Why is length not measured using hanspan or cubit?

Hands pan and cubit are not used as standard units of length because their sizes vary from person to person.so two different person may give different measurements for the same length, which is not desirable for a standard unit of length

acceleration = (change in speed) / (time for the change)

change in speed = (20-120)= -100 m/s.

1.5 min = 90 sec

acceleration = -100/90 = -1.11 m/s^2.

we don't care about the 150kg or the 2,500 meters. they don't make any difference.

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