One of the first things you learn in the study of physics is kinematics, the relationship between position, velocity and acceleration for a mass point, as described by the four kinematic equations of motion. In order to handle a non-constant force, or a force over a distance, the concept of work is introduced.
Work is defined as force times displacement.
More specifically, work is the component of an applied force in the direction of the displacement times the displacement. This is usually written as a dot product. If the force is not constant the work is an integral. In this section work is defined, and in a subsequent section work is connected to energy. Work and energy are powerful problem solving tools.