1. To understand pressure

2. Pressure
Small area of the blades creates a high enough pressure to melt ice and allow the skater to slide
A high pressure is produced by applying a force over a very small area

3. What is pressure? Key Definition
Pressure is the normal force exerted per unit cross sectional area.
Pressure = 𝐹𝑜𝑟𝑐𝑒 𝐴𝑟𝑒𝑎 ,
The force is at right angles to the area
SI units are Nm-2 or Pascals (Pa). 1Nm-2=1Pa
Pressure is highest when
The force exerted is large
The surface area over which the force acts is small

4. Calculate the pressure exerted by
A 300N force acting on a drawing pin with an area of 0.01mm2
A man with a mass of 85kg standing on a plank of wood with and area of 0.45m2

5. Derive a formula for the pressure at a depth h in a fluid
Liquid density = ρ
Area = A
Volume:
V=Ah
Mass: (hint: density formula)
m = Ahρ
Weight:
W = Ahρg
This is the force downwards h
So Pressure:
p = 𝐴ℎ𝜌𝑔 𝐴 =
hρg

6. So, in a liquid
P = ℎ𝜌𝑔
Pressure does not depend on cross-sectional area
P ∝ℎ, so pressure increases with depth
Denser fluids exert greater pressure

7. Upthrust, floating and Archimedes Principle
Consider a submerged rectangular block of wood of cross sectional area A, density of fluid is 𝜌. We can explain the buoyancy force or upthrust in terms of pressure differences on the upper and lower surfaces.
Lower pressure h
Area A x
Density of liquid 𝝆
Higher pressure

8. Key definition
Archimedes’ Principle states that the upthrust exerted on an object immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid displaced by the object.
An object will sink if the upthrust is less than the weight of the object.
For a floating object, such as a ship or person in water, the upthrust must equal the weight of the object and the weight of the floating object must be equal to the weight of the fluid it displaces.

9. Try this one
An oil tanker has a mass of 400,000 tonnes. It is 500m long and 40m wide. What is the minimum depth harbour it requires? Steps:
Calculate the downward force
State the upward force
Relating weight, cross-sectional area, density of sea water (1030kgm-3) and gravitational field strength, find the depth required.
Draw a diagram to show the forces acting on the boat.

10. Consider
The earth’s atmosphere exerts about 1.0x105 Pa on everything on it surface.
Homework
Research and write a paragraph describing how a gas exerts a pressure for homework
Revise for a test next week on everything we have covered since the last test
Complete the section of the homework booklet Density and Pressure and finish the Q on couples