The original vertical velocity = 0 (because the stone was dropped).
Assume that air resistance may be ignored.
Let x = the distance the stone drops in time t.
Then
x = 0*t + (1/2)*(9.8 m/s²)*(t s)²
= 4.9t² m
The height of the stone above ground is
h(t) = 900 - 4.9t² m
Answer: h(t) = 900 - 4.9t²
Conditions for equilibrium require that the sum of all external forces acting on the body is zero (first condition of equilibrium), and the sum of all external torques from external forces is zero (second condition of equilibrium). These two conditions must be simultaneously satisfied in equilibrium.
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