Question

Name the S.I. unit of time and define it.

Answer 1

Hint : S.I. unit is a method of expressing several quantities in terms of internationally accepted units. So the S.I. unit of time will be the unit that is used in as the internally accepted unit of time.

Complete step by step answer
The International System of Units, SI is a system comprising seven fundamental units and numerous derived units that can be represented in terms of the seven fundamental units.
A unit of time is any specific time interval, used as a standard way of expressing some duration.
The SI unit of time is seconds and it is written as ‘ $s$ ’.
Formerly, one second could be expressed as {\raise0.7ex\hbox{ 1 } \\!\mathord{\left/ {\vphantom {1 {86400}}}\right.} \\!\lower0.7ex\hbox{ {86400} }} part of a mean solar day. A mean solar day is the average period of rotation of the Earth on its axis. This duration can also be expressed in terms of $86400s$ .
One minute= 60 seconds;
one hour=60 minutes=3600 seconds and
one day= 24 hours.
Thus $86400$ seconds make up one day, i.e, $24$ hours which is also equivalent to $1440$ minutes.
In terms of ephemeris year, the fraction {\raise0.7ex\hbox{ 1 } \\!\mathord{\left/ {\vphantom {1 {31556925.9747}}}\right.} \\!\lower0.7ex\hbox{ {31556925.9747} }} of the tropical year for 1900 January 0 at 12 hours gives the ephemeris second.
The official SI unit of time is defined as “The duration of $9,192,631,770$ periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium atom.” In other words, one second can also be defined as the radiation frequency of the change of state of cesium atoms.

Note
Time expressed in seconds, symbol ‘s’, one of the seven fundamental units. Several S.I. units of quantities can be expressed derived from this.
An example is speed, which has SI unit $m{s^{ - 1}}$ . The S.I. units of energy, work and heat is ${m^2}kg{s^{ - 2}}$ .