Question

what are contact forces

Answer 1

some forces act with out touching, for example, you felt the force between two magnets ... they act even without touching. These are called non contact forces.

Contact forces require contact. for example: you are pushing table.
http://en.wikipedia.org/wiki/Contact_force

Answer 2

Contact forces are those forces which are a result of the contact between two objects.
eg: lifting a book, friction etc.

Answer 3

Contact forces are a little tricky because they don't directly involve any of the four fundamental forces (gravity, electromagnetic, weak and strong nuclear). They are the direct result of the Pauli exclusion principle.

What happens is that the outer electron orbitals of the atoms in both surfaces come into contact -- being to overlap. But as you know, the Pauli principle says that electrons in the same region of space must have distinctly different kinetic energies. Hence for the outer electron orbitals to overlap, some of the electrons need to be promoted to much higher kinetic energies. That is, you require lots more energy to move the orbitals further into each other, which means there is a strong repulsive force. However, the origin of the force is in the exclusion principle. It would operate even if the electrons were uncharged, e.g. if you were pushing orbitals occupied by neutrons into overlap.

Contact forces are one of the few direct manifestations of quantum mechanics in our everyday lives.