# Sub Unit 2 – Fluid Force Pressure in Fluid Systems Objectives

 Date 11.11.2018 Size 2.59 Mb.

## Sub Unit 1.2 – Fluid Force

• Pressure in Fluid Systems

## Objectives

• Describe the four states of matter.
• Define density and pressure
• Explain why pressure in a fluid depends on depth in the fluid
• Explain why an object submerged in a fluid experiences a buoyant force
• Predict whether an object will sink or float in a given fluid.
• Explain how a force can be multiplied in a hydraulic lift.
• Explain where atmospheric pressure comes from.
• Describe how a barometer measures atmospheric pressure
• Explain the difference between absolute and gage pressure.

## States of Matter

• Matter can exist in four states:
• Solid (ice)
• Liquid (water)
• Gas (steam vapor)
• Plasma (Extremely hot ionized atoms)

## Fluids

• Fluids are materials that can flow, has no definite shape of its own, and conforms to the shape of its container.
• Liquids
• Gasses
• Fluid systems use both liquids (hydraulic) and gasses (pneumatic) to operate mechanical devices.

## City Water System

• Hydraulic system under pressure

## Density and Pressure

• Density is how much mass is contained in a given amount of space.
• Amount of matter per unit of volume.
• mass
• Density =
• volume
• m
• v
• r =
• (rho)

• English
• SI
• mass
• Density =
• volume
• ft
• lb
• m
• kg
• 3
• 3
• cm
• g
• 3
• or
• (slugs)

## Example

• The mass of 1 cm3 of water has a mass of 1g; therefore the density is 1g/1cm3.

## Weight Density

• Weight Density is the comparison of an object’s weight to it’s volume
• weight
• Weight Density =
• volume
• r =
• V
• w
• weight
• lb
• N
• ft
• 3
• m
• 3
• Units

## Weight Density of Water

• Water has a weight density of 62.4 lb/ft3.

## Pressure

• A force applied over a surface is pressure.
• force
• Pressure =
• area
• F
• P =
• A
• N
• lb
• ft
• 2
• m
• 2
• English
• SI
• Units

## Pressure Units

• N/m2 = 1 Pascal (Pa)
• 1000 Pa = 1kilopascal (kPa)
• lb/in2 = psi (pounds per square inch)

## Pressure and Depth

• Pressure increase with depth because of the additional weight of the fluid above.
• weight density
• height
• Pressure =
• r
• h
• w
• P =
• x
• x

## Buoyancy and Archimedes’ Principle

• Ptop = w x h
• Pbottom = w x (h + d)
• F = P x A
• Ftop = Ptop x A = (w x h) x A
• Fbottom = Pbottom x A = [w x (h+d)] x A
• Fbuoyant = Fbottom – Ftop = w Ad
• Fbuoyant = w x Vbrick = weight of water displaced

## Archimedes’ Principle

• An object immersed in a fluid has an upward force exerted on it equal to the weight of the fluid displaced by the object.
• Note: the buoyant force is based on the weight of the fluid displaced not on the weight of the object.

## Pascal’s Principle

• A change in pressure at any point in a confined fluid is transmitted undiminished throughout the fluid.
• P = F / A or
• F = P x A
• A = r2

## Atmospheric Pressure

• The weight of the air above an area.
• At sea level, a column of air extending up through the atmosphere, with a cross sectional area of 1m2, encloses about 10,000 kg of air.
• This air weighs about 1 x 105 N
• Therefore, atmospheric pressure is about 105 Pa or 100kPa at sea level.
• Decreases with altitude
• This is why your ears pop (equalization)

## Atmospheric Pressure

• Barometer – instrument used for measuring atmospheric pressure.
• At sea level the average atmospheric pressure is 101.3 kPa = 760mm of mercury = one atmosphere = 14.7 psi = 2117 lb/ft2

## Absolute and Gage Pressure

• Absolute pressure is the total pressure measured above zero (perfect vacuum).
• Gage pressure is the pressure measured above atmospheric pressure.
• Absolute pressure = gage pressure + atmospheric pressure
• Suppose a tire gage measures the pressure of a tire to be 30 psi;
• Absolute pressure = 30 psi + 14.7 psi = 44.7 psi
• The air inside the tire pushes out with a pressure of 44.7 psi. The atmosphere pushes in with a pressure of 14.7 psi. The difference is 30 psi – the gage pressure.

## Pressure is a Prime Mover

• Pressure acts like a force to cause movement.

## Summary

• Matter can exist in four states: solid, liquid, gas, and plasma.
• Liquids and gases are called fluids.
• The density of a substance is its mass per unit volume.
• The density of water is 1g/cm3.
• Weight density is weight per unit volume.
• Pressure is force divided by the area over which the force acts.
• We treat pressure as a scalar.
• In SI units, pressure is measured in pascals, where 1 Pa= 1 N/m
• Pressure increases with depth in a fluid.

## Summary

• For a given fluid, the pressure does not depend on the size or shape of the container.
• When an object is submerged in a fluid, an upward force is exerted on the object caused by the pressure difference between the top and the bottom of the object. This force is called a buoyant force.
• The buoyant force exerted on a submerged object equals the weight of the fluid displaced by the object.
• A pressure applied to a confined fluid is transmitted throughout the fluid.
• Atmospheric pressure is caused by the weight of the air above a given area.
• Atmospheric pressure can be measured with a barometer.
• Absolute pressure is the sum of the gage pressure and atmospheric pressure.