Engines Car ,Ships and Aeroplane : Transport medium on Road, Water and Air.

ENGINES FOR TRANSPORT:

Engine Page in my Physics-World Software

It is almost impossible to imagine now how difficult it used to be to move about the country . Before the railways were developed in the 19th century , land transport depended entirely on the horse. Seas were crossed by sailing ships. The need for the horse and the sail were abolished by the invention of an efficient steam engine by James Watt.

INVENTION OF THE STEAM ENGINE:

James Watt

Watt was born in 1736, in Greenock, Scotland . At 17 years, he learnt to make mathematical instruments and had a job as mathematical instrument maker to glasgow University. One of the job they asked him to do was to repair a model of Newcomen`s steam engine. This rather inefficient engine was used to pump water out of the mines.

Watt`s Steam Engine

Watt improved on Newcomen`s design and then he completly alter it . His new engine only a third of the fuel that Newcomen`s engine required. Watt also made his engine capable of rotary movement, using a crank and gear wheels. By 1782 his engine was working up to 40 machines in a factory. This was the beginning of the industrial revolution when workers began to be replaced by machines.

The earliest steam engines were stationary. The first attempt to use them for transport was in 1786, when the American John Fitch built a steamship . By 1850 propeller driven steamships were regularly crossing the Atlantic. 

Steam Engine 

The first successful steam train was built by George Stephenson in 1814. This began the era of land travel by railway.

INTERNAL COMBUSTION ENGINE:

The steam engine has a great disadvantage that it is neccessory to carry coal or wood around to make the fire. The search began for a lighter fuel and an engine in which to burn it. The result was the internal combustion engine. A gas version of this was made by the French-man Etienne Lenoir in 1860. The modern engine we use in our cars is based on the four-stroke engine , built by Nikolaus Otto in 1876. The cycle of strokes on which it works is called the Otto cycle.Modern car engines have 4, 6 or 8 cylinders. The more cylinders they have, the more smoothly the engines runs.

Modern Car Engine

The first practical petrol cars were built in Germany in 1885 by Gottleib Daimler and Carl Benz. In 1892 the German engineer Rudolf Diesel built an engine using oil as the fuel. This engine does not need sparking plugs.Instead, the explosion takes place when the oil is sprayed into the cylinder which contains highly compressed air, Diesel engines are used in buses, taxis, lorries, and some trains and boats.

Small petrol and Diesel engines are often two-stroke engines. In this type of internal-combustion engine there are no valves. The piston uncovers parts or holes in the cylinder wall as it moves up and down. Some motor cycles,cars,and lawn-mowers have two-strokes engines. You can recognize them by the "pop-pop" noise they make at each stroke.

OTTO-CYCLE:

Four Stroke Engine

The Otto-cycle On the first stroke the piston goes down and the inlet valve opens. Air and petrol vapour is sucked into the cylinder . On the second stroke piston rises compressing the mixture . The sparking plug fires and the mixture explodes.This pushes the piston down for the third strokes. On the fourth , the upward stroke, the exhaust valve open and the gases are pushed out . The cycle is then repeated over and over again.

                                                                                        3D animation showing 4-Stroke-Engine

Watt`s Steam engine: Steam from the boiler enters the cylinder through a valve and pushes the piston down. At the bottom of the stroke another valve opens and steam pushes the piston up .The up and down (resiprocating ) motion of the piston is converted to round and round (rotary) motion by a system of cranks and gears.

Diesel Locomotive

Electric Train.

The Diesel Locomotive is not as smooth as the electric train, but is cheaper to run because an electrified rail or overhead wire is not required.

CARS:

Car Page in my Physics-World Software

After Newcomen had invented the steam engine in 1712, attempt were made to harness this device to a cart in place of the horse. It was only with Watt`s improved engine that this became possible. The first successful power-driven cart was built in 1769 by Nicholas Cugnot. From then on , all over the world a great variety of extraordinary steam-powered vehicles were produced.

