Small ball photography

(1) As can be seen from the figure, at equal intervals, the distance and speed of the ball gradually increase, which is a linear motion with variable speed;

(2) At this time, the ball is subjected to vertical upward resistance and vertical downward gravity. At this time, gravity is greater than resistance, and the ball moves at variable speed under the action of unbalanced force;

(3) Because everything around the earth is influenced by gravity, and because the direction of gravity is always vertical downward, the ball finally landed because of the influence of gravity.

So, the answer is: variable speed; Unbalanced force; Under the action of gravity.