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Zibo Ermo Mathematics
When a, a and b reach the maximum speed v, the acceleration of a and b should be zero.

For AB as a whole: from the equilibrium condition,

kx-(m+M)gsinθ-μ(m+M)gcosθ=0,

So the spring is in compression at this time. So, A is wrong.

When B, A and B are just separated, the elastic force between AB is zero. According to Newton's second law,

Along the inclined plane, mgsinθ+μmgcosθ=ma,

Get a=gsinθ+μgcosθ,

According to Newton's second law, the accelerations of A and B are the same, so B is correct.

C, in the process of A and B from release to maximum velocity V, for AB as a whole, according to the kinetic energy theorem, we get

-(m+M)gLsinθ-μ(m+M)gcosθ? L+W bullet = 12(m+M)v2

Work done by the spring on A W =12 (m+m) V2+(m+m) GLS in θ+μ (m+m) GCOS θ? L, so c is wrong.

D, in the process of releasing from A and B to the maximum speed V, for B, according to the kinetic energy theorem, it is obtained as follows.

The work done by the resultant force on b is W =△Ek= 12mv2, so d is correct.

So: BD.