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Chapter 13 Liquids. Pressure. Pressure is defined as Pressure = Metric unit of pressure is Pascal. 1 Pascal = 1 Newtons per square meter Atmospheric pressure is about 100,000 Pascals. ( Force ). ( Area ). Sample Problem. “Gold” brick is roughly: 12 kilograms [Mass]

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chapter 13 liquids

Chapter 13Liquids

Physics 1 (Garcia) SJSU

pressure
Pressure

Pressure is defined as

Pressure =

Metric unit of pressure is Pascal.

1 Pascal = 1 Newtons per square meter

Atmospheric pressure is about 100,000 Pascals

( Force )

( Area )

Physics 1 (Garcia) SJSU

sample problem
Sample Problem

“Gold” brick is roughly:

12 kilograms [Mass]

120 Newtons [Weight]

(0.2 m)x(0.1 m) =

(0.02 m2) [Area]

Pressure on surface is

(120)/(0.02)

= 6000 Pascals

Note: Atmospheric pressure is about 100,000 Pascals so much more than pressure due to the “gold” brick.

Physics 1 (Garcia) SJSU

c heck yourself

B

A

B

A

Check Yourself

In which case is the pressure greatest?

Physics 1 (Garcia) SJSU

c heck yourself5

A

B

Check Yourself

In which case is the pressure greatest?

A

B

Physics 1 (Garcia) SJSU

demo bed of nails
Demo: Bed of Nails

One may safely lay or sit on a bed of nails, as long as there are enough nails so that the pressure, measured as force per nail, is small.

Weight of 150 pounds is distributed over 300 nails. Force per nail is ½ lb. Need 5 lb per nail to pierce skin.

Physics 1 (Garcia) SJSU

pressure in liquids
Pressure in Liquids

Pressure in a liquid depends on depth.

As with bricks, weight of what’s above determines pressure.

Low

Medium

Low

Medium

High

High

Physics 1 (Garcia) SJSU

demo pressure weight
Demo: Pressure & Weight

A can full of water has holes in the sides through which water comes out.

What happens when you drop the can?

The can is now in freefall and weightless. Water stops flowing as the can falls since the pressure was due to the water’s weight.

Physics 1 (Garcia) SJSU

buoyancy

Buoyancy

Weight

Buoyancy

Since pressure depends on depth, a submerged object has more force due to pressure below it than above it.

Net effect is to have a net upward force, which we call buoyancy.

Pressure

Pressure

Pressure

If weight exceeds buoyancy force then object sinks, otherwise it floats.

Physics 1 (Garcia) SJSU

buoyancy depth
Buoyancy & Depth

For a fully submerged object the buoyancy force does not depend on depth, even though pressure depends on depth.

1

Buoyancy

2

3

Buoyancy

4

5

Buoyancy

6

Physics 1 (Garcia) SJSU

archimedes principle
Archimedes’ Principle

Weight of liquid displaced by floating or submerged object equals the buoyant force on the object.

Physics 1 (Garcia) SJSU

demo take a dip
Demo: Take a Dip

1000 g

A 1 kg mass (1000 g) submerged in water displaces 150 cm3.

Water has density of 1 g/cm3 so displaced water has mass of 150 grams.

Buoyant force equals weight of 150 grams.

Beaker of

Water

1000 g

850 g

150 cc displaced

Physics 1 (Garcia) SJSU

demo floating in the tub
Demo: Floating in the Tub

Buoyant force depends only on the volume of fluid displaced, not on the volume of fluid in which the object is immersed.

Rubber duck floats the same in a little water or in a full beaker of water

Physics 1 (Garcia) SJSU

c heck yourself14

40 N

Water

10 N

Water

Check Yourself

Place block of wood in the water.

Scale reading goes up, down, or stays the same?

10 N Block

50 N

?

Physics 1 (Garcia) SJSU

c heck yourself15

40 N

Water

10 N

Water

Check Yourself

Use similar block of petrified wood.

Scale reading goes up, down, or stays the same?

50 N

30 N Block

?

Physics 1 (Garcia) SJSU

c heck yourself16
Check Yourself

Did the designers of this “water bridge” have to account for the weight of ships or just the water?

Physics 1 (Garcia) SJSU

density floating
Density & Floating

By Archimede’s principle, a solid object will float if the density of the object is less than the density of the liquid.

Billiard ball (4.0 g/cm3) floating in a cup of mercury (13.6 g/cm3)

Physics 1 (Garcia) SJSU

demo diet or regular
Demo: Diet or Regular?

What can you say about the density of diet cola as compared with regular cola?

Physics 1 (Garcia) SJSU

ships
Ships

Density of wood is about 0.5 to 0.8 g/cm3 so not surprising that wooden ships float.

Density of iron 7.9 g/cm3. How is it that a battleship can float?

Physics 1 (Garcia) SJSU

density floating revisited
Density & Floating, Revisited

When an object is not solid then it floats if the average density, (total mass)/(total volume), is less than the density of the liquid.

Iron ball, 90% hollow

Volume: 1000 cc

Mass: 790 g

Density: 0.79 g/cc

Solid iron ball

Volume: 100 cc

Mass: 790 g

Density: 7.9 g/cc

Physics 1 (Garcia) SJSU

iron ships floating sinking
Iron Ships, Floating & Sinking

Iron ship floats since it is hollow inside.

If water floods the inside then it sinks.

Physics 1 (Garcia) SJSU

floating liquid density
Floating & Liquid Density

The greater the density of a liquid, the greater the buoyant force on objects floating or immersed in the liquid.

Floating in Great Salt Lake, Utah is easy because the lake water is dense due to high concentration of salt.

Physics 1 (Garcia) SJSU

demo hydrometer
Demo: Hydrometer

Hydrometer is a calibrated float; denser the liquid, the higher the hydrometer floats.

Physics 1 (Garcia) SJSU

density winemaking
Density & Winemaking

“Take a look at Diagram A, on the left, which represents a test jar full of must before the yeast is pitched.

Do you see how the hydrometer is floating rather high? This is because the liquid is "heavy" with all the sugar... the hydrometer is pushed up because of this.

As the yeast turns the sugar into alcohol and carbon dioxide, the wine becomes lighter (alcohol weighs less per unit volume) and the hydrometer doesn't float as high as it once did. Diagram B (seen on the right) represents a wine that has fermented to dryness and is lighter than water.”

B

A

Physics 1 (Garcia) SJSU

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