Fluid Mechanics in a Closed

System

Pascal’s Law

states in a closed system, an

increase of pressure at any point will

produce an equal increase in the pressure at

every point in the liquid.

Difference between force and pressure:

Pressure is exerted on a unit area.

Consider a glass flask. Fill it to the top and

insert stopper into the neck. If you hit the top

of a 1 sq in stopper with an 18 lb force, what

pressure is exerted on the 25 sq. in bottom?

Pressure is transmitted through a liquid.

If the system is closed with equal piston area,

we will have equal inputs and equal outputs.

If the system is “U” shaped, the direction of

the force can be changed.

One force down = One force up

The shape of the container has no effect on

the pressure in it. We could connect the

equal piston through a “small hose” and

move our equal force anywhere we want it.

Suppose the pistons have different areas.

Then the forces are different and the

distance moved are different.

If we connect two pistons in a closed system,

the distance moved is inversely proportional

to their areas. Volume remains constant.

A

B

C

D

Force

500 #

25 #

Area

20 sq in

30 sq in

60 sq in

Pressure

Length

6 in

Volume

A

B

C

D

E

F

Force

11200#

13440 #

Area

5 sq in

12sq in

10sq in

Pressure

1600

psi

2240 psi

Length

12 in

Volume

51.4 cu

in

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