Horse Carriage of  Old Time

HORSELESS CARRIAGE:

The first practical steam vehicles, called "horseless carriages", were built in 1820. Good road had been built by Telford and McAdam for horse-drawn vehicles. The surfaces were ideal for steam carriages. The early vehicles resembled stahe coaches and carried goods and passengers in the same way. They travelled at about 30 mph. However in the 1865 Road Locomotives Act (called the Red Flag act) . The Government restricted their speed to 4 mph.This act slowed down car develoment in Britain very considerably.

Different Parts of Modern Car

In the 19th century , the trend was to developed light vehicles, which were easy to manoeuvre. One problem with a steam engine is that it requires a furnace of some sort of to raise steam. This means carrying large quantity of heavy and bulky coal. With the development of the oil industry , inflammable liquids such as petrol became available. These made it possible to do away with external combustion in engines. Internal-combustion engines were built , in which the petrol vapours explodes inside the cylinder.

The first successful gas engine was built by Etienne Lenoir in 1860. This inspired a German, Nikolaus Otto, to build a four stroke internal-combustion engine in 1876.

The world`s first practical petrol-driven cars were produced in 1885. In 1889 petrol cars were imported into into Britain and then speed limit was raised to 12 mph.

At the beginning of the twentieth century steam engine cars were still being produced. In 1906 the Stanley brothers in  America built the Stanley Rocket capable of travelling at 127 mph . However , steam cars were clumsy and expensive  to run and gradually disappeared.

                                                                                        3D Animation Showing Different Internal Parts of Car.

PETROL-DRIVEN CARS:

3D Models of Different Cars

                                                                                        3D animation showing Different Cars and Parts

Instead people concentrated on improving petrol-driven cars .

In 1907 Sir Henry Royce produced his first famous Silver Ghost

In 1907 Sir Henry Royce produced his first famous Silver Ghost. Between 1907 and 1930 car bodies became stronger and more stream-lined. Front suspesion was added and syncromesh gears were invented . Shock absorbers, windscreen wipers, and indicator become standard equipment  on every car . Major development since 1930 include the use of automatic gears and the invension of the Wankel engine.

ELECTRIC CARS:

The first electric cars appeared in 1891, made possible by Gaston`s Plant`s invention of  a storage battery. Though popular, they could only go short distances before the battery needed recharging. When petrol cars became self-starting , electric cars went out of favour . They are still used today for milk floats and other delivery vehicles. Interest in electric cars has revived because they donot pollute the air. It is hoped that new lighter types of batteries or fuel cells can be developed which will last as long as a full tank of petrol.

The first true automobile being driven in Vienna in 1950 on the 75th anniversary of its invention by Siegfried Marcus. Marcus invented the internal-combustion engine in 1864,

The petrol-driven car was invented by Siegfried Marcus In 1874. Though it could be driven under its own power it was heavy and clumsy .It was not a commercial success.

The 1911 Ford Model T

The 1911 Ford Model T was the most popular car made at the time. Between 1908 and 1927, fifteen million were sold.

WATER TRANSPORT:

Water Transport page in my Physics -World Software

The first people to travel by water probably used log as simple boats. They sat astride the log and used a piece of wood to paddle along. About 5000 years ago. The Egyptians began to build proper boats. Their first boats were made from bundles of reeds. Later they made stronger and bigger ships from short planks of wood. They learnt to use a sail so that the wind couldpush their ships along. They fixed a paddle near the black of the ship to steer it .In these ships, the ancient Egyptians made long journeys.

A roman gallery.These were among the earliest efficient sailing ships, and could also be rowed by tiers of slaves. Ships like this one were the warships of the roman Empire.

