MEASURING TIME

ALL  TIME  KEEPING  DEVICES depend   on  counting  a regularly -repeated .phenomenon .The  earliest  devices   were   based  on  daily  ,monthly, or  yearly  cycles  of  the  sun and  moon .Most  modern  clocks  are  based   on  repeated   mechanical  or  electronic  oscillations (vibrations). The  more  frequent   the  vibrations, the  greater  the  potential  accuracy of  the  clock.The  crystal  in  the quartz  watch   typically   vibrates  at  32,768  hertz.(32,768 times  a  second), so  it  keeps  better   time than  a   pendulum   clock ,whose   pendulum   typically   swings  twice  a  second .
The    most  accurate    timekeeping   devices  are  atomic   fountains ,which   are  based  on  oscillations  of  caesium    atoms .Global  communication  technologies  , such  as  computer networks   and broadcasting, rely  on  the  world  using   one accurate  time  standard.This  is   called  UCT.(UNIVERSAL  COORDINATE  TIME),and  is  based on   the  average  time  signal  received  from   over  200  atomic  clocks  worldwide .The    more  we    rely  on  high technologies   for  precise   timekeeping ,the  more  vulnerable  we  become  to  problems with  that  technology.
THE   ATOMIC  FOUNTAIN.
At  present  ,the  most  accurate  timekeeping  device  ,is  the  atomic  fountain.It  was  developed  by  Nobel  Prize-winning  scientists in  the  early  1990s   and  is  far  more  accurate than  a  standard  atomic clock.The   atomic  fountain  enable us  to  measure time  with  greater  accuracy , mainly  because  it  uses  cooler ,slower-moving  atoms  than  an  atomic  clock.Atomic fountain   devices  are  accurate  to  within  one ten-billionth  of a second  per day.
PENDULUM  CLOCK.
In  the   16th   century , the  Italian  scientist  Galileo Galilei  used  the   regular  swing  of  a  pendulum  to measure periods  of  time .He  suggested     that  a  pendulum  could  be connected  to  the  hands   of  a  clock to  turn  them  in   WE CAN NOT SEE AIR, BUT it is not just empty space .Air is a mixture of many gases.Air exerts pressure on us,but does not crush us .In principle ,we are living on the floor of a kilometer -thick ocean of air known as the air cover or the atmosphere of the Earth.Since ,air is a matter ,it can also carry things ;leaves, in autumn ,birds,aircraft,hot air balloons,and many more things .However ,all objects need propulsion to lift from the ground-birds beat their wings, Aircraft depends on propellers or jet engines. FLYING OF AIRCRAFT- Air crafts need a combination of procedures to fly in the air .They need a propulsion system- a propeller or jet engine .They also need a lift , which comes from wings .While flying , the forward thrust causes the air to flow at great speeds along the wings.The curved shape of the wings and their slightly tilted position give rise to a low pressure above the aircraft and high pressure below it ..This is how the aircraft is pushed upwards .The pressure difference thus generates a buoyancy ,which is greater than the weight of the aircraft ,which pulls it down .The aircraft will fall if the buoyancy is less. HOT AIR BALLOON FLOATS IN THE AIR.- Air at 90 degree centigrade filled in the hot air balloon .The hot air keeps the balloon in the air because its density is lower than that of the surrounding cold air .Before starting the balloon , a gas burner is ignited below the opening of the huge balloon cover and the flames heats up the air in the balloon .When the balloon is rises to the desired height , the burner is switched off, if one wants to the further , it is started again .The balloon must have a minimum size so that it can over come the force of gravitational attraction of the Earth.Thus hot air balloon are able to float in air . PROPELLER WORKS BY SCOOPING AIR. A PROPELLER, IN an aircraft scoops air , as it scoop water in a ship., as in a wind mill,the blades of a propeller are arranged in the form of a star around a rotating shaft..They are shaped and aligned in such a way that the air falls diagonally on them and flows along them .This create a strong air flow that gives the aircraft the required thrust..This principle is used not just by smaller aircraft but also by hovercrafts which can travel over meadows or frozen water. CANDIES ARE SERVED BEFORE AN AIRCRAFT TAKES OFF.-This is because when an aircraft takes off ,some passengers experience a crackling sound in the Ears or get earache .The reason is that the inner ear does not adapt itself quickly to the lower air pressure caused by the take off.A simple candy helps in avoiding this discomfort because while sucking a candy , we need to swallow often .This helps the small muscles to expand the connection between the ear and the nose .In a way , they are ventilated. This leads to a quicker adaption of the inner ear to the outer air pressure and reduce the pain. WE ARE NOT CRUSHED BY THE AIR PRESSURE.-The air exerts pressure on us .The reason why we are not crushed by the air pressure is that the same air pressure is also present in the hollow cavities of our body :in the lungs and in the tiny chambers of our bones.With the same pressure in and around us , we do not feel the weight of the air at all. FACTS- The normal aircraft cannot fly in outer space .Because there is no air in space to generate lift on the wings there will be no buoyancy to overcome the force of gravity acting downwards. HOT AIR BALLOONS PROPELLER AIR CRAFT FUNCTION   regular  steps.
 MEASURING  TIME