The light bar AB is 2L long, the A end is connected with a fixed shaft, the B end is fixed with a ball with a mass of 2m, and the middle point C is fixed with a ball with a mass of m. The AB bar can be wound around the A end. (1) During the rotation, the angular velocities of the two balls A and B are the same. If the speed of the ball is 0 and the speed of the ball is 0, then there is.

v C = 1 2 v B

The system composed of a, b and rod has the conservation of mechanical energy. According to the law of conservation of mechanical energy, the reference plane with zero potential energy is selected as the lowest point, including

E 1 =mg？ L+2mg？ 2L=5mgL，

E 2 = mgl+12mV 2c+12mV 2B.

And e1= e 2.

The above four equations can be obtained simultaneously: v B = 2 10gL 3.

By the formula: v B =ω? 2L

Solution: ω= 10g 9L

(2) Increment of mechanical energy of B-end ball: △ E = 1.22mV2B-2mg? 2l = 49mg L.

Answer: When the AB lever rotates to the vertical position, the angular velocity ω =10g9l; The mechanical energy increment of B-end ball is 49 mgl.