(1) Let two balls roll off the same arc trajectory at the same height, so that the two balls roll off the trajectory at the same time with the same speed. Ball A makes a flat throwing motion, and ball B makes a uniform linear motion. When two balls meet, it means that the horizontal direction of flat throwing motion is uniform linear motion. When two balls roll down at the same time, they can still collide, which means that the horizontal direction of flat throwing motion is always a uniform linear motion.

Therefore, the answer is: uniform linear motion; Flat throwing motion is a uniform linear motion in the horizontal direction.

(2) Therefore, the flat throwing motion of an object includes:

Vertical direction: h = 9l = 12gt2, vy=gt.

Horizontal direction: 6L=v0t

The speed at which A reaches point P is V = V20+V2Y.

L = 5 cm, which leads to v = 10 m/s. 。

So the answer is: 10.