The main concepts in physics are often very simple; it takes only a few words to express them and they can be applied to lots of situations. However, ‘simple’ does not mean ‘easy’. Some concepts are quite abstract – such as force, energy and voltage. Scientists had to use their imagination to conceive such concepts. Other scientists then spent years working, experimenting, testing and refining the concepts until they finally reached the established concepts that we use today.

Isaac Newton’s work on motion is a good example. Newton published his ideas in a book; the book’s title translates as Mathematical Principles of Natural Philosophy. 
Newton wanted to develop an understanding of the idea of ‘force’. You may have been told in your early studies of science that ‘a force is a push or a pull’. Newton’s idea was that forces are interactions between bodies and that they change the motion of the body that they act on. Forces acting on an object can produce acceleration. For an object of constant mass, this acceleration is directly proportional to the resultant force acting on the object. That is much more like a scientific definition of force.