Different Water Vehicles

Clipper ships and steamships:

Over the years, ship design was improved. More masts and sails were added until the ships could move very fast .The fastest of all sailing ships was called the clipper.It was long and narrow and had three tall masts, with up to six sails on each mast. The cutty Sark, a famous clipper, sailed from Australia to England in 69 days. The usual time was 100 days.

3D Picture of Large Passenger Ship

The steamship was the next improvement in ship design.An American John Fitch, used a steam engine to drive a ship in 1786, His ship , the Experiment, had oars that were driven by steam , Soon afterwards Paddlesteamers were used . These had a paddle wheel , like the wheel of a water mill, at the side or back of the ship.

3D Picture of Large War Ship

In 1894, A British engineer built a new kind of steam ship engine.It was called the steam turbine , steam turbines use less fuel and go faster than paddlesteamers. The first steam turbine ship crossed the Atlantic in 1904. Soon all large ships were using turbines as engines.

Different Water Transport Vehicles 

                                                                                    3D animation showing Moving Ship On Sea

Steam turbines are still used in some ships today. Another type of engine often used is called the Diesel engine.The Diesel engine also powers some land vehicles, such as lorries and buses.

Kawasaki_heavy_industries cylinder_Diesel_Ship_Engine

THE HOVERCRAFT AND THE HYDROFOIL:

Some of the ships used today are quite different from the ships of 100 years ago. These are the ships of 100 years ago. These are the hovercraft  floats on a cushions of air which lifts it above the water.The hovercraft is driven forward by large propellers that spin in the air above the craft. Hovercraft can travel on both land and sea. The hydrofoil has small wings on its underside .These lift the craft out of the water when it is moving.

US marines hovercraft.

Other ships have push through the water and this slows them down. Hovercrafts and hydrofoils can move more swiftly because they travel above the water. They cannot travel over rough seas , and so are used for short journeys in quite waters.

Hydrofoil 

Submarines:

Under the sea , the ubmarine is used , The first submarine was built by David Bushnell in 1775.It was used during the American revolution when the Americans tried to attach a mine to the hull of a British ship .It was moved along by propeller-like screws that were turned by hand from within the submarine.

Submarine

The most modern submarines have small nuclear reactors which use plutonium or Uranium as fuel. They boil water to turn turbines which drive the propeller.The great advantage of a nuclear reactor is that it uses only a tiny quantity of fuel to produce a lot of power.So nuclear submarines can travel completely around the world without surfacing.

AIRCRAFT:

Aeroplane and Air Transport page in my Physics-World Software

People have always dreamt of being able to fly .Many flying machines and devices have been tried .The first successful balloons were launched by the Montgolfier brothers in 1782. They were filled withhot air, which being lighter than the cooler surrounding air lifted the balloon off the ground. In 1783 their balloon carried people through the air for the first time, although the balloon was tethered to the ground. In 1785 Blanchard and Jefries crossed the channel in a hydrogen filled balloon.Hydrogen is lighter than air and so rises up through the air.

Gliders, copying the principle of flight used by birds, were tried at the end of the nineteenth century. The wings of a glider are shaped so that as they pass through the air an upward force called the lift is created. A wing with this shape is called an aerofoil.

Gliders rely on the wind to give them the force or thrust to move forwards. The invention of the internal combustion engine made powered flight possible.

The Wright brothers learnt to fly gliders and they used this experience to make aeroplanes powered by petrol engines

The Wright Brothers:

The Wright brothers learnt to fly gliders and they used this experience to make aeroplanes powered by petrol engines.Their first successful flight took place on the 17th December 1903. The plane flew for 12 seconds covering 40 metres at a height of about 3 metres. The plane had a wood and canvas frame built round the engine.

At first flying was regarded as a dangerous sport.

At first flying was regarded as a dangerous sport. Plane were built for enthusiasts. In the first World War. However , they were used for dropping bombs. After the war it was realized how valuable they were. Planes were soon improved, wood and canvas giving way to metal , particularly aluminium. Instead of being open they had enclosed bodies, with seating for a passenger as well as a pilot.

