Question

HOW IS THE VALUE OF ACCELERATION DUE TO GRAVITY DERIVED?

Answer 1

the value of acceleration due to gravity is 9.8

Answer 2

     We can take a good solid object of having a small mass m , that has a good shape like sphere.   We can climb to the top of a tall building like 50 meters.  Then drop the object.  We can keep some metallic sheet on the floor so that the object makes sound when it touches the ground.  We note down by watch either at  the top or at the bottom of the building the time duration for the fall.
  Then we use the equations of kinematics  s = 1/2 g t^2 to find the g.
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we can drop a known object with known volume and mass , in  a deep tank containing water.  The acceleration of the object is less in water.  We can measure the time duration for descent of the object.  Using Pascal's law and Archimedes law , we can find the gravity.
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Experiment in a laboratory can consist of  a set up for the above with calibrated instruments.