Help I am under lots of pressure!

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Help I am under lots of pressure!
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Topics that have been covered

• Forces
• Particulate model of matter
• Mass, weight and density

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Air has weight
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Pressing Problem

• Suppose the thin film of ice gave way and this
  girl has fallen through the ice in the pond.
  There is no means of getting help to her other
  than yourself. Would you walk or crawl to her?

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Walking

• Let your weight be 55kg. The force on the ice
  will then be 550N whether your walk or crawl.( g
  is 10 N/Kg)
• Weight will mostly be on one foot at any one
  time. We supposed that your foot is rectangle.

Foot
0.10m
0.20m
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Crawling

• Assume you lie flat on your stomach when crawling
  and we assume your body to have the shape and
  measurements shown.

BODY
0.25m
1m
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Comparison

• To make a fair comparison we will have to
  consider the same area of ice in each case.
• 1m2 will be a convenient area.

• For crawling,
• A force of 2200N will be exerted on every square
  metre of ice.

• For walking,
• A force of 27500N will be exerted on every square
  metre of ice.

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Definition of pressure

• Pressure is defined as force per unit area. (here
  1m2)
• Pressure, P Force, F
• Area, A
• Where force F is in newtons (N)
• area A is in m2
• pressure P is in pascal (Pa) or N/m2

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Semi trailer
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Who will you bring as your dance partner?
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Bed of nails
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Suction cup

• Suction cup

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Atmospheric pressure

• Atmosphere is a layer of air surrounding the
  Earth
• Extends up to 1000km above the surface of the
  Earth
• Weight of the atmosphere exerts a pressure on the
  surface called atmospheric pressure.
• The pressure exerted by air molecules at sea
  level is 1.013 x 102 Pa
• This is equivalent to placing a 1kg mass(10N)
  weight on an area of 1cm2

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Drinking from a straw

• Atmospheric pressure is acting on the surface of
  water inside the cup.
• When you suck, you are reducing the pressure in
  your mouth.
• As a consequence, the
• atmospheric pressure
• pushes the water from
• the cup into your mouth.

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Rubber sucker

• Rubber suckers are used at home to hang clothes
• The suckers are pressed in to force out the air
  within to create a partial vacuum
• The greater external
• atmospheric pressure acts
• on it to keep it in place

Partial vacuum
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Question???

• How will a swimmer feel in his ear drums as he
  dives deeper into a swimming pool?

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Work problem

• Find the pressure acting on a diver when he is at
  the surface
• Given density of water 1000kg m-3
  acceleration due to gravity 10ms-2 atmospheric
  pressure 1.01x 105 Pa

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Work problem

• Find the pressure acting on the same diver when
  he is 15 m in the water