Aeroplane Engine

In 1919 Alcock and Brown were the first to fly the Atlantic nonstop. In 1934 a DC2 was used to carry passengers from England to Australia. In 1939 Pan American provided the first transatlantic passenger device.

Air Transport Page

The second World War and after:

During the second world war , planes were the most vital part of the flighting force. During these six years , neccessity led the great advances in the development of aircraft.The main advances were the use of monoplanes (single wing) in place of biplanes ( double wing) and the development of the jet engine .By the end of the war both sides were using jet fighters. After the war all these developments were used to provide modern passenger aircraft.For example , The Boeing 707 was a development of the B29. The aircraft used to drop the atom bomb on Japan.

Small Aeroplane.

In fixed wing aircraft , lift is obtained from stationary wings. A propeller or jet produces the forward thrust.These aircraft need long runways and have high landing speed.

Big Passenger Aeroplane

Supersonic aircraft can fly faster than the speed of sound. Which is about 760 mph(1200 km per hour) .Concorde can fly at twice the speed of sound.

The first manned flight of the Montgolfier hot-air balloon took place on 21 November 1783.

The first historic flight made by the Wright brothers on 17 December, 1903. They made four more flights that day.

                                                                                     3D Animation Showing  Aeroplane Movement

Length, Time, Velocity, Weight : Four Physical Quantities Which are measure very Frequently

Length,Time, Weight Measuring Instruments

LENGTH:

Length Page in my Physics-World Software

When we talk about a length we always compare one measurement with another.If you say you are taller than someone else you mean that the distance from the soles of your feet to the top of your head is greater than that distance on the other person. If you wanted to find out if you are taller than a pen friend in another town it would not be possible to compare your heights by standing side by side .Instead you would both use a scale such as a ruler or a tape measure to compare heights.In this case you would each compare your own height with a standard length - a foot or metre.It would impossible to use this method if the standard of lenght in one place was different from the standard in the other.So it is most important to make sure that the standard length is the same everywhere.

YARDS AND METRES:

British Imperial System.

The imperial British system of measurement of length was based on the yard but in 1963 the yard was defined in terms of the meter.

Imperial standards of length 1876 in Trafalgar Square, London.

The first accurate standard yard was made in 1878 .It was a bronze bar exacly one yard long kept by the government.This was called the primary standard. From it lengths a yard long were measured onto other rods.These rods kept in the laboratories throughout the country, were called secondery standards.

In 1963 the yard was defined in terms of metre, one yard being equal to 0.9144 metre. The metre is the standard unit of length used in Europian countries.

Metric Imperial US Customary Units.

In 1927 the metre was redefined as the length of a platinum iridium bar kept in Paris.This bar was also found to vary in length in tiny amounts. So in 1960 a new type of definition was internationally agreed.This is based on the wavelength of the light emitted by atoms .This length never varies ; the same atom under the same condition always emits light of indentical wavelength.The atom chosen is an isotope of gas Krypton.The metre is now defined as 1,650,763.73 wave-length emitted by this atom under certain conditions.

Astronomers sometimes use another method of measuring of lengths when discussing the distance of stars .This is the lightyear and is an enormous distance.It is the distance travelled by light in one year .Light travels at a velocity of 300 million metres per second . As there are about 31.5 millions seconds in a year , one light year is 9460 million million metres of about 6 million million miles.  

CALLIPER & MICROMETER

CALLIPERS & MICROMETER:

Callipers are used for measuring diameters, either internal or external. The distance between the jaws of the callipers is read on a scale marked on the instrument.

Callipers are used for measuring diameters, either internal or external

A micrometer is an instrument for measuring small distances very accurately. It consists of a sleeve which rotates on a screw inside it . 

A micrometer is an instrument for measuring small distances very accurately

                                                                                    3D Animation Showing the Work of  CALLIPERS & MICROMETER



As the sleeve is rotated it moves forward until the tip of the micrometer just touches the object being measured. The width of the object is then given on the scale.

