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Force and
Pressure (Liquid Pressure
(at rest) (Context
Snorkel usually…
Force and
Pressure
Pressure,
Surfaces
Definition - Pressure
Step on sharp object barefooted - painful due to pressure
High amount of force (weight) on small area (tip)
Pressure is from objects exerting forces on each other
Pressure depends on force magnitude and area of contact
Examples
Caterpillar tracks, fitted to vehicles for sand, mud, snow
Contact area incr. compared to just wheels, decr. Pressure
Therefore Pressure = force per unit area
P = F/A
Hospital circumstances
Sharp knife cuts more easily than a blunt one
Surgical knives therefore are very sharp
Sharper = smaller contact A = greater Pressure
Liquid Pressure
(at rest)
Context
Snorkel usually used closer to surface to breather
Deep-sea diver would find that water PSI is to great
Chest can't expand against water PSI
Examples
Holes in water bottle at different depths/heights
One's nearer top have weaker jets of leakage
Same depth leads to same PSI (and radius of jets)
U-shaped tube
Pour water into one side of tube, oil poured into other
When liquid settles, water level is higher
This means oil is less dense (higher volume for same PSI)
Liquid Column Pressure
Pressure at bottom of column changes w/ height and density
GFS also taken into account to make:
Pressure(Pa) = height(m) x density(kg/m3) x GFS(N/kg)
p = hpg
Atmospheric
Pressure
Definition
Earth atm - layer round earth, 100 kPa at sea level
(100 kN per m2) due to air particles colliding w/ surface
No. of collisions per second being very large
atm PSI decr. w/ incr. in altitude, less air particles hi-er up
atm extends to >100 km into space, PSI changes w/ weather
Fine weather normally means high PSI
Uses
Rubber suction cap sticks to tile since atm PSI
atm PSI is on the outside, not inside, acting only outside
Drinking straw works when air inside sucked out
atm PSI outside straw on liquid pushes it in straw
High Altitude
Death zone, >8000 m above sea level
mt Everest summit, 8848 m above sea level
Summit PSI, 30 kPa; O2 conc. decr. w/ altitude
Commercial jets have air-filled pressured cabins
Emergency O2 masks used if air lost at hi altitude
Pressure adjusted to below sea level atm,
PSI should never be <70 kPa (12 km altitude = 30 kPa)
Model
atm is thin layer round Earth
Density of air assumed to be the same at any altitude
Therefore weight, amount of air decr. w/ altitude
Upthrust &
Flotation
Aqua mobility
People normally feel lighter under water
People w/ mobile problems normally move
more easily under water; H2O exerts upward force
Called Upthrust
Explanation
Water level rises when object is submerged:
H2O displacement, more displacement = more upthrust
Water displaced = object volume if fully submerged
PSI incr. w/ depth therefore PSI under > PSI on top
R force = upthrust
Float or Sink
Ship w/ cargo float lower in water w/ incr. load
More displacement = more upthrust
Upthrust should equal weight or it sinks
Float - weight equals upthrust
Sink - weight > upthrust
Density test
Floaters are less dense than water
Due to P = hpg:
Sinkers are more dense since their weight
is greater than weight displaced,
Weight is then greater than upthrust given
Floaters' weight is less than weigh displaced etc.