Pressure

What do we already know about pressure?

• Pressure is a force which acts at right angles on an object

• Pressure is exerted downwards by a solid object• If the area a force acts on is small, a greater

pressure will be given• If the same force was applied to a larger area the

pressure will be reduced.• Gases and liquids exert pressure in all directions

How do we calculate pressure?

• Pressure can be calculated by dividing the force by the area

• Pressure is measured in Pascals (Pa)• We sometimes see pressure being measured in

N/m2 or N/cm2

• 1Pa is the same as 1N/m2

Pressure = force area

Sensible shoes?• The force of our body

pushing down will exert a pressure

• The surface area of our shoes in contact with the ground will affect the pressure

• These stiletto shoes will concentrate the force over a small area

• These shoes will give a larger pressure and would not be suitable for walking on soft ground

More sensible shoes• These boots have a large

surface area • The weight of a person is

spread out over the larger surface

• The pressure exerted by the person is smaller than if they were wearing stiletto shoes

• A small pressure means you will not sink into soft ground

Examples of where a large area is needed to reduce

pressure.Caterpillar tracks on diggers and

on tanks

Tyres on a tractor

Skis and snowboards

snowshoes

Examples of where a small area is needed to increase the pressure

• Nails• Needles• Kitchen knives • Scissors• Drawing pins• Spade

Questions on pressure

1) What combination of force and area give a high pressure?

2) What combination of force and area give a low pressure?

3) What is the formula for calculating pressure?

4) What units are pressure given in?

Answers:1) Large force and

small area2) Small force on large

area3) Pressure = force /area4) Pascals (Pa)

Practice using the formula for pressure

1. A box weighs 500N. It’s base has an area of 5 m2. What pressure does it exert?

2. Another identical box is placed on top of the first box. What is the pressure now?

3. A snowboarder weighs 600N. The area of the snowboard is 0.5m2. What is the pressure on the snow?

Answers:1. 100 Pa

2. 200 Pa

3. 120 Pa

Liquid pressure• Liquids are made up of particles• The particles can move in all directions• The pressure exerted by a liquid acts

in all directions too• Liquids with a high density will exert a

higher pressure• As you go deeper, the pressure

increases• This is due to the weight of all the

water above pushing down

Water pressure and body pressure

• Water pressure acts in all directions on this fish

• The fish’s body pushes back with an equal and opposite pressure

List the fish under the highest pressure to the lowest pressure

• Flipper

• Bubbles

• Snapper

• Angel

Angel

Snapper

Bubbles

Flipper

Water pressure and scuba divingWhen this deep sea diver

goesdeeper, the water pressure on his body increases. This will causehis blood pressure to increase. The increased pressure inside his body causes gas exchange in his lungs tohappen at a greater rate. It also causes nitrogen to go into the blood with the oxygen.This can cause problems like euphoria, and the diverCan’t think clearly and can forget basic safety rules.

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’.

A cure for the bends

• A 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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