Height after ts seconds is given by the expression
[tex]H = 610-4.9t^2[/tex]
Taking derivative with respect to time
[tex]\frac{dH}{dt} =0-9.8t[/tex]
This is the value of velocity after time t seconds, negative sign means velocity is towards downward direction.
v = 9.8t
After 7 seconds, v = 9.8*7 = 68.6 m/s
Speed after 7 seconds after it is dropped = 68.6 m/s
