Chapter 6: Fluid Mechanics

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Oct 24, 2013 (3 years and 9 months ago)

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Chapter 6: Fluid Mechanics

Answers to Chapter 6 Review Questions

1.

Liquid and gas, which both flow.

2.

The volume decreases; the mass remains the same; the density increases.

3.

Mass density is mass/volume; weight density is weight/volume.

4.

Force is pu
sh or pull, or an interaction; pressure is push or pull
per amount of
surface area
.

5.

Both directly proportional; more depth, more pressure. More density, more
pressure.

6.

Twice as deep produces twice the water pressure. Greater pressure at the
same dept
h in salt water because it is denser.

7.

The same.

8.

Initial flow is perpendicular to the surface of the container.

9.

Because pressure up against bottom is
greater than pressure down against top.
Pressure is a function of the depth.

10.

Same.

11.

An imm
ersed body is buoyed up by a force equal to the weight of the fluid it
displaces.

12.

Both imply water displacement and submerged means completely immersed
with complete volume displacement.

13.

Same.

14.

1 kilogram. 10 N (or more precisely, 9.8 N).

15.

1/
2 L. Weight of 1/2 L of water, 5 N (or more precisely, 4.9 N).

16.

Depends only on weight of water displaced.


Volume of object, because amount of buoyant force depends on that volume
of water displaced.

17.

Floating. Then buoyant force and weight of obje
ct have the same magnitude.

18.

Yes, for the volume of the submerged object is the same volume of fluid
displaced. The weight of this displaced fluid is the buoyant force.

19.

When floating, buoyant force depends on both, because weight of object =
Elimina
te Autoformatting weight of displaced fluid. In either case the net
force on the object is zero, meaning weight = buoyant force.

20.

100 tons; 100 tons.

21.

Distance between molecules is greater in a gas.

22.

By twice.

23.

Increases by twice.

24.

“Pressure



volume” for a quantity of gas at one time is equal to any
“different pressure


different volume” at any other time.

25.

Weight of air in the atmosphere.

31.

Pushed, because atm pressure is doing the pushing. When you suck on the
straw, all you do is red
uce the pressure within the straw, allowing the push
of the atm to raise the fluid level in the straw. See Figures 6.26 and

6.27.

32.

Atm pressure will push water only as high as 10.3 m.

33.

1 N. If buoyant force decreases, balloon moves downward. If it in
creases, the
balloon moves upward.

34.

Bernoulli’s Principle briefly: Where the speed of a fluid increases, internal
pressure in the fluid decreases.

37.

They all experience reduced atmospheric pressure on their top surfaces. The
greater pressure beneath p
roduces a net upward force

lift.

Solutions to Chapter 6 Problems

1.

How much pressure do you experience when you balance a 5
-
kg ball on the tip of your finger, say of area 1 cm
2
?

A 5
-
kg ball weighs 49 N, so the pressure is 49 N/cm
2

= 490 kPa.

2.

A 6 kg
piece of metal displaces 1 liter of water when submerged. What is its density?

Density =
m/V

= 6 kg/1 liter = 6 kg/liter. (Since there are 1000 liters in 1 cubic meter, density may be
expressed in units kg/m
3
.

Density = 6 kg/1 liter


1000 liter/m
3

= 6000

kg/m
3
, six times the density of water.)

7.

An ice cube measures 10 cm on a side, and floats in water. One cm extends above water level. If you shaved off the 1
-
cm
part, how many cm of the remaining ice would extend above water level?

10% of ice extends above water. So 10% of the 9
-
cm thick ice would float above the water
line; 0.9 cm. So the
ice pops up. Interestingly, when mountains erode they become lighter and similarly pop up! Hence it takes a
long time for mountains to wear away.