Transcript Document

Chapter 12: Gravity,
Friction, and
Pressure
12.1 Gravity is a force exerted by masses
12.2 Friction is a force that opposes motion
12.3 Pressure depends on force and area
12.4 Fluids can exert a force on objects
Pressure describes how a force
is spread over an area

Pressure - a measure of how much
force is acting on a certain area
Describes how “concentrated” a force
is
 Pressure = Force / Area P = F/A
Pressure equals force per unit area
 Cat laying on lap, then stands up

• Feels like more force, but force is still the
same, just over a different area  different
pressure
A Bed of Nails
Would the pressure
be greater, less, or
the same if there
were only one nail
in each board?
Pressure

To increase pressure (P = F/A)





Increase force (F)
Decrease area (A)
Thumbtack: decreased area – can pierce the
wall!
Snowshoes: can walk across snow and not
sink!
Unit: Pascal (Pa) = pressure exerted by one
newton (1 N) of force on an area of one
square meter ( 1 m2)
1 Pa = 1 N/m2
Calculating

Hiker weighing 500 N is wearing snowshoes an area of 0.2 m2
s. What pressure is being exerted on the snow?
What do you know?
What do you want to
find out?
Formula:
Substitute:
Calculate and
simplify
Check units
Answer
Pressure in Air

Air does have weight (force), and so
exerts a force on you at all times
Sea level pressure = 100,000 pascals
= “1 atmosphere” = 1 atm
 Every square centimeter of your body
feels a force of 10 N (2.2 lb)
 10N/cm2 = 10N/.0001 m2
=100,000N/m2 = 100,000 pascals

Pressure in Air


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
Changing Elevation: air has mass, so it has weight
 Higher elevation, shorter column of air above you, so
less air, less mass, less force per same area = less
pressure
Changing density: Air is compressible
 Air at the top of a column presses down on the air
below it, so is more dense (more molecules per
volume (d = m/V)
Effects on Pressure: Denser air, increases collision
and force  increase pressure
Increase elevation, pressure in your ear drum
continues to push out with the same force that
balanced the air pressure at lower elevations
Atmospheric Pressure
The weight of a one-square meter
column of air 30 miles tall is
about 100,000 N, or about
25,000 pounds.
---------------------------------------------Why doesn't the pressure of the
atmosphere break windows?
---------------------------------------------At what temperature would
water boil in a vacuum?
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