How to distinguish between the end of the first second, the beginning of the third, within the second and within the first two seconds

How to distinguish between the end of the first second, the beginning of the third, within the second and within the first two seconds

The end of the first second and the beginning of the third second refer to the time, which is a time point. In the first two seconds, the second second refers to a time period. But in the second second second, it only refers to one second, while in the first two seconds, it refers to two seconds

The velocity of a small ball at the end of the first second, the second and the third seconds and the velocity of the ball falling within the first second, the second and the third seconds are determined
(it's better to have formulas and processes,
The last is to find out the displacement of the fall

This is actually the law of free falling motion
Because V = GT,
So V1: V2: V3 = GT1: GT2: GT3 = T1: T2: T3 = 1:2:3
Because s = 1 / 2GT ^ 2
So ^ s ^ 1:2:2 / S ^ 2:2 = 2:1 / S ^ 2

8r+3s=2r-6s=3 s= r=

8r+3s=3 (1)
2r-6s=3 (2)
(1) Multiply by 2
16r+6s=6 (3)
(3) + 2
18r=9
r=1/2
Substituting (2) to get:
1-6s=3
6s=-2
s=-1/3
So:
s=-1/3
r=1/2