The reason why the second is squared in the denominator of the unit for acceleration due to gravity is because acceleration is a rate of change of velocity. In other words, it is how quickly the velocity of an object is changing. When we say that an object is accelerating at 9.8 meters per second squared, we mean that its velocity is increasing by 9.8 meters per second every second.
The second squared in the denominator of the unit for acceleration due to gravity is a reminder that acceleration is a second-order quantity. This means that it depends on both the velocity and the time. In other words, the acceleration of an object is not just determined by its velocity, but also by how quickly its velocity is changing.
