In classical physics force is:
$$F=\frac {dp}{dt}$$ 
How about [quantum mechanics](http://en.wikipedia.org/wiki/Quantum_mechanics)?
In Old Quantum Mechanics momentum is: $p=\hbar \cdot k$ so force will be: 
$$F=\hbar \frac {dk}{dt}$$
what does $\frac {dk}{dt}$ mean?