Physics problem in senior high school: The polished rod AB is 2L long, the A end is fixed at the stern, the B end is fixed with a ball with a mass of 2m, and the midpoint C is fixed with a ball with a mass of m (to be supplemented). Conservation of system mechanical energy

2 mgx 2l+mgL =( 1/2)mVCVC+( 1/2)(2m)VBVB

VB=2VC

Under the root symbol, VC= 10gL/9.

The angular velocity under the root sign W=VC/L= 10g/(9L).

The mechanical energy increment of the ball at the B end is (1/2) (2m) vbvbvbvb-2mg2l = (1/2) (2m) x10g/(9l) x4-4mgl.

=(4/9)mgL