# Tag Info

## When to Use this Tag

Use when discussing the origin or action of classical forces, i.e. quantities causing an acceleration of a body. If you already know all forces relevant to your question and are now interested in the dynamics of the problem, use instead. You might also want to tag the question as one of the other tags mentioned in . If you know the source of a (classical) force, tag the question as , , etc., too.
For a quantum field theoretic approach to forces, use either and/or .

## Introduction

Newton’s Laws are based on the idea of forces, quantities that cause the body they act on to accelerate proportional to their magnitude and parallel to their direction. Forces can have many different origins, but most of these can be traced back to four fundamental forces: gravity, electromagnetism, the weak interaction and the strong interaction. Only gravity and electromagnetism can be described in a classical setting.

The most important property of the forces acting on a body is that they sum up. That is, to find the net force acting on a body (the $\vec F$ in Newton’s second law), one can sum over all external forces $\vec F_i$ acting on said body:

$$\tag{1} \sum_i \vec F_i = m \vec a \quad.$$

It is often difficult to find the forces relevant to a problem at hand. Lagrangian mechanics solves this by deriving the equations of motion (such as (1)) from the variational principle.