1. 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
1
Safety.
• Beware slipping
on wet floor
2. 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.
2
Safety.
• SCUBA diving is
amazing but requires
proper training.
3. 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
3
4. 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.
4
