Pressure, Buoyant Force and Density
Fluids and forces can be a bit tricky at times.
If you break their brhaviors down to a few key concepts, it’s really not all that daunting.
All fuids exert a downward pressure that increases with depth.
The reason is fairly simple – gravity pulls fluids downward. Fluids are made of molecules, which are made of matter, and therefore have mass. As a consequence of gravity, they also have weight.
As fluid molecules stack on top of one another, their incremental weights add up…the farther down a column of a fluid you travel, the greater the mass of molecules above, and the greater the pressure (weight) exerted downward.
This pattern is true of the atmosphere, where atmospheric pressure is the lower at higher elevations than at lower elevations, even though we rarely think of air having weight.
As you move upward along the mountain in the animation above, notice the number of air particles surrounding each hiker – they decrease with elevation.
This pattern is also true in regards to water. Water pressure increases significantly with depth – much more so in fact than air pressure – because of two main factors: density and, well, basic addition.
To begin with, water is much denser than air. It therefore exerts a greater downward pressure than air (there are, simply, more molecules sacked together in a similar quantity of water than in air).
The dramatic increase in water pressure is also due in part to basic addition – above water, is air. The “weight” of air is weighing down on water as well, adding to the pressure water exerts with depth.
Just as fluids exert downward force, they also exert an upward force on all objects in them – called buoyant force.
Archimedes discovered that the upward force exerted by a fluid is equal to the weight of the water that is displaced by an object in that fluid.
Objects heavier than this force will sink, objects that are lighter will float.
This concept is well related to density.
Objects that have a higher density than the fluid in which they are placed will sink, and those with a lower density will float.
Knowing this, we can use density calculations to determine the behavior of objects in a fluid – a nifty skill if you’re engineering a boat, say.
The formula for calculating density is D = M/V (or, density = mass/volume).
Because the relationship between mass and volume determines the density of objects, we can adjust either variable to affect overall density (and consequently affect the behavior of an object in a fluid).
Ways to affect density:
- Change the shape: If you change the shape of an object without changing its mass, you effectively change the surface area and overall volume, thereby changing the density. If you increase the volume of an object, you decrease the density and vice versa.
This is exactly how ships work – were the materials used in huge ships to be compressed into a lower volume, they would sink. But, because their overall surface area is large enough to compensate for their high masses, the float! - Change the mass: If you change the mass of an object without changing its volume, you can drastically affect density. Increased masses will increase density; decreased masses decrease density.
This process is how submarines work. Subs have ballast tanks that fill with water (increasing their mass) when submarines dive. Conversely, they push water out of the vessel when rising to the surface.
Objects can also behave in a different way (regarding buoyancy) in fluids. When objects float, they are considered to have positive buoyancy. When they sink, negative buoyancy.
If the density of an object is the same as the fluid in which it exists, it is considered to have neutral buoyancy, and neither floats, nor sinks, but flinks.
This is how submarines “hover” in columns of water – they equalize their density to the surrounding fluid.
Tags: Buoyancy, Density, Fluids, Phsysics, Pressure
Posted in Forces & Motion, Physics & Physical Science
