Physical Quantities and measurement - II simply explained

1. Physical Quantities
and measurement - II
By Sukanya KB

2. Measurement of
time
 The accurate measurement of time helps us to schedule
and coordinate our daily activities.
Time
Time is defined as the interval between two events
We measure time with the help of mean solar day.
Mean solar day is the time taken by the earth to complete
one rotation about its own axis.

3. Units of time
 The standard unit of time in SI System is
‘Second’ (s).
 The bigger units of time are minute, hour, day,
month and year.
Units of time and their symbols
Unit Second Minute Hour
Symbol s min h

4. Relationship between units of
time.
1 minute = 60 seconds
1 hour = 60 minutes = (60×60) seconds = 3600 seconds
1 mean solar day = 24 hours = (24×3600) seconds =86400 seconds
1 year = 365 ¼ days = 1461/4 ×86400 seconds = 31557600 seconds

5. Numerical
 How many hours are there in 5 days and six
hours?
 1 day = 24 hours
 So 5 days is = 5×24 hours = 120 hours
 Thus 5 days and 6 hours = 120 hours + 6
hours = 126 hours.

6. Instruments used to measure
time
 Earlier people estimated time by looking at
the sun and the moon.
 The jantar mantar of Jipur and Delhi are
examples of Sundials

7.  Later time was measured with the help of
water clock, candle clock or sand clock.
 Then pendulum clocks were used.
 Now a days electronic or digital watches and
clocks are being used which are very
accurate.
 Stop watches are used to record the time
interval that has lapsed between two events.

9. Two convention for measuring
time.
 12 hour clock
 The 12 hour clock is the most commonly used
convention,
 Here, the day (24 hours) is divided into two
periods – the anti meridiem. (am) meaning
before midday , and post meridiem (pm)
meaning post midday.
 Eg; 6:20 am time in the early morning
 6:20 pm time in the evening.

10. 24 hours clock
 We do not use am or pm
 the time interval after midnight to 12 noon is
recorded as 0000 to 1200 hours
 Similarly 1201 to 2400 (0000) hours is used to
denote the time interval from 12 noon to midnight.
 Eg; if train is departed at 0750 hours, this means
the train departs at 7: 50 in the morning.
 To denote the same time in the evening, the time
mentioned would be 1950 hours.

11. Measurement of temperature
 Temperature is the degree of hotness or
coldness of a body.

12. Units of temperature
 The standard unit of temperature in SI System
is Kelvin.
 Kelvin unit is used for scientific
measurements.
 Commonly used units of temperature are
degree Celsius and degree Fahrenheit.
Units of temperature and their symbols.
Unit kelvin Celsius Fahrenheit
Symbol K ºC °F