Physics Form 4

Form 4 Chapter 3: Forces and Pressure

3.1 Understanding Pressure

Pressure1. Pressure is defined as a perpendicular force acting per unit area of a surface.

P= FA

2. The SI unit of pressure is Pascal (Pa). 1 Pa is equal to 1 Newton per meter square (N/m2)3. Factors that affect the pressure acting on a surface:

a) Magnitude of the Force The larger the force, the higher the pressure

b) Contact areaThe larger the contact area, the lower the pressure

Examples of Application of High Pressure Examples of Application of Low Pressurea) Sharp Knifeb) Ice Skatec) Sole of shoes with spike

a) Foundation of Buildingb) Snow shoesc) Tyre of tractord) Feet of Elephant

Examples:1. The weight of a wooden block with dimensions as shown in the figure is 12N. Calculate the

maximum pressure that the block exerts on the floor.

Solution:Maximum pressure happened when the contact area is the smallest,Area= 0.2 x 0.3 = 0.06 m2

Pressure= 12N

0.06m2 = 200 Pa.

3.2 Understanding Pressure in Liquids

Pressure in Liquids1. Pressure in liquid is as a result of the weight of the liquid acting on the surface of any objects in

the liquid.2. Pressure of a liquid is directly proportional to :

a) the depth, hb) the density of the liquid, ρ ("rho")c) the gravitational field strength, g

3. Formulae:a) Pressure caused by a liquid

P=hρgb) Pressure in a liquid

P=hρg+Patm

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Patm = Atmospheric pressure, 1.01 x 105

Deriving the Formula for Pressure in Liquid

Volumeof LiquidColumn ,V=AhMass of LiquidColumn,m=ρV= ρAhWeight of Liquid Column,W=mg=ρAhg

Pressure ,P= Weight of Liquid ColumnArea of the base of LiquidColumn

P=WA

P= ρAhgA

P=hρg

*PRESSURE IN LIQUID DOES NOT DEPEND ON THE SURFACE AREA*

Characteristic of Pressure in Liquid1. The pressure of liquid increase with depth2. The pressure of liquid acts in all direction.3. The pressure of liquid does not depend on the area of its surface.4. The pressure of liquid does not depend on the shape and size of the container.5. The pressure of liquid at the same level in the same liquid are equal.

U-Tube1. Usually , a U-Tube is used to compare and

measure density of liquids.2. The density of the 2 liquids is related by the

equation:h1 ρ1=h2ρ2

Application of Liquid Pressure1. Dam2. Submarine3. Measuring Blood Pressure4. Intravenous Transfusion5. Water Tower

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3.3 Understanding Gas Pressure and Atmospheric Pressure

How Gas Pressure produced in a Closed Container?1. Gas molecules inside the container are in constant

and random motion.2. As a result, the gas molecules collide on the inner

wall of the container.3. After colliding on the wall, the gas molecules bounce

off, and the direction change creating a change of momentum to the molecules of the gas.

4. The change of momentum produces a force on the wall.

5. The force per unit area is the pressure on the wall.

Factors Affecting the Air or Gas Pressure1. Pressure increases when the density of gas

increases.

2. Pressure increases when the temperature increases (kinetic energy of the molecules increases)

Atmospheric Pressure1. On the surface of the earth, there is a thick layer of gas called atmosphere. The atmosphere

consists of various types of gas called the atmospheric gas.2. The atmospheric gases collide on the surface of the earth and hence exert a pressure on the

surface of the earth, called the atmospheric pressure.3. The atmospheric pressure can be measured in the unit of atm, mmHg or Pa. The atmospheric

pressure at sea level is taken to be 1 atm, which is approximately 760 mmHg or 101,000 Pa.

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Proof of Existence of Atmospheric PressureThe existence of the atmospheric pressure can be proved by the following experiments: Crushing can experiment Water cover with cardboard does not flow

out

Unit Used to Measure Atmospheric Pressure1. Pascal (Pa)

1Pa = 1 N/m2 2. Standard Atmospheric Pressure (atm)

1atm = 101,325 Pa3. mmHg

1 atm = 760 mmHg = 76cmHgInstrument used to Measure Atmospheric Pressure: Simple barometer Fortin barometer Aneroid barometer

Simple Barometer

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Characteristic of the Mercury Barometer

Applications of Atmospheric Pressure

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Instruments Used to Measure Gas PressureThe pressure of the gas in a container can be measured by using :1. Manometer2. Bourdon Gauge3. Fortin Barometer4. Aneroid BarometerManometer Bourdon Gauge

3.4 Pascal's Principle1. Pascal's Principle states that the pressure exerted on an enclosed liquid is transmitted equally

throughout the liquid.2. Pascal's principle is also known as the principle of the transmission of pressure in liquid.Experiment :When the plunger is pushed in, the water squirts equally from all the holes. This shows that the pressure applied to the plunger has been transmitted uniformly throughout the water.

