Topic 15 Flashcards
Why are oxygen masks needed in these
situations?
Air pressure and density decrease as altitude increases
(less air above) so there is less oxygen in each breath of
air (see next slide).
Why do bags in aeroplanes look full
When the food was packed, the
pressure inside and outside the
bag was the same. At a higher
altitude the pressure outside the
bag is less so pressure inside
pushes the bag out.
cutting with spoon vs knife
A knife has a smaller surface area than a spoon so the
pressure is greater for the same force.
What is the relationship between pressure and surface area?
Pressure is inversely proportional to surface area. As
surface area increases, pressure decreases. Pressure is how
concentrated a force is.
pressure is mesured in
pascals
(pa)
compare birds
The bird in the air is only under atmospheric pressure. The weight on it is only from
the air above.
The bird in the water is under both atmospheric pressure AND water pressure.
There is still the air above it, but also now water as well, so the bird feels both
pressures.
pressure depends on:
Pressure depends on the depth and density of the fluid.
At sea level, water is much denser than air so if you go
under water the pressure is much greater than in air.
The increase in pressure as you go down in water is
greater than as you go down in the atmosphere.
Note: Sea water is more dense than fresh water.
pressure key points
Pressure is exerted by
fluids.
Pressure acts normal to
surfaces in all directions.
Pressure from fluids
inside your body is the
same as atmospheric
pressure.
Pressure increases with
depth – weight of fluid
above.
Atmospheric pressure is not proportional to the height of
the atmosphere above.
Air is a gas so it is compressible. When the height is
greater there is less air above and it is also less dense
because the air is less compressed.
How do football boots work to stop
players slipping?
The players weight is spread over
the bottom of the boots. The area of
the studs is less than the area of the
players feet meaning the weight is
spread over a smaller area. The
pressure under the studs is greater
so they dig into the ground.
Why are liquids incompressible?
Particles in liquids are close together so
they are virtually incompressible.
The pressure in a liquid is transmitted
equally in all directions so the force
exerted at one point on a liquid will be
transmitted to other points in the liquid.
Describe how pressure in a fluid depends on depth
and density. Try to give an example.
Depth – the deeper you go the more weight of fluid is
above you to exert pressure. E.g. lower pressure at
higher altitude as there is less air above you.
Density – the greater the density of the fluid the greater
the pressure. E.g. pressure in water is greater than in air.
suringe thing
The pressure is the same throughout the fluid so a
force is transmitted from one syringe to the other. The
force on the larger syringe is greater but it moves a
smaller distance.
car on pistons
The area of the smaller
piston is less that that of
the large piston so the
force on the larger
piston is greater
because pressure is the
same throughout the
liquid in the system.
= force x distance, distance moved by the
smaller piston is greater than the distance moved by the
large piston so force on larger piston must be greater for
the work done to be the same (energy is conserved).
why did spring A need more force t extend
- stifness
- material
- thickness
