Question: Why my friction curve is discontinuous?

I want to simulate the model as show in Fig.1 

Fig. 1

Its equation of movement is 


and its mathematical solution can be caculate as Fig. 2

Fig. 2

I created a model (Fig. 3) to simulate it

Fig. 3

and the component of the friction was definited as


However, the result is  discontinuous (Fig. 4)

simulation result

Could you please to explain the reason of discontinuous?

think you very much.


Here is the .msim file



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