The acceleration of an object thrown straight up in the air near the surface of the Earth at the very top of its path is approximately 9.8 m/s² downwards.

Here's why:

* Gravity is the only force acting: At the top of its path, the object momentarily stops moving upwards before it begins to fall back down. The only force acting on it at this point is gravity.

* Constant acceleration due to gravity: Gravity pulls objects towards the Earth with a constant acceleration, which we typically approximate as 9.8 m/s² near the Earth's surface.

Important Note: This acceleration is directed downwards, even though the object is momentarily at rest. The acceleration is what will cause the object to start moving downwards again.