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 = force
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
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 area
2) Small force on large
area
3) Pressure = force /area
4) 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?
Answers:
1. 100 Pa
2.
Another identical box is placed on
top of the first box. What is the
pressure now?
2. 200 Pa
3.
A snowboarder weighs 600N. The
area of the snowboard is 0.5m2.
What is the pressure on the snow?
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 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
Angel
• Flipper
Snapper
• Bubbles
• Snapper
Bubbles
• Angel
Flipper
Water pressure and scuba
diving
When this deep sea diver
goes
deeper, the water pressure on
his body increases. This will
cause
his blood pressure to increase.
The
increased pressure inside his
body
causes gas exchange in his
lungs to
happen at a greater rate. It
also
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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