1. © Edco 2010 Exploring Science Physics Pressure Learning outcomes In this section, you will learn: – The definition and unit of pressure. – How to calculate pressure from force and area. – About the pressure on on object in a liquid. – That our atmosphere puts pressure on our planet. – That there is a connection between atmospheric pressure and the weather.

2. © Edco 2010 Exploring Science Physics The definition and unit of pressure Pressure measures the force acting on one unit of area. Force is measured in Newtons (N) and area is measured in square metres (m 2 ), so the unit for pressure is N/m 2. This unit is also called the Pascal (Pa).

3. © Edco 2010 Exploring Science Physics Calculating pressure We calculate pressure by dividing force by the area that the force is acting on.

4. © Edco 2010 Exploring Science Physics Examples of calculating pressure If a force of 600N acts on a rectangular area of length 3m and width 2m, what is the pressure on this area? Area = 3m x 2m = 6m 2

5. © Edco 2010 Exploring Science Physics Pressure in liquids and gases When a book lies on a table, it puts pressure on the table because of the force of its weight. But objects are not the only things that apply pressure. Liquids and gases apply pressure to objects in them. There is one important difference between the pressure from objects and from liquids and gases. Liquids and gases apply pressure in all directions on the object in them.

6. © Edco 2010 Exploring Science Physics Connection between pressure and depth in a liquid A good example of pressure in a liquid is the pressure on a diver in water. The pressure on top of the diver comes from the weight of water above the diver. Thus, the pressure increases with the depth.

7. © Edco 2010 Exploring Science Physics Atmospheric pressure The atmosphere surrounding earth is made up of gases. These gases exert a pressure just like a liquid. Standing on the surface of the earth, we have a great weight of gases on top of us, just like a diver in water. The weight of these gases on a square metre of the earth is called atmospheric pressure.

8. © Edco 2010 Exploring Science Physics Measuring atmospheric pressure One simple way of measuring the pressure due to our atmosphere is to measure the height of a column of mercury that it can hold up in a tube. This is what happens in a barometer.

9. © Edco 2010 Exploring Science Physics Atmospheric pressure and weather The weather is complex, but there is a connection between atmospheric pressure and the weather. In general, low pressure will bring stronger winds and rain, but high pressure will bring settled, dry weather. Weather charts often have lines on them showing places that have the same atmospheric pressure.

10. © Edco 2010 Exploring Science Physics Summary: Key points Pressure is the amount of force acting on a unit of area. Pressure is calculated using the formula The unit of pressure is N/m 2, also called a Pascal (Pa). The pressure in a liquid is exerted in all directions.

11. © Edco 2010 Exploring Science Physics Summary (continued) The pressure in a liquid increases with depth. The atmosphere is made of gases that exert pressure on the earth. Atmospheric pressure is measured by a barometer. Atmospheric pressure is a major influence on the weather.