1. Questions 1. A car travels 240 km in 8 hours. What is its speed?
2. A person runs 200m in 25s.What is their speed?
3. A car travels at 10 m/s for 3 minutes. How far does it travel?

2. Pressure Why do some shoes damage floors?
Why can you press a drawing pin into the wall, but not your finger?
Why is an armchair more comfortable than a stool?

3. Learning Objective
To understand pressure
Learning Outcomes
All Can calculate pressure
Most Can describe the effect of pressure and use correct units
Some Can explain the effects using scientific terminology

4. What is pressure?
Why would a lady in high heels standing on your foot hurt more than an elephant standing on your foot?
The elephant has a larger weight than the lady, but the contact area between its feet and the floor is far larger.

5. High pressure and low pressure
A force spread over a larger area produces a lower pressure.
Photo credit: © 2008 Jupiterimages Corporation
Which type of footwear would be better for standing on a muddy field – the flat-soled shoes or high-heeled boots?

6. High pressure and low pressure
In contrast, the soles of the high-heeled boots have a smaller surface area and so exert a higher pressure.
These shoes are likely to sink into soft ground.
The shoes have flat soles, so they spread the person’s weight over a large surface area.
These boots exert a low pressure on the ground.
Photo credit: © 2008 Jupiterimages Corporation

7. Calculating pressure F P A force pressure = area
Pressure is the force per unit area and is calculated using this formula: F P A
pressure =
area
force
This can also be represented in a formula triangle.
Pressure is measured in newtons per square metre (N/m2), which are also called pascals (Pa).
Pressure can also be measured in newtons per square centimetre (N/cm2).

8. Pressure calculation
How much pressure is the woman exerting on the floor?
Weight of woman = 500 N
Area of contact with floor = 20 cm2
Force
Pressure =
Area
Photo credit: © 2008 Jupiterimages Corporation
500 = 20
= 25 N/cm2

9. Pressure Calculation
A person of weight 600N is wearing shoes with a total area of 450cm2. What pressure is exerted on the floor?

10. Using low pressure
A force spread over a large area produces low pressure. This is used in skis and snowboards.
The large surface area of the board means the snowboarder exerts very little pressure on the snow.
This means he slides over the top of the snow and does not sink into it.

11. Using high pressure
A force concentrated on a small area produces high pressure. This is used in things like pins and sharp knives.
The force applied to the tiny surface area of the end of a pin creates a high enough pressure to puncture even wood.
Photo credit: © 2008 Jupiterimages Corporation
The narrow blade of a knife means that it exerts a high pressure, making it easier to cut fruit and vegetables.

12. Pressure in a liquid low pressure high pressure
Pressure in a liquid acts in all directions and increases with depth.
You can investigate this by using a needle to make holes at different heights in a plastic bottle, before filling it with water. The water escaping from the lower holes will travel further because it is at a higher pressure.
low pressure
high pressure

13. Overcoming pressure underwater
When entering deep water, a diver will experience pressure from all sides.
At greater depths, this pressure becomes too much for the body to handle.
So how do we overcome this pressure?
Submarines use extremely strong materials to withstand the huge forces. This means some can dive to the very deepest points of the ocean.
Photo credit: © 2008 Jupiterimages Corporation

14. Hydraulics
Hydraulic systems use the principle that pressure is transmitted throughout a liquid.
They are used to transfer movement from one part of a machine to another without linking the parts mechanically.
All hydraulic systems use two pistons linked via a pipe which carries special oil called hydraulic fluid.
force
applied
here
force transferred
here
Pressure inside all parts of the
hydraulic system is the same.

15. Hydraulic brakes
All hydraulic brake systems (like those used in cars) use a small master piston and bigger slave pistons.
The master piston is used to apply a force. This puts the liquid under pressure. The pressure is transmitted to the pistons on all four wheels of the car. 15

16. What does this demonstration show?

17. Explain why the can is crushed when air is removed
Air Pressure
What is air pressure?
Why is air pressure lower at the top of a mountain?
Vacuum pump
Explain why the can is crushed when air is removed

18. Questions
A box has an area of 4cm2 and a weight of 2N. What is the pressure it exerts?
A drawing pin is pushed into a wall with a force of 10N. The area of the pin is cm2 . What is the pressure it exerts?
A skier has a weight of 600N. They are on skis with a total area of 0.5m2 . What is the pressure they exert?
Copy and complete….
A sharp knife is easier to use because it creates a _____ pressure. This is because the ________ is acting on a small _________