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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

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### Sub Unit 1.2 – Fluid Force

Pressure in Fluid Systems

• 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.

• Matter can exist in four states:

• Solid (ice)

• Liquid (water)

• Gas (steam vapor)

• Plasma (Extremely hot ionized atoms)

• 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.

Hydraulic system under pressure

• Density is how much mass is contained in a given amount of space.

• Amount of matter per unit of volume.

mass

Density =

(rho)

volume

m

r =

v

mass

Density =

• English

volume

SI

(slugs)

kg

lb

g

or

3

3

ft

3

m

cm

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

• Weight Density is the comparison of an object’s weight to it’s volume

weight

Weight Density =

volume

weight

lb

N

r =

w

3

V

ft

3

m

Units

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

• A force applied over a surface is pressure.

force

Pressure =

area

Units

F

lb

N

P =

2

2

A

ft

m

English

SI

• N/m2 = 1 Pascal (Pa)

• 1000 Pa = 1kilopascal (kPa)

• lb/in2 = psi (pounds per square inch)

• Pressure increase with depth because of the additional weight of the fluid above.

Pressure =

weight density

x

height

P =

r

h

x

w

• Ptop = rw x h

• Pbottom = rw x (h + d)

• F = P x A

• Ftop = Ptop x A = (rw x h) x A

• Fbottom = Pbottom x A = [rw x (h+d)] x A

• Fbuoyant = Fbottom – Ftop = rw Ad

• Ad = Vbrick

• Fbuoyant = rw x Vbrick = weight of water displaced

• 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.

• 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 = p r2

• 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)

• 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 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 acts like a force to cause movement.

• 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 ofwater 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.

• 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.