2 What do we already know about pressure? Pressure is a force which acts at right angles on an objectPressure is exerted downwards by a solid objectIf the area a force acts on is small, a greater pressure will be givenIf the same force was applied to a larger area the pressure will be reduced.Gases and liquids exert pressure in all directions
3 How do we calculate pressure? Pressure can be calculated by dividing the force by the areaPressure is measured in Pascals (Pa)We sometimes see pressure being measured in N/m2 or N/cm21Pa is the same as 1N/m2Pressure = forcearea
4 Sensible shoes?The force of our body pushing down will exert a pressureThe surface area of our shoes in contact with the ground will affect the pressureThese stiletto shoes will concentrate the force over a small areaThese shoes will give a larger pressure and would not be suitable for walking on soft ground
5 More sensible shoes These boots have a large surface area The weight of a person is spread out over the larger surfaceThe pressure exerted by the person is smaller than if they were wearing stiletto shoesA small pressure means you will not sink into soft ground
6 Examples of where a large area is needed to reduce pressure. Caterpillar tracks on diggers and on tanksTyres on a tractorSkis and snowboardssnowshoes
7 Examples of where a small area is needed to increase the pressure NailsNeedlesKitchen knivesScissorsDrawing pinsSpade
8 Questions on pressure Answers: Large force and small area Small force on large areaPressure = force /areaPascals (Pa)What combination of force and area give a high pressure?What combination of force and area give a low pressure?What is the formula for calculating pressure?What units are pressure given in?
9 Practice using the formula for pressure A box weighs 500N. It’s base has an area of 5 m2. What pressure does it exert?Another identical box is placed on top of the first box. What is the pressure now?A snowboarder weighs 600N. The area of the snowboard is 0.5m2. What is the pressure on the snow?Answers:100 Pa200 Pa120 Pa
10 Liquid pressure Liquids are made up of particles The particles can move in all directionsThe pressure exerted by a liquid acts in all directions tooLiquids with a high density will exert a higher pressureAs you go deeper, the pressure increasesThis is due to the weight of all the water above pushing down
11 Water pressure and body pressure Water pressure acts in all directions on this fishThe fish’s body pushes back with an equal and opposite pressure
12 List the fish under the highest pressure to the lowest pressure AngelFlipperBubblesSnapperAngelSnapperBubblesFlipper
13 Water pressure and scuba diving When this deep sea diver goesdeeper, the water pressure onhis body increases. This will causehis blood pressure to increase. Theincreased pressure inside his bodycauses gas exchange in his lungs tohappen at a greater rate. It alsocauses nitrogen to go into the bloodwith the oxygen.This can cause problems like euphoria, and the diverCan’t think clearly and can forget basic safety rules.
14 Problems for scuba divers As a diver ascends the water pressure will decrease, and the diver’s blood pressure will decrease.The nitrogen which dissolved in the blood easily under high pressure will come back out.If the diver decompresses too quickly the nitrogen leaves the blood so quickly it will bubble and fizz like carbon dioxide from a bottle of pop.This is known as the ‘bends’.
15 A cure for the bendsA diver can go into a decompression chamber to avoid getting the ‘bends’The pressure in the chamber is controlled to change slowly from high pressure to a low pressure.This allows the diver’s pressure to decrease slowly to avoid nitrogen leaving the blood so quickly.
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