Pressure Flashcards
what is pressure?
how concentrated the force is
what is the equation for pressure?
pressure (N/cm2 or N/m2) = force (N) ÷ area (cm2 or m2)
[force is the weight of the object]
what is a Pascal?
N/m2
names after Pascal, who discovered pressure
what is the traingle for pressure?
calculate the pressure exerted by a force of 30N acting on an area of 2m2
p = f ÷ a
30 ÷ 2
= 15 N/m2
calculate the pressure exerted on a force of 20N acting on a area of 160m2
p = f ÷ a
20 ÷ 160
= 0.125 N/m2
what is the total force on an area of 3m2 on which there is a pressure of 15N/m2?
f = p x a
15 x 3
= 45 N
what is the total force on an area of 12m2on which there is a pressure of 25Pa?
f = p x a
25 x 12
= 300 N
atmospheric pressure is 100000Pa
calculate the force exerted by the air on a window of area 15m2?
f = p x a
100000 x 15
= 1, 500,000 N
a car weighs 15000N
what is the force acting on each tyre in the weight is evenly distributed among the four tyres?
15000N ÷ 4
= 3750 N
a car weighs 15000N
we have worked out that the tyres each have a force of 3750N
if each tyre has an area of contact with the ground of 60cm2, calculate the pressure of the air in the tyre
p = f ÷ a
3750 ÷ 60
= 62.5 N/cm2
why must a knife be sharp to cut easily?
a sharp knife has a small surface area in contact with the object being cut
p = f ÷ a
force is concentrated over a small area
therefore the pressure is large, making it easy to cut
why do eskimos wear snowshoes?
snowshoes have a large surface area in contact with the snow
p = f ÷ a
force (person’s weight) is spread out over a large surface area
therefore pressure is low, so the eskimo doesn’t sink
what causes air pressure?
when air particles collide with the surface, they rebound and move in the opposite direction
each collision exerts a tiny force on the surface
there are so many particles colliding that overall they exert a significant force, and hence a pressure, on the surface
how can air pressure increase?
more particles result in more collisions; therefore, pressure increases
hotter air has faster moving particles, so the number of collisions per second increases, therefore pressure increases
greater frequency of collisions = greater pressure
what is a manometer
a simple device to measure the pressure of gas
how does a manometer work?
one end is open to the atmosphere (Pa of atmosphere is 101,000 Pa)
the other end is attached to the gas to be investigated
what is the Pa of atmosphere?
101,000 Pa
what is the equation for differnece in pressure
ΔP = Δh x density x g
difference in pressure (in kg/m3) = differnece in height of water (in metres) x density (kg/m3) x strength of gravity (10N/g)
a manometer containing water is attached to a gas supply, producing a difference in water level of 15cm
calculate the difference in pressures between the two sides and the actual value of the pressure of the gas being tested
ΔP = Δh x density x g
ΔP = 0.15 x 1000 x 10
= 1500 Pa
if the ΔP ws worked out to be 1500 Pa calculate thepressure of gas
101,000 (Pa in atmosphere) + 1500
= 102,500 Pa
what is a barometer?
a device used to measure air pressure
how does a barometer work?
the air pushes down on the mercury forcing it up the tube
when the weight of the mercury in the tube balances the force exerted by the air on the mercury the column of mercury settles down
if the air pressure goes up the column of mercury is taller
on an average day air pressure is 101,000 Pa
density of mercury is 1300 kg/m3
g = 10N/kg
calculate Δh
Δh = ΔP ÷ density x gravity
101,000 ÷ 1300 x 10
= 0.74m (2 d.p)
air is pumped into a bike tyre and the pressure inside the typre increases
explain how pressure is produced inside the tyre and why it increases
inside the tyre is air
air particles collide with each other, changing direction and moving in random direction
each small collision exerts a tiny force
an increase of frequency of collision, which happens as more air is pumped in, means a greater overall force is exerted, a pressure
describe the motion of the molecules in a gas
lots of collisions of particles against container, then particles change direction
particle direction is random
(Brownian motion)
explain how molecules exert a significant force
each collision exerts a small force
many collisions per second exert a significant force
in which direction does pressure act on a diver in water that is not moving
in all directions
pressure is exerted on everyone of the earth
explain what causes this pressure
the particles in the air are colliding
they exert a pressure/force on our body
name five characteristsic of hydrostatic pressure
pressure increases with depth
pressure increased with density
pressure on a submerged object always acts perpendiclarly to its surfae
pressure is transmitted through a liquid
it doesn’t depend on the shape of the object
a student fills a cyclinder with water and watches the water emerge from three spouts
explain your diagram
pressure increases with depth
Pressure in a fluid is/is not dependant on its surface area
Pressure in a fluid is not dependant on its surface area
In which direction does pressure in a fluid act?
pressure in a fluid acts in all directions
