19-sep-15 physics 1 (garcia) sjsu chapter 13 liquids

Apr 19, 2023 Physics 1 (Garcia) SJSU

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]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.


Check Yourself

In which case is the pressure greatest? A B

A B


Check Yourself

In which case is the pressure greatest? A B

AB


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.


Pressure in Liquids

Pressure in a liquid depends on depth.

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

High

Medium

Low

High

Medium

Low


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.


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.

Buoyancy

Weight

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

Pressure Pressure

Pressure


Buoyancy & Depth

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

1

2

3

4

5

6

Buoyancy

Buoyancy

Buoyancy


Archimedes’ PrincipleWeight of liquid displaced by floating or submerged

object equals the buoyant force on the object.


Demo: Take a Dip

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.

1000 g

1000 g

850 g

150 cc displaced

Beaker ofWater


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


Check Yourself

?

Place block of wood in the water. Scale reading goes up, down, or stays the same?

50 N

10 N Block

10 N Water

40 N Water


Check YourselfUse similar block of petrified wood. Scale reading goes up, down, or stays the same?

?

50 N

30 N Block

10 N Water

40 N Water


Check Yourself

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


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)


Demo: Diet or Regular?

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


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?


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

Solid iron ballVolume: 100 ccMass: 790 gDensity: 7.9 g/cc

Iron ball, 90% hollowVolume: 1000 ccMass: 790 gDensity: 0.79 g/cc


Iron Ships, Floating & Sinking

Iron ship floats since it is hollow inside.

If water floods the inside then it sinks.


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.


Demo: Hydrometer

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


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.”

www.grapestompers.com

A B

19-sep-15 physics 1 (garcia) sjsu chapter 13 liquids

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