Scale and Protector

ELECTROMAGNETIC SPECTRUM:

The electromagnetic spectrum

The electromagnetic spectrum . Radio wave have much longer wavelengths than light waves which in turn have greater wavelengths than x-rays and gamma rays . The visible part of the spectrum is very narrow compared with other parts.

TIME:

Time Page in my Physics-World Software

In ancient times,before clocks were invented,people knew that the seasons came and went at regular intervals. The sun also rose and set regularly creating night and day. Far back in prehistory, people realized that time could be measured using these regular events. The length of time between the appearance of two summers was called a year. The time between one sunrise and the next was called a day.Another way of measuring time was by noting the occurrence of a new moon.This happens at regular intervals of about 30 days.When some one said "many moons have passed" he or she meant that that the moon had waxed and waned many times.The word month comes from the word moon.

We know know that these regular events are due to movements of the earth and moon.The moon waxes and wanes as it moves round or orbits the earth. A day is the time taken for the earth to spin round on its own axis it is also travelling around the sun. A year is the time that its takes for the earth to make one complete circuit of the sun.A year is the time that its takes for the earth to make one complete circuit of the sun.It actually takes us 365(1/4) days to travel completely round the sun.So every fourth year we have a leap year of 366 days.The extra day occurs as the 29 th February.This avoids the calendar getting out of step with the movements of the earth and sun.

A day is divided into 24 equals divisions of an hour.Each hour is divided into 60 minutes and each minute into 60 seconds.

MEASURING DEVICES OF TIME:

Picture of Sundial

One of the first devices used to measure time was a Sundial.As the earth spins round on its axis a shadow cast by the sun gradually moves round.This movement was mapped  out and used  for telling the time.Other method of measuring depend on the processess that occur at regular rates. A simple example is burning down of a candle .If you have two similar candles you can make a candle clock.Measure the time it takes for the first candle to burn down a measured distance. It take say 3 hours to burn 3 cm. You now know the 1 cm is burned in one hour.Mark your second candle with 1 cm  marks. If you now light your second candle you able to tell how much time has passed by seeing how many marks heve disappered.Another regular process used to measure time is the passing of sand through a small hole .An egg timer works in this way. A larger version of the egg timer is called an hour glass.

Pendulum Clock

The next regular  process use to measure time was the swing of pendulum.This is the weight at the end of a stick or string.Pendulums of the same length will always take the same time to swing from one side to the other.No matter howfar they travel.To obtain different periods of swing you need pendulums of different lengths .A pendulum about 25 cm long swings both way in a second. A 1 metre pendulum takes roughly twice as long (about 2 seconds) . The pendulum of Big Ben in the houses of parliament.London is 4 metres long .It takes 4 seconds to swing back and forth.

Fashion Watches Quartz Watches Wrist Watches Lady's Watches Jewelry Watches.

The clocks that scientists us to measure time are based on the vibrations of  atoms . These vibrations are very regular and atomic clocks are correct to one second in 3000 years.Quartz digital watches measure time by counting the atomic vibrations of a quartz crystal.

Quartz digital watches

EARTH ORBIT:

Earth travels round the sun it creates night and day

As the earth travels round the sun it creates night and day .When one half of the earth is in darkness the other half is in the sunlight . As the earth moves around the sun it is also spinning on its own axis. Taking about 24 hours to complete an entire revolution.As the sun`s ray shinestedily from one direction, the spin of the earth has the effect of creating day and night on the earth surface. It takes the earth 365 days, 5 hours, 48 minutes and 46 seconds to complete each orbit around the sun. The earth travels around the sun . The earth travels around the sun at a speed of 106 000 kph (66 000 mph).

SEASON:

Four Seasons

                                                                                           3D Animation Showing The Four Seasons of Earth due to its Rotation and Revolution

MARCH 21 ST-- Spring in the northern hemisphere and autumn in the southern.

