Which fluid produces the greater buoyancy force

Why is it that some things float and others do not? Do objects that sink get any support at all from the fluid? Is your body buoyed by the atmosphere, or are only helium balloons affected Figure?

Answers to all these questions, and many others, are based on the fact that pressure increases with depth in a fluid. This means that the upward force on the bottom of an object in a fluid is greater than the downward force on top of the object. There is an upward force, or buoyant force , on any object in any fluid Figure.

The buoyant force is always present, whether the object floats, sinks, or is suspended in a fluid. This change in pressure and associated upward force on the bottom of the cylinder are greater than the downward force on the top of the cylinder. The differences in the force results in the buoyant force. Horizontal forces cancel. To answer this question, think about what happens when a submerged object is removed from a fluid, as in Figure.

If the object were not in the fluid, the space the object occupied would be filled by fluid having a weight. The buoyant force on an object equals the weight of the fluid it displaces. This principle is named after the Greek mathematician and inventor Archimedes ca. Since this weight is supported by surrounding fluid, the buoyant force must equal the weight of the fluid displaced.

The force that provides the pressure of a fluid acts on a body perpendicular to the surface of the body. In other words, the force due to the pressure at the bottom is pointed up, while at the top, the force due to the pressure is pointed down; the forces due to the pressures at the sides are pointing into the body. Since the bottom of the body is at a greater depth than the top of the body, the pressure at the lower part of the body is higher than the pressure at the upper part, as shown in Figure.

Therefore a net upward force acts on the body. This upward force is the force of buoyancy, or simply buoyancy. If you drop a lump of clay in water, it will sink. This upward force is the force of buoyancy, or simply buoyancy. Some say it all started in a bathtub. To hear this story, watch this video or explore Scientific American to learn more. If you drop a lump of clay in water, it will sink. But if you mold the same lump of clay into the shape of a boat, it will float.

Because of its shape, the clay boat displaces more water than the lump and experiences a greater buoyant force, even though its mass is the same. The same is true of steel ships. The average density of an object is what ultimately determines whether it floats. The reason is that the fluid, having a higher density, contains more mass and hence more weight in the same volume. The buoyant force, which equals the weight of the fluid displaced, is thus greater than the weight of the object.

Likewise, an object denser than the fluid will sink. In Figure We can derive a quantitative expression for the fraction submerged by considering density. The fraction submerged is the ratio of the volume submerged to the volume of the object, or. The volume submerged equals the volume of fluid displaced, which we call V f l V f l.

This gives. Since the object floats, its mass and that of the displaced fluid are equal, so they cancel from the equation, leaving. Here we are given the maximum volume of water the steel boat can displace.

The buoyant force is the weight of this volume of water. The mass of water displaced is found from its relationship to density and volume, both of which are known. That is,. The maximum buoyant force is ten times the weight of the steel, meaning the ship can carry a load nine times its own weight without sinking.

A piece of household aluminum foil is 0. Use a piece of foil that measures 10 cm by 15 cm. Test your prediction. The average density of an object is what ultimately determines whether it floats. If its average density is less than that of the surrounding fluid, it will float. This is because the fluid, having a higher density, contains more mass and hence more weight in the same volume.

The buoyant force, which equals the weight of the fluid displaced, is thus greater than the weight of the object. Likewise, an object denser than the fluid will sink. In Figure , for example, the unloaded ship has a lower density and less of it is submerged compared with the same ship loaded.

We can derive a quantitative expression for the fraction submerged by considering density. The fraction submerged is the ratio of the volume submerged to the volume of the object, or. The volume submerged equals the volume of fluid displaced, which we call.

Now we can obtain the relationship between the densities by substituting into the expression. Since the object floats, its mass and that of the displaced fluid are equal, and so they cancel from the equation, leaving.

We use this last relationship to measure densities. This is done by measuring the fraction of a floating object that is submerged—for example, with a hydrometer.

It is useful to define the ratio of the density of an object to a fluid usually water as specific gravity :. Specific gravity is dimensionless, independent of whatever units are used for. If an object floats, its specific gravity is less than one. If it sinks, its specific gravity is greater than one. Moreover, the fraction of a floating object that is submerged equals its specific gravity. Scuba divers try to obtain this state so that they can hover in the water. We measure the specific gravity of fluids, such as battery acid, radiator fluid, and urine, as an indicator of their condition.

One device for measuring specific gravity is shown in Figure. Specific gravity is the ratio of the density of an object to a fluid usually water.

What is her average density? Entering the known values into the expression for her density, we obtain. Her density is less than the fluid density.

We expect this because she floats. Less obvious examples include lava rising in a volcano and mountain ranges floating on the higher-density crust and mantle beneath them. Even seemingly solid Earth has fluid characteristics. One of the most common techniques for determining density is shown in Figure.

An object, here a coin, is weighed in air and then weighed again while submerged in a liquid. The density of the coin, an indication of its authenticity, can be calculated if the fluid density is known. This same technique can also be used to determine the density of the fluid if the density of the coin is known. We can derive a quantitative expression for the fraction submerged by considering density.

The fraction submerged is the ratio of the volume submerged to the volume of the object, or. Density and Submersion : An unloaded ship a floats higher in the water than a loaded ship b. This gives. Since the object floats, its mass and that of the displaced fluid are equal, and so they cancel from the equation, leaving.

Privacy Policy. Skip to main content. Search for:. Learning Objectives Calculate the direction of the buoyancy force. Key Takeaways Key Points The buoyancy force is caused by the pressure exerted by the fluid in which an object is immersed. The buoyancy force always points upwards because the pressure of a fluid increases with depth.

Key Terms buoyant force : An upward force exerted by a fluid that opposes the weight of an immersed object. Archimedes principle : The buoyant force exerted on a body immersed in a fluid is equal to the weight of the fluid the body displaces.

Learning Objectives Identify factors determining the buoyancy force on a completely submerged object. Key Takeaways Key Points If an object is completely submerged, the volume of the fluid displaced is equal to the volume of the object. The buoyancy force on hot-air balloons, dirigibles and other objects can be calculated by assuming that they are entirely submerged in air.

The buoyancy force does not depend on the shape of the object, only on its volume. Key Terms Archimedes principle : The buoyant force exerted on a body immersed in a fluid is equal to the weight of the fluid the body displaces. Learning Objectives Express the relationship between the buoyancy force and the weight for a floating object. Licenses and Attributions. CC licensed content, Shared previously.