Consider the situation of the previous problem. The force on the central charge due to the shell is

We know that electric field lines originate from a positive charge and terminate towards a negative charge. Now, the charge Q outside will try to nullify the field lines that emerge from charge q1, and hence, negative charge will be induced on the face nearer to q1. This will induce positive charge on the farther face of the sphere. Hence, a force acts on the central charge from left to right, that is, from negative to positive.

1