Important definination and notes

Time
It is defined as the interval between the  two events.
The SI Unit of time is Second. 

Mean Solar Day
The time taken by Earth to rotate around its axis is called 1 Mean Solar Day.
1 Mean Solar Day = 24 hr 
                              = 24 x 3600 s
                              =  86400 s.

1 Second

It is defined  as 1/86400 of a mean solar day.
1  minute =60 second (sec)

1  hour   =3600 second (sec)

Length
The distance between two points (ends) is known as length. Its SI Unit Is Meter.

Volume
The space occupied by a body is called volume. Its SI Unit is Cubic Meter (m³).  
Note *

Volume of a regular shape is calculated by using their respective formula i.e.  V= LXBXH

Volume of a irregular body is measured by displacement method. 
                                                                                                                      
Area
The total space covered by a body is called area. Its SI Unit is square meter (m²).
Note *
Area of a regular body is found by using their respective formula i.e. A = LXB 

Area of a irregular body is formed by plotting in a graph

Multiples and sub-multiples of

METER
mm   cm   deci    m    deca   hm    km


1 Meter           = 10 decimeters (dm)
1 Meter           = 100 centimeters (cm)
1 Meter           = 1000 millimeters (mm)
10 Meter         = 1 decameter (dm)
10 decameter  = 1 hectometer (hm)
10 hectometer = 1 kilometer (km)

KILOGRAM

mg   mm  deci   g  deca   hg  kg

1 Tonne      = 1000 kilograms (kg)

1 Quintal    = 100   kilograms (kg)

1 kilogram  = 1000  grams (g)

1 gram        =  1000 milligram (mg)