a stone is thrown from the top of a cliff above the sea at an angle of 30 degrees to the horizontal. it has an initial speed of 60m/s and is in flight for 8 secs. when finding the height of the cliff, why is the acceleration value -9.8 when the stone travel both up and down?

Answer 1

There is only one force acting on it during flight (ignoring air friction). That is gravity, m g.
Gravity does not push up.
Second law, force = m a
only force is m g
so
m g = m a
g is down so a is down