Coulomb's Law states that the force of friction between two surfaces in contact is directly proportional to the normal force between them. Therefore, the tip of a scanning probe microscope will only experience friction when it is in contact with the surface. To prevent the tip from crashing into the surface, the normal force must be kept sufficiently small. This can be achieved by using a soft cantilever with a low spring constant.

To calculate the critical normal force, we use the following formula:

```

F_n ≤ μF_t

```

where:

- F_n is the normal force between the tip and the surface

- F_t is the tangential force applied to the tip

- μ is the coefficient of friction between the tip and the surface

For a typical scanning probe microscope, the coefficient of friction is between 0.2 and 0.5. If we assume a tangential force of 1 nN, the critical normal force would be between 0.2 nN and 0.5 nN. This is a very small force, and it is easy for the tip to crash into the surface if the normal force is not controlled properly.

To avoid crashing the tip, the scanning probe microscope must be operated in a non-contact or intermittent contact mode. In non-contact mode, the tip is held a few nanometers above the surface, and it does not touch the surface at all. In intermittent contact mode, the tip is brought into contact with the surface, but it is immediately retracted before it can crash into the surface.