9 1 Density and Pressure Density The density

Density • The density of a substance of uniform composition is defined as its

Density, cont. • The densities of most liquids and solids vary slightly with changes

Specific Gravity • The specific gravity of a substance is the ratio of its

Pressure • The force exerted by a fluid on a submerged object at any

Measuring Pressure • The spring is calibrated by a known force • The force

Example – The Water Bed. A water bed is 2. 00 m on each

Variation of Pressure with Depth • If a fluid is at rest in a

Pressure and Depth • Examine the darker region, assumed to be a fluid –

Pressure and Depth equation • • Po is normal atmospheric pressure – 1. 013

Pascal’s Principle • A change in pressure applied to an enclosed fluid is transmitted

Pascal’s Principle, cont • The hydraulic press is an important application of Pascal’s Principle

In a car lift in a service station, compressed air exerts a force on

Pressure Measurements: Manometer • One end of the Ushaped tube is open to the

Absolute vs. Gauge Pressure • The pressure P is called the absolute pressure –

Pressure Measurements: Barometer • Invented by Torricelli (1608 – 1647) • A long closed

Pressure Values in Various Units • One atmosphere of pressure is defined as the

Blood Pressure • Blood pressure is measured with a special type of manometer called

Archimedes • 287 – 212 BC • Greek mathematician, physicist, and engineer • Buoyant

Archimedes' Principle • Any object completely or partially submerged in a fluid is buoyed

Buoyant Force • The upward force is called the buoyant force • The physical

Buoyant Force, cont. • The magnitude of the buoyant force always equals the weight

Buoyant Force, final • The buoyant force is exerted by the fluid • Whether

Archimedes’ Principle: Totally Submerged Object • The upward buoyant force is B = ρfluid.

Totally Submerged Object • The object is less dense than the fluid • The

Totally Submerged Object, 2 • The object is more dense than the fluid •

Archimedes’ Principle: Floating Object • The object is in static equilibrium • The upward

Archimedes’ Principle: Floating Object, cont • The forces balance • – Neglects the buoyant

You purchase a “gold” crown at Crowns-Я-Us. You hang it from a scale and

A raft is constructed of wood having a density of 600 kg/m 3. Its

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9 -1 Density and Pressure

Density • The density of a substance of uniform composition is defined as its mass per unit volume: • SI unit: kg/m 3 (SI) – Often see g/cm 3 (cgs) • 1 g/cm 3 = 1000 kg/m 3 Section 9. 2

Density, cont. • The densities of most liquids and solids vary slightly with changes in temperature and pressure • Densities of gases vary greatly with changes in temperature and pressure • The higher normal densities of solids and liquids compared to gases imply that the average spacing between molecules in a gas is about 10 times greater than the solid or liquid Section 9. 2

Specific Gravity • The specific gravity of a substance is the ratio of its density to the density of water at 4° C – The density of water at 4° C is 1000 kg/m 3 • Specific gravity is a dimensionless quantity Section 9. 2

Pressure • The force exerted by a fluid on a submerged object at any point is perpendicular to the surface of the object • The average pressure P is the force divided by the area Section 9. 2

Measuring Pressure • The spring is calibrated by a known force • The force the fluid exerts on the piston is then measured

Example – The Water Bed. A water bed is 2. 00 m on each side and 30. 0 cm deep. What is its weight and what pressure does it exert on the floor?