JUNE 21 ST --   The north pole is tilted towards the sun and it is in summer in the northern hemisphere, winter in the summer.

SEPTEMBER 23 RD-- Autumn in the northern hemisphere and spring in the southern hemisphere.

DECEMBER 21 ST -- The south pole is tilted towards the sun it is summer in the southern hemisphere.

WATCH:

Fashion Watches Quartz Watches Wrist Watches Lady's Watches Jewelry Watches.

A quartz digital watch keeps time by counting the atomic vibrations of small quartz crystal. In watches and small clocks the pendulam is replaced by a hairspring . This makes a balance wheel is kept in the motion by the impulse it receives from the mainspring through the escape wheel.

VELOCITY AND ACCELERATION:

Velocity and Acceleration Page in my Physics-World Software

Imagine two towns that are 60 miles apart. Two cars drive the distance between these towns.One goes all the way at 60 miles per hour (mph) and the other goes at 30 mph.Obviously the one that travels faster will get there more quickly.

VELOCITY:

Car Velocity

When we say that a car moves at 60 mph we mean that it takes it one hour to travel a distance of 60 miles.Therefore it its get from one town to the other in one hour.The second car only travels at 30 mph so that in one hour it has gone 30 miles so that in one hour it has gone 30 miles .Its take 2 hours to go the 60 miles .We said that the speed or velocity of the first car is greater than that of the second car. In one hour the first car travels 60 miles whereas the second car only travels 30 miles.Velocity measures the distance that some thing moves in a certain times, or how fast it moves.Besides miles per hour velocity is also measured in metres per second or kilometres per hour.

It is quite easy to work out how fast something moves.If a car travels 100 miles in 4 hours how far does it travel in one hour? The answer is 100 divided by 4 (100/4) , or 25 miles.In other words the car`s velocity is 25mph.

Next time you travel by car you can work out the average speed for the journey.Find out from the mileometer how far you have travelled.Velocity is equal to distance divided by time,so divide this distance by the length of time you have been travelling. A motorway journey of 110 miles may take 2 hours.The average speed is 55mph.For a town journey you may only go 20 miles in 1 hour so your velocity is 20 mph.

Look at the speedometer during this journeys. Sometimes it may read 60 mph,sometimes 30 mph . It may even read 0 mph when you are stuck in a traffick jam.This is because the speed of a car changing all the time .The speedometer shows how fast the car is moving at any point.The speed of a car over a whole journey is the average speed.

Various Speed of Different Cars

Average velocity is:

         

         s

v = ----- 

         t

(here v = velocity, s = distance covered by an object,  t = time taken to cover the distance)

ACCELERATION:

Velocity and Acceleration

When a car is standing still it does not have any velocity at all. As it starts to move it goes faster and faster.This is called acceleretion.If a car goe from 0 miles per hour to 50 miles per hour it has accelereted.A racing car can move from 0 miles per hour to 50 mph very quickly . An old car may take much longer to reach this speed.The racing car has more acceleretion.Acceleretion is the rate at which something changes its speed.The opposite of acceleretion is called deceleration-the rate of slowing down. If we assume that an moving object at time t0 present at A point where its velocity is v0 then at the time t moving object present at B point where it velocity is v then the average acceleration of an object is:

           v - v0

f = ----------------

        t - t0

                                                                                  3D Animation Showing Relationship Between Velocity and Acceleration

HI-SPEED CARS:

High Speed Racing Car

Most ordinary car can reach a speed of between 70 and 100 mph , but cars powered by jet engines can go much faster.In the 1983 the British driver Richard Noble raised the world land speed 633.4 mph .

ANIMAL SPEED:

Animals show wide difference in the speeds at which they can travel .A snail can move at about 1 millimetre per second(3.9 yard per hour). The fastest land animal is the Cheetah which has been known to move at 26 metres per second (60 mph) over short distance. A race horse can gallop 40 mph. The fastest speed ever measured for a man running was almost 27 mph.

