At t sec after liftoff, the height of a rocket is 8t^2ft. How fast is the rocket climbing 3 sec after liftoff?

Answer 1

... which makes the height of the rocket at t=3s equal to 72 ft, its velocity is 48 ft/sec at that point, and the net acceleration (second derivative of displacement, or first derivative of velocity) is 16 ft/s^2

Answer 2

Velocity is the derivative of distance.

In this case velocity is 16t.

This, of course. ignores the downward force of gravity.