Variation of Pressure with Depth • If a fluid is at rest in a container, all portions of the fluid must be in static equilibrium • All points at the same depth must be at the same pressure – Otherwise, the fluid would not be in equilibrium – The fluid would flow from the higher pressure region to the lower pressure region Section 9. 4

Pressure and Depth • Examine the darker region, assumed to be a fluid – It has a cross-sectional area A – Extends to a depth h below the surface • Three external forces act on the region Section 9. 4

Pressure and Depth equation • • Po is normal atmospheric pressure – 1. 013 x 105 Pa = 14. 7 lb/in. 2 • The pressure does not depend upon the shape of the container Section 9. 4

Pascal’s Principle • A change in pressure applied to an enclosed fluid is transmitted undiminished to every point of the fluid and to the walls of the container. – First recognized by Blaise Pascal, a French scientist (1623 – 1662) Section 9. 4

Pascal’s Principle, cont • The hydraulic press is an important application of Pascal’s Principle • Also used in hydraulic brakes, forklifts, car lifts, etc. Section 9. 4

In a car lift in a service station, compressed air exerts a force on a small piston with a radius of 5. 00 cm. The second piston attached to the lift has a radius of 15. 0 cm. What force must the air produce to lift a 13, 300 N car? What is the air pressure required? Show that the work done by the pistons is the same.

Pressure Measurements: Manometer • One end of the Ushaped tube is open to the atmosphere • The other end is connected to the pressure to be measured • If P in the system is greater than atmospheric pressure, h is positive – If less, then h is negative Section 9. 5

Absolute vs. Gauge Pressure • The pressure P is called the absolute pressure – Remember, P = Po + ρ gh • P – Po = ρ gh is the gauge pressure Section 9. 5

Pressure Measurements: Barometer • Invented by Torricelli (1608 – 1647) • A long closed tube is filled with mercury and inverted in a dish of mercury • Measures atmospheric pressure as ρgh Section 9. 5

Pressure Values in Various Units • One atmosphere of pressure is defined as the pressure equivalent to a column of mercury exactly 0. 76 m tall at 0 o C where g=9. 806 m/s 2 • One atmosphere (1 atm) = – 76. 0 cm of mercury – 1. 013 x 105 Pa – 14. 7 lb/in 2 Section 9. 5

Blood Pressure • Blood pressure is measured with a special type of manometer called a sphygmomanometer • Pressure is measured in mm of mercury Section 9. 5

Archimedes • 287 – 212 BC • Greek mathematician, physicist, and engineer • Buoyant force • Inventor Section 9. 6

Archimedes' Principle • Any object completely or partially submerged in a fluid is buoyed up by a force whose magnitude is equal to the weight of the fluid displaced by the object Section 9. 6

Buoyant Force • The upward force is called the buoyant force • The physical cause of the buoyant force is the pressure difference between the top and the bottom of the object Section 9. 6

Buoyant Force, cont. • The magnitude of the buoyant force always equals the weight of the displaced fluid • The buoyant force is the same for a totally submerged object of any size, shape, or density Section 9. 6

Buoyant Force, final • The buoyant force is exerted by the fluid • Whether an object sinks or floats depends on the relationship between the buoyant force and the weight Section 9. 6

Archimedes’ Principle: Totally Submerged Object • The upward buoyant force is B = ρfluid. Vobjg • The downward gravitational force is W = mg = ρobj. Vobjg • The net force is B – W = (ρfluid-ρobj)Vobjg Section 9. 6

Totally Submerged Object • The object is less dense than the fluid • The object experiences a net upward force Section 9. 6

Totally Submerged Object, 2 • The object is more dense than the fluid • The net force is downward • The object accelerates downward Section 9. 6

Archimedes’ Principle: Floating Object • The object is in static equilibrium • The upward buoyant force is balanced by the downward force of gravity • Volume of the fluid displaced corresponds to the volume of the object beneath the fluid level Section 9. 6

Archimedes’ Principle: Floating Object, cont • The forces balance • – Neglects the buoyant force of the air Section 9. 6

You purchase a “gold” crown at Crowns-Я-Us. You hang it from a scale and its weight is 7. 84 N. You then weigh the crown while in a bucket of water, and the scale reads 6. 86 N. Is the crown pure gold?

A raft is constructed of wood having a density of 600 kg/m 3. Its surface area is 5. 70 m 2 and its volume is 0. 60 m 3. When placed in fresh water, to what depth is the bottom of the raft submerged?