13.13
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, wh…
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’s weight, the object remains suspended at that depth in the fluid.
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Q1: Why does pressure increase with depth in a fluid?
Pressure increases with depth because the weight of the fluid above pushes down on the fluid below. The greater the depth, the more fluid weight accumulates overhead, creating higher pressure at lower levels. This pressure variation is fundamental to understanding how buoyant forces arise on submerged objects.
Q2: What causes the buoyant force on an object in a fluid?
The buoyant force results from the pressure difference between the bottom and top of a submerged object. Since pressure increases with depth, the object experiences greater pressure at its bottom than at its top. This pressure difference produces a net upward force known as the buoyant force, which is always present whether an object floats, sinks, or remains suspended.
Q3: How does an object's weight compare to buoyant force to determine if it floats or sinks?
If the buoyant force exceeds the object's weight, it rises to the surface and floats. If the buoyant force is less than the object's weight, the object sinks. When the buoyant force equals the object's weight, the object remains suspended at that depth. This relationship between density and archimedes principle determines whether objects float or sink in fluids.
Q4: Why does a floating object's buoyant force decrease when it protrudes above the fluid surface?
When an object protrudes above the fluid surface, air replaces the submerged volume at the top. Since air exerts much lower pressure than the fluid below, the pressure difference between the object's bottom and top decreases. Consequently, the buoyant force decreases until it equals the object's weight, allowing it to float in equilibrium.
Q5: Do objects that sink experience any buoyant force from the fluid?
Yes, sinking objects always experience a buoyant force. The buoyant force is present on all objects in a fluid, regardless of whether they float, sink, or remain suspended. For sinking objects, the buoyant force is simply less than the object's weight, resulting in a net downward force that causes the object to descend.
Q6: How can a submarine control whether it floats or sinks?
A submarine adjusts its buoyancy by controlling the amount of water in its ballast tanks. Adding water increases the submarine's overall weight, making the buoyant force less than its weight, causing it to sink. Removing water decreases its weight, allowing the buoyant force to exceed its weight, enabling it to rise and float at the surface.
Q7: What does the trajectory of fluid from container holes reveal about pressure variation?
Fluid exits holes at different heights with varying trajectories. Holes deeper in the container produce fluid that travels farther horizontally before falling, indicating greater pressure at greater depths. This demonstration directly shows how pressure increases with depth and explains why objects experience greater pressure at their bottom surfaces than at their tops.