Motion and Time
Class 7 Arjun
Motion
- the objects which change their positions with respect to time and the objects at rest are said to be in state of motion
- state of rest and motion are relative terms
Slow and Fast motion
- compared with different objects with respect to the distance covered in unit or same time
Speed
- how fast an object moves is measured by speed
- distance travelled
- time taken to cover the distance
- ie. distance covered by the object in unit time (metre/sec)
- speed = distance / time
Measurement of time
- interval or duration between 2 events in known as time
- earlier, people used to calculate time using natural phenomena or events
- sunrise and sunset = one day
- new moon and next new moon = one month
- one complete revolution by the earth around the sun = one year
Water clocks
Sundial
- first used by romans
- length of the shadow cast by the object changes with the time of the day
Sand clocks
- fixed quantity of sand passes through narrow gaps in certain fixed interval
Modern clock
- atoms and molecules of the substances vibrate about a mean position
- like oscillation of the crystal like quartz
National physical laboratory
- delhi
- time keeping service
- clock with accuracy of one millionth of a second
Sawai Jai Singh II
- ruler of amber (jaipur)
- title "Sawai" - one and a quarter - from Aurangazeb
- observatories in - Delhi, Jaipur, Varanasi, Ujjain, Mathura to measure time
- sundial in jaipur - accuracy of 2 seconds
- sundials cannot work after sunset
- his observatories were called as Jantar Mantar
Timer
- special type of clock to control the sequence of an event
- traffic signals, automobiles, washing machine, oven, explosives
Unit of time
- SI unit - second
- one complete revolution by the earth around the sun = one year
- one microsecond - one millionth of a second
- one nano second - one billionth of a second
- centuries and millenniums -
- in sports, timer of one tenth or one hundredths are used to measure accuracy
Simple pendulum
- all clocks use some periodic motion
- consists of small mass called BOB (metallic ball or stone) suspended by a non extensible string
- rest in its mean position
- begins to move to and fro from the mean position
- this motion is known as oscillatory or periodic motion
- when it reaches the mean position, then one oscillation is completed
- that is the distance covered by the bob between 2 extreme position on the either side of the bob and returing to the original mean position
- time taken for one oscillation is known as time period
Characteristics of pendulum
- time depends on the length - greater the length greater the time period
- does not depend on the mass of the bob used within the reasonable limit
- does not depend upon the extent to which the bob is displaced
Measuring Speed
- speed = distance / time
- SI unit = meter / second
- conversion from m/sec to km/hr then multiply with 18 / 5
- for reverse multiply by 5 / 18
- Speed of light is 3 * 10^8 m/sec or 300 million meter/sec
- speed of earth around the sun = 30km/sec
Average speed
- total distance travelled / total time taken
Speed measuring devices
- speedometer
- odometer
Uniform and non uniform motion
- when a object covers equal distance in equal time, then it is uniform motion
- covers unequal distance in equal time is known as non uniform motion
Distance time graph
- uniform motion - straight line
- non uniform motion - non straight line