RELATIVE VELOCITY:

Picture of Relative Velocity among the A, B , C cars

                                                                                    3D Animation Showing  Relative Velocity of Three Car A,B,C .

Two car named A and B coming to each other from opposite direction at a velocity of 50 km/hr and 60 km/hr respectively.In respect of A and B car`s drivers, both of them assume that other car is more faster than his car. In this instance one car`s driver assume that other car  coming toward his car in a velocity of 50 km/hr + 60 km/hr = 110 km/hr . This is (110 km/hr) the relative velocity in respect of each A and B car. If another car name C come into velocity of 70 km/hr in the same direction of car A (50 km/hr) and overtake it . After this incident driver of A car assume that car C come into very slow velocity of 70 km/hr - 50 km/hr  = 20 km/hr and move in front of the car A . Driver of car C assume that car A move backward at a velocity of 20 km/hr.

WEIGHT:

Weight  Page in my Physics-World Software

Pick up a brick in one hand and block of wood of about the same size in the other. The brick will feel heavier.It has more weight than the wood. The earth is pulling the brick with a larger force.It does this because the brick has more mass than the block of wood.In  other words the total amount of matter in the brick is greater than in the wood. We can see and feel that different materials have different masses even if they are the same size.The mass of a cube of gold would be nearly 20 times the mass of an ice cube of the same size.

Weighing Scale

                                                                                 3D Animation Showing the Use of Weight Scale

DENSITY:

The lightness or heaviness of a substance for a given volume is called density.It depends on how tightly the molecules in the substance are packed.

We say that brick has greater density than wood .The stony particles of brick are heavy nad are packed more tightly than the fibres of wood.

Imagine a lift. When the lift is empty, its density is low. As the lift begins to fill with people its density increases until it is full.It is then that its density is greatest. Its size has remained the same throughout but its mass has increased.

Different affects of water density on Different Materials 

Density is the mass of a certain volume of material.If a cube is 1 cm by 1 centimetre(1 cm3) . 1 cm3 of water weighs 1 gram.We say its density is 11.3 g per cm3 .The molecules of air are not very tightly packed and so its density is very low ,about 0.0012g per cm3.

Imagine a peice of material to be made up of little cubes of the same size.If you know how many of these cubes there are and weight of just one of them .You can work out the weight of the piece.The number of small 1 centimetre cubes is the total volume of the piece.

Equal weights of different material may have different volume.If you have a pair of old balance scales,put an iron kilogram weight on one side .Measure a kilogram of sugar on the other.You can see that a kilogram of sugar has a much larger volume than 1 kg of iron .This is because sugar has amuch lower density than iron. Measure Kilograms of other materials and see how their volumes vary .The smaller the volume for a given weight ,The greater the density.

Steel ships have an overall density of less than 1 kg per cm3 because their inside contain so much air.They can therefore float in water

Materials less dense than water can float.Icebergs which have a density less than that of water can float in it.Steel ships have an overall density of less than 1 kg per cm3 because their inside contain so much air.They can therefore float in water. A peice of solid steel does not float .A peice of solid steel does not float.People are about the same density as water so they can float or swim without sinking.

The density of a material is very important.We would not want to use a very dense material to make an aeroplane because it would make the plane too heavy.So an alloy , or mixture of metals is used The alloy consists mainly of mainly of aluminum because it is not very dense.Other metals in this mixture give it strength.Model aeroplanes are made up of balsa wood which also has very low density.Lead is used as the sinker on the end of a fishing line because it is very dense. 

SINK & FLOAT:

Sink and Float 

Some materials sink in water because their densities are greater than the density than water.Others will float . This shows that they are less dense than water. Weight some objects that look the same size ,if they have different weights then they have different densities.