Force and Pressure

UNDERSTANDING PRESSURE ,[object Object],P = pressure, Pascal (Pa) F = forces, N A = area, m 2 ( Unit: N m -2 ) State relationship between pressure and area. ,[object Object],[object Object],[object Object],[object Object]

APPLICATION INVOLVING HIGH PRESSURE A sharp knife has a very small surface area on its cutting edge so that high pressure can be exerted to cut the meat. Nails, needles and pins have very sharp ends with very small surface areas . When a force is applied to the head of a nail, the pressure will drive its sharp end into a piece of wood easily. The studs on a football boot have only a small area of contact with the ground . The pressure under the studs is high enough for them to sink into the ground , which gives extra grip.

APPLICATION INVOLVING LOW PRESSURE Skis have a large area to reduce the pressure on the snow so that they do not sink in too far. A tractor moving on soft ground has wide tires to reduce the pressure on the ground so that they will not sink into the ground. A wide shoulder pad of a heavy bag will reduce the pressure exerted on the shoulder of the person carrying the bag.

EXAMPLES ,[object Object],[object Object],[object Object]

UNDERSTANDING PRESSURE IN LIQUID ,[object Object],[object Object],What is pressure in liquid? P = pressure, Nm -2 / Pa h = depth, metre (m)  = density, kgm -3 g = gravitational field strength, Nkg -1 Formula pressure in liquid

[object Object],[object Object],[object Object],Relate depth to pressure in a liquid . ,[object Object],[object Object],[object Object]

Relate density to pressure in a liquid ,[object Object],[object Object],[object Object],900 kg/m 3 1000 kg/m 3

Exercises ,[object Object],[object Object],[object Object],[object Object],[object Object]

Characteristics of Pressure in Liquid ,[object Object],[object Object],[object Object],[object Object],[object Object]

[object Object],[object Object],[object Object],Application of pressure in liquid a) Construction of dam

[object Object],[object Object],Application of pressure in liquid b) Thick wall built in submarine

[object Object],[object Object],Application of pressure in liquid c) Blood transfusion

UNDERSTANDING GAS PRESSURE AND ATMOSPHERIC PRESSURE

UNDERSTANDING GAS PRESSURE AND ATMOSPHERIC PRESSURE ,[object Object],[object Object],What you feel when you squeezed a balloon filled with air?

[object Object],a) Gas Pressure Wall of container ,[object Object],[object Object]

b) Atmospheric Pressure ,[object Object],[object Object],[object Object],Characteristics of Atmospheric Pressure ,[object Object],[object Object],[object Object],[object Object],[object Object]

At sea level , the atmospheric pressure is about 100 000 Pa (1×10 5 Pa) 1 atmosphere (1 atm) = 10 5 Pa / 10 5 Nm -2 At 5600 m above sea level, the atmospheric pressure is about 50 000 Pa (5×10 4 Pa) The pressure in the space around satellite is almost zero . An aircraft travelling at an altitude of 11000 m will experience an atmospheric pressure of 25 000 Pa (2.5×10 4 Pa) Atmospheric pressure varies with height

Activities to show existence of atmospheric pressure ,[object Object],[object Object]

Application of atmospheric pressure ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

1) The fan sucks out the air when it is switched on. 2) Space in x become partially vacuum. 3) The higher atmospheric pressure forces the air and dust particles into the dust bag 4) Dust particles are trapped but the air still can flow through the exit. x c) Vacuum cleaner

Instrument for measuring Gas Pressure a) Bourdon Gauge

Instrument for measuring Atmospheric Pressure a) Aneroid barometer ,[object Object],b) Fortin barometer ,[object Object],[object Object]

c) Mercury barometer ,[object Object],[object Object],[object Object],[object Object],Vacuum

Atmospheric pressure and its units a) At a sea level, the atmospheric pressure : 1 atm = 760 mm Hg = 76 cm Hg = 0.76 m Hg b) Atmospheric pressure in unit Nm -2 or Pa : = h  g =0.76 × 13600 × 9.8 =10 5 Nm -2 / 10 5 Pa ,[object Object],[object Object],[object Object],c) Atmospheric pressure in unit metre water:  P atm = 10 m water

Archimedes’ Principle Archimedes’ Principle state that “ When an object is wholly or partially immersed in a fluid, the buoyant force is equal to the weight of fluid displaced by the object.

When in fluid, the object experiences two forces : (a)The actual weight which acts downwards (b) The buoyant force which acts upwards . ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],V F b

Apparent weight, W 2 Actual weight, W 1 0.40 N 0.67 N Buoyant force, F b //Upthrust = Actual weight, W 1 – Apparent weight, W 2 = Weight of water displaced

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Examples 35 N 35 N 3.5×10 -3 m 3 3.5×10 -3 m 3

[object Object],[object Object],Application of Archimedes’ Principle a) Submarine How does a submarine sink? How does a submarine rises? ,[object Object]

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],b) Ship (Plimsoll line)

[object Object],[object Object],[object Object],[object Object],[object Object],c) Hot-air balloon

d) Hydrometer ,[object Object],[object Object],[object Object],[object Object],Small reading Large reading

PASCAL’S PRINCIPLE ,[object Object],What will happen when you squeezed the toothpaste?

[object Object],[object Object]

Pascal principle is widely used in a hydraulic system. Hydraulic system Output force = output piston area Input force input piston area F 2 = A 2 F 1 A 1

A 1 d 1 = A 2 d 2 ,[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object],[object Object]

Applications of Pascal’s Principle in Everyday Life a) Hydraulic jack ,[object Object],[object Object]

b) Hydraulic brake ,[object Object]

Bernoulli’s Principle Bernoulli’s principle states that when the speed of a fluid increases the pressure is decreases and vice versa . ,[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],V  , P  V  , P   Patm  Patm

[object Object],[object Object],[object Object]

Applications of Bernoulli’s Principle

Carburettor ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

Insecticide sprayer ,[object Object],[object Object],[object Object],[object Object]

Bernoulli’s principle applied to fluid flow in tubes ,[object Object],[object Object]

Aerofoil ,[object Object],[object Object],[object Object]

Force and Pressure

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Force and Pressure