Some physicists have pointed out that we do not know what force is. The most common definition of force in textbooks has been criticized for more than two centuries. Many studies have shown that the concept of force is a problem for teaching. How to conceive force on the basis of the concepts and criticism of force in the works of Newton, Euler, d'Alembert, Lagrange, Lazare Carnot, Saint-Venant, Reech, Kirchhoff, Mach, Hertz and Poincare is the question of the present article. This part of the article is followed by an overview of definitions of force in contemporary textbooks. In the next part, an answer to the question is given: how to understand force within the framework of the laws of motion and in applications. Finally, some educational implications are considered.