In most situations, forces can be grouped into two categories: contact forces and field forces. Contact forces occur as a result of direct physical contact between objects. Field forces, however, act without the necessity of physical contact between objects. They depend on the presence of a "field"in the region of space surrounding the body under consideration. You can think of a field as a property of space that is detectable by the forces it exerts. Scientists think there are only four fundamental force fields in nature. These are gravitational, electromagnetic, strong nuclear, and weak nuclear fields.

The gravitational field is responsible for the weight of a body. The forces of the electromagnetic field include those of static electricity and magnetism; they are also responsible for attraction among atoms in bulk matter. Both the strong nuclear and the weak force fields are only effective over distances roughly equal to a length of scale no larger than an atomic nucleus (10⁻¹⁵ m). Their range is so small that neither field has influence in the macroscopic world of Newtonian mechanics.

Contact forces are fundamentally electromagnetic. When adhesive tape sticks to a piece of paper, the atoms of the tape get intermingled with those of the paper, causing a net electromagnetic force between the two objects. However, in the context of Newtonian mechanics, the electromagnetic origin of contact forces is not an important concern.

This text is adapted from Openstax, University Physics Volume 1, Section 5.1: Forces.