressure at the top and the bottom surface of the object, leading to an upward force known as the buoyant force. If the buoyant force is greater than the object's weight, the object rises to the surface and floats. If the buoyant force is less than the object's weight, the object sinks. If the buoyant force is equal to the object's weight, the object remains suspended at that particular depth. The buoyant force is always present, whether the object floats, sinks, or remains suspended in the fluid.
This text is adapted from<a href='https://openstax.org/books/university-physics-volume-1/pages/14-4-archimedes-principle-and-buoyancy'> Openstax, University Physics Volume 1, Section 14.4: Archimedes&#8217; Principle and Buoyancy. </a>

When an object is placed in a fluid, it either floats or sinks. All objects in a fluid experience a buoyant force. For example, a metal ball sinks, while ...

Education

JoVE Core

Unit 1

Fluid Mechanics

KEY TERMS AND CONCEPTS

buoyant force in sinking and floating

Buoyant force explains why objects float, sink, or stay suspended in a fluid. A fluid can be a liquid or a gas, and every object placed in it experiences this upward force. A metal ball sinks, while a rubber ball floats. A submarine also changes its buoyancy to move up or down.

The force comes from pressure changes in the fluid. Fluid pressure increases with depth because the weight of the fluid above also increases. That means the bottom of an object feels more pressure than the top. This pressure difference creates the upward buoyant force.

Buoyancy depends on how the upward force compares with the object’s weight. If the buoyant force is greater than the weight, the object rises and floats at the surface. If it is less than the weight, the object sinks. If the two forces are equal, the object stays suspended at that depth.

The buoyant force is always present, even when an object sinks. So sinking objects still get support from the fluid. Understanding this idea helps explain why some objects float, why some do not, and how submarines can control their motion in water.

Buoyant Force in Sinking and Floating

characteristics of fluids

Fluid Behavior Under Force

Fluid behavior under force differs from the behavior of solids. When a force acts parallel to the top surface of a solid, the solid resists it because of internal friction between its layers. This is called shearing resistance. If the force is removed, the shearing forces help the solid return to its original shape. Other deformation forces can also change the shape of a solid for a short time, as long as the force stays below a threshold magnitude. Solids usually keep their shape, which makes...

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density

Density, Floating, and Specific Gravity

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pressure of fluids

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variation of atmospheric pressure

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pascal's law

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application of pascal's law

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archimedes' principle

Buoyant Force from Fluid Pressure

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Fluid pressure changes when a liquid accelerates inside a moving container. A beaker placed in an elevator that moves upward with constant acceleration is one example. In this setup, the pressure and buoyant force equations must be modified. To find the pressure difference, the liquid is modeled as a thin vertical cylinder with height h and an infinitesimal cross-sectional area, ΔS. Three vertical forces act on this small fluid element. The liquid below the bottom surface pushes upward, the...

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Buoyant force explains why objects float, sink, or stay suspended in a fluid. A fluid can be a liquid or a gas, and every object placed in it experiences this upward force. A metal ball sinks, while a rubber ball floats. A submarine also changes its buoyancy to move up or down. The force comes from pressure changes in the fluid. Fluid pressure increases with depth because the weight of the fluid above also increases. That means the bottom of an object feels more pressure than the top. This...

contact angle

Wetting and Surface Curvature

Wetting and surface curvature describe how a liquid behaves where it meets a solid. When a solid is dipped into a liquid, the liquid surface can bend near the contact line. In some cases, the liquid rises along the solid. In others, the surface becomes convex or depressed near the solid. This behavior comes from cohesive and adhesive forces. Adhesive force is the attraction between molecules of different materials, such as a solid and a liquid. Cohesive force is the attraction between...

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rise of liquid in a capillary tube

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Bernoulli's equation links fluid pressure and fluid speed in a static, incompressible fluid. For horizontal flow, where gravitational energy density stays constant, the equation is especially useful. In that setting, the result is known as Bernoulli's principle. Bernoulli's principle says that fluid pressure drops when speed increases, and pressure rises when speed decreases. This pressure-speed relationship helps explain how moving fluids behave in simple flow situations. It also provides a...

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energy conservation and bernoulli's equation

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Bernoulli’s Equation in Streamlined Flow

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viscosity

Viscosity and How Fluids Flow

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stokes' law

Terminal Velocity in a Viscous Fluid

Viscous forces act like friction in a liquid. They resist the relative motion of molecules and pull back on a solid body moving through the fluid. The size of this force depends on the body’s shape and size, its speed, and the liquid’s viscosity, density, and temperature. For a spherical solid object, the force in a fluid is described by Stokes’ law. This law applies only at low Reynolds number, when the flow is laminar and not turbulent. Experiments show that the force is proportional to the...

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key terms and concepts

Explore how fluid pressure creates buoyant force and why objects float, sink, or stay suspended. Includes real examples like metal balls and submarines.

Buoyant Force in Sinking and Floating | JoVE K12

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Buoyant Force in Sinking and Floating

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