what is pressure?

how concentrated the force is

what is the equation for pressure?

pressure (N/cm^{2} or N/m^{2}) = force (N) ÷ area (cm^{2}^{ }or m^{2})

[force is the weight of the object]

what is a Pascal?

N/m^{2}

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 2m^{2}

p = f ÷ a

30 ÷ 2

= 15 N/m^{2}

calculate the pressure exerted on a force of 20N acting on a area of 160m^{2}

p = f ÷ a

20 ÷ 160

= 0.125 N/m^{2}

what is the total force on an area of 3m^{2} on which there is a pressure of 15N/m^{2}?

f = p x a

15 x 3

= 45 N

what is the total force on an area of 12m^{2 }on 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 15m^{2}?

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 60cm^{2}, calculate the pressure of the air in the tyre

p = f ÷ a

3750 ÷ 60

= 62.5 N/cm^{2}

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/m^{3}) = differnece in height of water (in metres) x density (kg/m^{3}) 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/m^{3}

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