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?

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

Did you know?

In the scenario described, the acceleration value of -9.8 m/s² is used when calculating the height of the cliff because it represents the acceleration due to gravity. Gravity always acts in the downward direction, causing objects to accelerate towards the Earth at a rate of 9.8 m/s². When the stone is thrown, its initial upward velocity opposes gravity's pull, leading to a decrease in upward velocity until it reaches its highest point. From there, gravity begins to dominate, causing the stone to accelerate downwards. By considering the acceleration due to gravity as -9.8 m/s² for both the upward and downward motion, the height of the cliff can be accurately determined using the given information of angle, initial speed, and time of flight.