Forces affect every moment of our life. Our bodies are held to the Earth by force, and they are held together by the forces of charged particles. When we open a door, walk down a street, lift a fork, or touch a baby's face, we are applying force. Our body's atoms are held together by electrical forces, and the core of an atom, called the nucleus, is held together by the strongest force known to us—nuclear force.

The study of motion is called kinematics, but kinematics only describes the way objects move—their velocity and their acceleration. Dynamics is the study of how forces affect the motion of objects and systems. It considers the cause of the motion of objects and systems of interest, where a system is anything being analyzed. To understand this, we need a working definition of force. An intuitive definition of force—that is, a push or a pull—is a good place to start. We know that a push or a pull has both magnitude and direction (therefore, force is a vector quantity). Thus, we can define force as the push or pull on an object with a specific magnitude and direction. Force can be represented by vectors or expressed as a multiple of a standard force.

The SI unit of force is called the newton (abbreviated N), and 1 N is the force needed to accelerate an object with a mass of 1 kg at a rate of 1 m/s²: 1 N = 1 kg·m/s².

This text is adapted from Openstax, University Physics Volume 1, Section 5.0: Introduction and Section 5.1: Forces.