the transfer function of the whole electrical/mechanical system. That is, the output (in the S domain) as a function of a transfer function (in S) times the electrical input, in the S domain. It sounds like you are applying a step input voltage to the motor, and when you disconnect the power, that is a step input also, just one of opposite sign compared to the input one. If it were possible to, you would apply a reverse voltage to the motor, to make it a dynamic brake, as others have called it. The voltage versus time function would be found by taking the inverse laplace transform of the desired output (in S) divided by the transfer function (in S).

Taking a wild guess here, and imagining the motor as a first-order system, one where a step input voltage produces a velocity proportional to (1 - e^(-kt)). I would guess that you would apply an extremely high reverse voltage, for a fraction of time, then the voltage would rapidly drop off to zero, exponential decay. This should keep the motor from starting to accelerate in the reverse direction, and still reduce the motor's momentum in the forward direction, as quickly as possible.

The 8051 output pins are 0/1, so have a second pin that when turned on, it switches a transistor on, and that discharges a capacitor. The capacitor's discharge should give the exponential decay you need. Email me and I'll try to help further.