∴AB=5，

Let ⊙P be tangent to the straight line AB after x seconds, the crossing point P is perpendicular to AB, and the vertical foot is Q, then PQ =1;

(1) When ⊙P is tangent to the left straight line AB, AP=4-x,

From △APQ∽△ A Bo

APAB=PQBO, which is 4? x5= 13，

The solution is x = 73.

(2) When ⊙P is tangent to the right straight line AB, AP = x-4;

From △APQ∽△ A Bo

APAB=PQBO, which is x? 45= 13,

The solution is x = 173.

So fill in 73 or 173.