## Question 8: The human pulse and the swing of a pendulum are possible time units. Why are they not often used?

##### Time standard

Any repetitive phenomenon occurring in the nature can be used as time standard.

Standard time unit
Do you know what it means to be a time unit?
Our present time unit is second. We know it has a clear definition. If human pulse is chosen to be a time unit, then it will be the time for one heartbeat, and instead of second, might have name like 1 beat etc. Similarly, if pendulum is chosen to be a unit standard, then instead of seconds, we might be saying as 1 period time or 2 period time!

(Example of constantly repeating phenomenon in nature may be the revolution of earth around the sun). Human pulse and pendulum (time period of) are of them.
However, to choose any such phenomena as time standard needs them to be uniform, unaffected in a particular environment and easily accessible. Human pulse and pendulum do not fulfill this criteria.

• Human pulse varies from person to person. Different persons have different pulse rate. So first to decided whose pulse is to be the time standard. Moreover, human pulse can also change even for a single person; depending on age, mood, orientation position, health etc etc. So even in the case of the same person, the time unit may not be the standard. Similarly, a person will not live forever and the time standard has to be changed after his/her passing away. Therefore, human pulse is not uniform, affects easily in the changed conditions and not easily accessible and so cannot be used as standard time unit.
• Pendulum cannot be used as standard time unit. The time period of a pendulum depends upon the length of the pendulum and gravitational acceleration. Both of them change when the environment is changed. Temperature affects the length of a substance. So, for the same pendulum, the length may vary from place to place in the world or change of season. (Remember there is big variation in temperature around the globe. Your pendulum in the North Pole will give different result when brought to the sizzling African deserts). Similarly, the value of gravitational acceleration, g, also varies with height. The same pendulum has different time periods at Karachi and Kalam.

So, neither human pulse nor the pendulum is good to be taken as time units.

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