Chapter 10 – Fluids and Pressure

When you drink liquid through a straw, which of the items listed below is primarily responsible for this to work?

60

0

0

5

Wat

er p

ress

ure

gravi

ty

iner

tia

Atmosp

heric

pre

ssure

mas

s

20% 20% 20%20%20%1. Water pressure

2. gravity

3. inertia

4. Atmospheric pressure

5. mass

A blood platelet drifts along with the flow of blood through an artery that is partially blocked. As the platelet moves from the wide region into the narrow region, the blood

pressure:

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5

incr

ease

s

decre

ases

Stays

the

sam

e

Drops

to ze

ro

25% 25%25%25%1. increases

2. decreases

3. Stays the same

4. Drops to zero

How is the smoke drawn up a chimney affected when there is a wind blowing outside?

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5

Smoke

rise

s m

ore ra

p...

Smoke

is u

naffe

cted

b...

Smoke

rise

s m

ore s

lo...

Smoke

is fo

rced

bac

...

25% 25%25%25%1. Smoke rises more rapidly in the chimney.

2. Smoke is unaffected by the wind blowing.

3. Smoke rises more slowly in the chimney.

4. Smoke is forced back down the chimney.

Answers1. 4 – The atmospheric pressure pushes on the fluid in the glass and since the gas

at the top of the straw is less than atmospheric pressure, the fluid rises. Or…the pressure in the fluid in the straw = Ptop+gh and at the level of the top of the fluid in the glass the pressure must equal Patm. Since you’re using the straw, Ptop is now less than atmosphere, so there must be height in the fluid of the straw so that pressure at the level of the top of the glass is the same everywhere.

2. 2 – the velocity in the constricted region increases (continuity equation) and the pressure drops following Bernoulli’s principle. Thus the pressure on either side of the constriction is higher than inside the constriction; it is this pressure difference that speeds up and slows down the fluid as it enters and leaves the constriction. It’s true that having narrow arteries raises your overall blood pressure, but that is not the question here. Here, we’re comparing the pressure inside and outside the narrow region.

3. 1 – Bernoulli again. As wind passes over the top of the chimney it lowers the air pressure. Thus the air pressure at the top of the chimney is less than the pressure at the bottom and the smoke feels an upward force from the pressure difference.