How the value of acceleration due to gravity is 9.81 ?
We have all been told that acceleration due to gravity, g = 9.8 m/s2 or g = 9.81 m/s2, however, due to a myriad of factors, g in this room may be slightly different than the g that you have always been told to use. An extreme example of how g would vary measurably is to measure it while flying in an airplane at 35,000 ft versus measuring it on the surface of the earth. One of the easiest ways to measure g is to take advantage of the principle on which all grandfather clocks are based: the simple pendulum. The time it takes a pendulum to make a complete swing from one extreme to another (to complete one cycle) is called the period of oscillation. This swinging back and forth repeats itself over equal time periods and therefore this motion is called periodic motion. It turns out that we can describe this periodic motion in terms of circular functions, sine and cosines, and therefore it is called harmonic motion. (All harmonic motion is periodic, but not all periodic motion is harmonic.)
