# Explain the relationship between fluid pressure and buoyancy force

### Pressure and buoyancy

Buoyancy arises from the fact that fluid pressure increases with depth and from Since the "water ball" at left is exactly supported by the difference in pressure Objects of equal volume experience equal buoyant forces. What is it's density?. The buoyancy force always points upwards because the pressure of a fluid . We will explore this further as we discuss applications of the principle in . Express the relationship between the buoyancy force and the weight for a floating object. So why do fluids exert an upward buoyant force on submerged objects? by simply taking the difference between the magnitudes of the upward force F u p We can relate these forces to the pressure by using the definition of pressure P = F.

Anything that can flow -- such as a gas -- is a fluid, and can create buoyant force.

Buoyancy is caused when areas of higher pressure beneath an object exert force upward toward areas of lower pressure. The amount of buoyant force that a fluid exerts, however, is determined by the object's volume and according to Archimedes' principle.

Pascal and Pressure Before you can understand how differences in fluid pressure can affect buoyancy, you need first to understand how pressure behaves in fluids.

Pascal's principle states that when pressure is changed at any location within a closed system, that pressure change will be felt equally at every point within that system and in all directions. This principle is that which allows hydraulic systems to function. It also dictates that within a body of fluid where there aren't any additional factors that affect pressure, the pressure will remain constant and even.

On Earth, however, there is usually at least one other force that causes a variance in the pressure of a fluid, and that force is gravity. Depth and Difference Gravity pulls downward on everything that has mass. Therefore, when gravity pulls downward on a body of fluid, the weight of the fluid in the upper parts of the body pile upon the the fluid in the lower parts, creating a grade of increasing pressure as you move downward within that fluid.

For example, if you dive deep into a lake, you will feel increasing pressure in your ears -- and perhaps even against your body -- the deeper you dive. If you stop swimming downward, the higher pressure below you will push you back up toward the area of lower pressure. This is very large, but it is not usually noticed because there is generally air both inside and outside of things, so the forces applied by the atmosphere on each side of an object balance.

It is when there are differences in pressure on two sides that atmospheric pressure becomes important.

### How Do Differences in Fluid Pressure Create Buoyancy? | Sciencing

A good example is when you drink using a straw: If point 2 lies a vertical distance h below point 1, there is a higher pressure at point 2; the pressure at the two points is related by the equation: Note that point 2 does not have to be directly below point 1; it is simply a vertical distance below point 1.

This means that every point at a particular depth in a static fluid is at the same pressure. Pascal's principle Pascal's principle can be used to explain how hydraulic systems work.

A common example of such a system is the lift used to raise a car off the ground so it can be repaired at a garage. Pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid, as well as to the walls of the container.

## What is buoyant force?

In a hydraulic lift, a small force applied to a small-area piston is transformed to a large force at a large-area piston. If a car sits on top of the large piston, it can be lifted by applying a relatively small force, the ratio of the forces being equal to the ratio of the areas of the pistons.

Even though the force can be much less, the work done is the same. Work is force times the distance, so if the force on the large piston is 10 times larger than the force on the smaller piston, the distance it travels is 10 times smaller. Measuring pressure The relationship between pressure and depth is often exploited in instruments that measure pressure.

Two pressure gauges based on this principle are the closed-tube manometer and the open-tube manometer, which measure pressure by comparing the pressure at one end of the tube with a known pressure at the other end. A standard mercury barometer is a closed-tube manometer, with one end sealed. The sealed end is close to zero pressure, while the other end is open to the atmosphere, or is connected to where the pressure is being measured. Because there is a pressure difference between the two ends of the tube, a column of fluid can be maintained in the tube, with the height of the column proportional to the pressure difference.

If the closed end is at zero pressure, then the height of the column is proportional to the pressure at the other end. In an open-tube manometer, one end of the tube is open to the atmosphere, and is thus at atmospheric pressure.

The other end is connected to a region where the pressure is to be measured. Again, if there is a difference in pressure between the two ends of the tube, a column of fluid can be supported in the tube, with the height of the column being proportional to the pressure difference.

The actual pressure, P2, is known as the absolute pressure; the pressure difference between the absolute pressure and atmospheric pressure is called the gauge pressure. Many pressure gauges give only the gauge pressure. According to legend, this is what Archimedes' cried when he discovered an important fact about buoyancy, so important that we call it Archimedes' principle and so important that Archimedes allegedly jumped from his bath and ran naked through the streets after figuring it out.

An object that is partly or completely submerged in a fluid will experience a buoyant force equal to the weight of the fluid the object displaces.

The buoyant force applied by the fluid on the object is directed up.

**Fluids in Motion: Crash Course Physics #15**

The force comes from the difference in pressure exerted on the top and bottom of an object. For a floating object, the top surface is at atmospheric pressure, while the bottom surface is at a higher pressure because it is in contact with the fluid at a particular depth in the fluid, and pressure increases with depth.

For a completely-submerged object, the top surface is no longer at atmospheric pressure, but the bottom surface is still at a higher pressure because it's deeper in the fluid.