Hydraulic System

A hydraulic system applies Pascal's principle in its working mechanism. It can be used as a force multiplier.

F1

A1=F2

A2

Change of Oil Level in a Hydraulic System

In the diagram to the left, when piston-X is pressed down, piston-Y will be push up. The change of the piston levels of the 2 pistons is given by the following equation.

h1 A1=h2 A2

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3.5 Archimedes' Principle

Archimedes' Principle

1. Archimedes' Principle states that when a body is wholly or partially immersed in a fluid, it experiences an upthrust equal to the weight of the fluid displaced.

2. Upthrust/Buoyant force is an upward force exerted by a fluid on an object immersed in it.

F=ρVgF =Upthrust/Buoyant Forceρ = Density of the liquidV = Volume of the displaced liquidg = Gravitational field strength

*Revision Note: ρ=mV *

Law of Flotation

1. The principle of floatation states that when an object floats in a liquid the buoyant force/upthrust that acts on the object is equal to the weight of the object.

2. If the weight of the object > upthrust, the object will sink into the fluid.

Examples of Law of Flotation:

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Applications of Archimedes' Principle1. Submarine

A submarine floats in the sea because the buoyant force acting on the submarine is the same as the weight of the submarine.

When water enters the ballast tank of a submarine, the weight of the submarine becomes greater than the buoyant force. Thus, the submarine dives into the sea.

When the compressed air forces water out of the ballast tank, the weight of the submarine becomes less than the buoyant force. Thus, the submarine rises.

2. Hot Air Balloon

a. Hot air expand and large volume of air is displaced.b. Hot air lower density than the surrounding air. When buoyant force is greater than the weight

of the balloon, the balloon start rising up.c. When the height increases, the density of air decreases. Thus, buoyant force is less.d. Balloon remain stationary in the air when buoyant force is equal to the weight of the balloon.

3. Ship

a. A ship, though very heavy, floats in the sea. This is because the volume of water displaced by the ship is sufficiently large to have a weight equal to the weight of the ship.

b. Although the ship is constructed of metal, which has greater density than water, its shape is hollow so that the overall density of the ship is less than the sea water.

c. A ship will submerge deeper in fresh water because the density of fresh water is less than seawater

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4. Hydrometer

a. Hydrometer is an instrument used to measure the relative density of liquids.b. The lead shots are placed in the bulb to weight it down and enable the hydrometer to float vertically in

a liquid.c. Liquid of lower density, a greater volume of liquid must be displaced for the buoyant force to equal the

weight of the hydrometer and so it sinks.d. Liquid density high = hydrometer float highere. Liquid density low = hydrometer sinks lowerf. The scale of the hydrometer stem can be calibrated to give reading for density in units of g/cm3.

3.6 Understanding Bernoulli's PrincipleBernoulli's Principle states that as the speed of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases.

Examples of Bernoulli's Principlea. Air is blown across the top of the paper.b. The flow of air at high speed creates a

region of low pressure across the top of the paper.

c. The still air beneath the paper is at a higher pressure.

d. Therefore a net upward force lifts the paper.

a. Water flow faster in narrow tube than in wider tube.

b. The pressure at B is the lowest because the water flow the fastest at B. Therefore the water level is the lowest at B.

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a. When air is blown through the tube, the narrow section B has a lower pressure.

b. The external atmospheric pressure pushes the water in B to its highest level.

a. When the bunsen burner is connected to a gas supply, the gas flow at high velocity.

b. Outside air is drawn in and mixes with the gas.

c. Mixture of gas and air enables the gas to burn completely.

a. When a plane wing in the form of an aerofoil moves through air, the flow of air over the top has to travel faster to cover a longer distance and creates a region of low pressure.

b. The flow of air below the wing is slower resulting in a region of higher pressure.

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Physics Form 4

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