The concept of force is central to the study of all natural phenomena that involve some kind of interaction between two or more objects regardless of whether visible motion occurs. For example, architects and civil engineers want their structures to stand firm against the forces of gravity, wind, and earthquakes. On the other hand, automotive engineers need to know how best to accelerate a car, brake it to a safe stop, and smoothly change its direction. Students need to know that balanced forces keep an object from changing its velocity and that changes in the velocities of objects are caused by unbalanced forces.

There are only four known fundamental forces: gravitational forces, electromagnetic forces, and two nuclear forces known as the strong and the weak forces. Gravitational force is the attraction all objects with mass have for one another. The common experience of gravity on Earth is only one example; the other forces of pushing and pulling are elastic forces caused by electromagnetic interactions between atoms and molecules being pushed together or pulled apart. The large, repulsive electrical forces between the positively charged protons in the nucleus of an atom are balanced against the stronger, attractive nuclear forces that hold the atom together.

Students learned in grade two that the way to change how something is moving is to give it a push or a pull (e.g., apply a force). In grade four the study of magnets, compasses, and static electricity gave students experience with electromagnetic forces. In grade seven students learned about motion and forces, which involved comparing bones, muscles, and joints in the body to machines.