Pressure

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Pressure

K Warne

http://www.warnescience.net/




?? Questions ??Why?• Do drawing pins work?

• is it easier to walk through deep snow using snowshoes –or ski’s?

• Is it easier to cut with a sharp knife?

• Dangerous to put aerosol cans into a fire?

• Does it hurt your ears when you swim deep underwater?

• Could you get sucked out of a airliners window if it breaks?

Answer: PRESSURE!





PressurePressure is a measure of the force exerted per unit area.

ForcePressure =

area

Units: force - newtons (N)

area - meters squared (m2)

Pressure: N/m2 = pascal (Pa)

Calculate the pressure exerted by a 50N force

on a 0.1m2 area.

Pressure = force/area

= 50/0.1

= 500 Pa (N per m2)

F

P xA





?? Answers??Why?• Do drawing pins work?

• is it easier to walk through deep snow using snowshoes – or ski’s?

• Is it easier to cut with a sharp knife?

• Dangerous to put aerosol cans into a fire?

• Does it hurt your ears when you swim deep underwater?

• Could you get sucked out of a airliners window if it breaks?

BECAUSE• Small area of point creates HIGH PRESSURE – large surface on

thumb low pressure

• Large surface area reduces pressure – feet do not penetrate snow! You walk on top!

• Small surface of sharp edge creates HIGH PRESSURE.

• Cans pressurized gas – pressure increases with temp – explode!

• Pressure increases with depth.

• High pressure inside – low pressure outside – gas flows from HIGH to LOW PRESSURE.





• To calculate your area - convert to m2.

Estimated area lying

down.

194x75=14550cm2

14550/10000 = 1.455m2

Body


= your mass x 10

area of your body

= 60x10/1.455

= 412 Pa

194cm

75cm

100 cm = 1m

(100)2 cm2 = 1m2

10 000cm2 = 1m2

Pressure exerted by your body





Pressure of an object


=(weight)/(l x w)

=

2.5kg

Rectangle/Square

W = 20cm

L = 60cm

Calculate the pressure exerted on the ground by each of these objects.

(Length measurements must be in meters.)

Circular Based

100gr

r = 30cm


=(weight)/(∏x r 2)

=





Pressure of an object

Pressure = force/area base

The area of the base in contact with the ground must be used to calculate the pressure.

Rotate each of the bricks (B & C) to show how they need to be placed so as to exert the MOST (B) and LEAST (C) pressure.

A

B

C





An elephants Pressure

Calculate the pressure

exerted by an elephant

which has a mass of 2.3

tonnes (1tonne = 1000kg)

and a foot diameter of 50

cm.

Force =

Area =

Pressure =

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rights reserved. © Peter Davey, Bruce Coleman, Inc.





Stillettos

• Calculate the pressure

exerted by a 50 kg lady

wearing stiletto heels

having an area of 1cm2 .

Force =massx10

=

Area =


=Pa





Liquid pressure• Observation: Water squirts our equally far in all

directions at the same depth.

Liquids cannot be compressed! Because of this fact;

• Pressure is the same at all points at the same depth

• Pressure is exerted EQUALLY in all directions.

B C

PA = PB = PC





Liquid Pressure

The pressure increases

with depth.

PA …. PB ….PC

A

B

C

Calculate how much the

pressure increases for every

metre of depth in water. (use

g = 10 N.kg-1)

p =





Liquid pressure• Pressure is the same at all points at the same depth

• Water squirts our equally far at the same height!

A B C

PA = PB = PC

The hydrostatic pressure at any given depth is found by

p = x g x h= density of liquid (1g/cm3

or 1000 kg/m3 water)

g = gravitational

acceleration (10 m/s2)

h = height of column of

water (depth)

Show that p =

hg and that the

units are still

N/m2 (Pa)





Liquid pressure• Pressure is the same at all points at the same depth

A B C

PA = PB = PC

The pressure at any given depth is

found by

p = F /A

= weight /A

= mxg /A

= xv xg /A

= x(Axh)xg /A

p = x h x g

p = F /A

F = weight

Weight = m x g

m = density () x volume

Volume = area x h

Show that p = hg and that the units are still N/m2 (Pa)

Units (Kg/m3) h(m) g(N/Kg) = (Kg/m3).(m).(N/Kg) = N/m2

HYDROSTATIC

PRESSURE DENSITY OF

THE LIQUIDDEPTH AT

WHICH P

MEASURED

GRAVITATIONAL

ACCELERATION





Liquid Pressure

A B

Only the depth affects pressure.

The pressure at A and B would be equal!

Calculate the pressure at the

bottom of each of these containers

if the liquid has a density of 1500

kg/m3. The diameter of A is 5m

and B is 50cm and both are 2.75m

deep.

p = hg = (1500)(2.75)(10) = 4 125 Pa

2.75m





Liquid PressureThe TOTAL pressure at any point in a liquid includes the

atmospheric pressure exerted on the liquid surface!

p = atmos + hg = ……………………= …………………. Pa

2.5m

Atmospheric pressure 100 000 Pa

Hydrostatic pressure = hg



Hydraulic

Calculations

…………..force

……………..

FORCE!

……….

distance

………

distance




Hydraulic Pressure

• Pressure applied to the liquid is exerted ……………… throughout the liquid ( ).

Force = ………….. x ……….

• The large piston has a

greater area so experiences a greater ……....

• The smaller piston however moves a greater …………. to displace the same ……….

Hydraulic Press

Liquid (hydraulic fluid) -

cannot be compressed.

ForceFORCE!



Hydraulic Calculations

A builds a hydraulic lift to lift 1250N on a surface area of

12m2, if he exerts a force on the liquid over an area of

0.5m2, what is the magnitude of the force he exerts.

F1

A1

F2

A2

=

F2

……=

F2

……=

F2 =

P1 = p2Summary:

1. F1 =1250N,

A1 = 12m2

2. F2 = ?

A2 = 0.5m2

…….

…..

…….

……..

………………… = …………….

Hi -

This is a SAMPLE presentation only.

My FULL presentations, which contain loads more slides (with all the gaps filled in) as well as

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Have a look and enjoy!

Keith Warne




Pressure

Education

liquid pressure pressure

depth pressure

force pressure

1m2 pressure

gas pressure

object pressure

pressure feet

elephants pressure