FUNDAMENTAL Physics – Matters: 2.7 Pressure and Depth

Pressure and Depth

Learning objectives: How does the pressure change with depth in a liquid? What is the relationship between fluid pressure and

depth? How cool is SCUBA diving? When is the next PADI OW course at IS?

Demo 1

The greater the depth the futher the water squirts from the holes.

The pressure is greater at greater depth .

Demo 2

At the same depth, the water squirts out really fast in all directions. The pressure acts equally in all direction in a fluid at a given depth.

Demo 3 – Pascal’s Vases

This demo shows us that the shape or width or angle of the water column does not change the height of the liquid.

Therefore shape or width does not change pressure.

The only thing that will change the pressure of a given liquid is the DEPTH!

Examples of Pressure and Depth

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Safety.

Beware slipping on wet floor

FUNDAMENTAL Physics – Matters: 2.7 Pressure and Depth

Example 1 – Damn Walls

Dam Walls are built thicker and stronger at the lower depths, because that is where the pressure is greatest.

Example 2 – SCUBA Divers.

As they descend, SCUBA divers experience greater pressure and must equalise their air spaces.

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Safety.

SCUBA diving is amazing but requires proper training.

FUNDAMENTAL Physics – Matters: 2.7 Pressure and Depth

The Physics of Pressure and Depth

Consider a column of water pressing down on a surface:

w

l

h

What is the pressure exerted on the bottom surface?

Pressure = Force / Area = weight / Area

But weight = mass x gravity

Therefore Pressure = mass x gravity / l x w

But mass = density x volume

Therefore Pressure = density x volume x gravity / l x w

But volume = l x w x h

Therefore Pressure = density x l x w x h x gravity / l x w

Pressure = density x gravity x depth

P = e x g x h

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FUNDAMENTAL Physics – Matters: 2.7 Pressure and Depth

Practice Question:

1. A column of mercury is 760mm high. What pressure does it exert at the base?The density of mercury is 13,300kg/m3.

P = p x g x h

= 13,300 x 10 x 0.76

=101,080 Pa

2. A diver descends to a depth of 25m in seawater, which has a density of 1030kg/m3. What is the pressure exerted by the water?

25 x 1030 x 10= 257500 Pa

3. In the future a moon base is built on the moon (funny old thing) which has large pressurised domes. Gravity on the moon is 1.6N/kg. The ‘Lunarnauts’ decide to build a 3m deep swimming pool. What is the pressure exerted from the water at the bottom of the pool?

3 x 1.6 x 1000=4800 Pa

4. You ride the express elevator to the top of the IFC2. What is the change of pressure? Height = 187m. Density of Air is 1.3kg/m3. 1.3 x 10 x 187= 2431

Homework P65 Q1 – 3 & revise for the matter test.

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