Q:

Why is acceleration constant in this example?

I was shown this video by our professor:
https://www.youtube.com/watch?v=8A7DiGzJUvg&feature=youtu.be
And I do not understand why the acceleration is constant. Our professor told us that a freely falling object has constant acceleration of 9.8 meters per second squared. My first question is if an object is considered freely falling when it is thrown upwards, or if it only becomes "freely falling" when it..actually is falling? Secondly, if acceleration measures the rate of change of the velocity, then from looking at the example in the video, the velocity goes down to 0 when it reaches maximum height. Shouldn't changing from an non-zero velocity to a zero velocity be considered a change in acceleration, since it is slowing down at that time? If I understood this correctly, why is the acceleration constant throughout the video?
At this point I re-watched the video and realized it says "straight line motion" which I'm not sure if they mean a horizontal line [kinda confusing since it is vertical so I feel like it makes more sense to think of it as it being thrown so I guess the question of freely falling doesn't apply, but either way at one point the velocity reaches 0 so why doesn't the acceleration change at all?

A:

The phrase freely falling really means freely moving, that is nothing is pushing or pulling the object while it's moving. So the object is freely falling even when it's rising! Like all things Physics has its own special terminology and I'm afraid you just have to get used to it.
Re the acceleration: the acceleration is the change of velocity with time. If I throw a stone upwards at 98.1 m/s then (ignoring air resistance) in the first second it slows by 9.81 m/s to 88.29 m/s, in the second second it slows by 9.81 m/s to 78.48 m/s and so on until after 10 seconds its velocity has slowed to zero. Each second it slows by the same 9.81 m/s, so the acceleration has the constant value of -9.81 m/sec$^2$ (the - sign means the acceleration is downwards).

