## 9.1 What is a force?

You can think of a force as a push or a pull, but really forces describe *interactions* between two objects. Forces cause *changes* in motion.

Generally speaking, we have have two models that we use to understand interactions between things: the classical model and the modern model. The classical model was developed over many centuries, and was really solidified in the 1600’s and 1700’s. The modern model came about as a result of radical shifts in our understanding of the universe, starting in the early 1900’s; this model is continuously being explored and refined based on new research.

### 9.1.1 The classical model

In the classical model, we describe the interactions between macroscopic objects (essentially anything large enough to be seen by the naked eye) by specifying a number of specific, unrelated, forces. For example, some of the forces used in the classical model are:

- Gravity
- Tension
- Drag
- Thrust
- Friction
- Buoyancy
- Electric force
- Magnetic force

You may find that you’re familiar with some of these; the classical model does an excellent job of describing the sort of interactions that we humans deal with on a day-to-day basis. The classical model is what we will use in this class, since we focus on macroscopic phenomena.

### 9.1.3 The modern model

In the early 1900’s, our understanding of the universe began to change. The theory of relativity took steps to explain phenomena dealing with very massive and fast-moving objects, and quantum mechanics did the same to explain interactions at a microscopic level. Both theories have been shown to be remarkably accurate, and both can be very counter-intuitive, seeming to fly in the face of the classical model that is based on our day-to-day experiences.

The modern model uses four *fundamental* forces to explain all interactions:

- Gravitational force
- Electromagnetic force
- Strong nuclear force
- Weak nuclear force

The two of these that we notice directly are the first two—gravity and the electromagnetic force.

Gravity is the attraction between any two objects based based on their mass; we’ll look at this in more detail later in this chapter.

The electromagnetic force is the interaction between any two objects based on their electric charge. Since the macroscopic objects that we deal with in our everyday lives are made up of many many many atoms (which in turn are made up of charged particles), this is the force that is responsible for all the forces described in the classical model, the only exception being gravity. Take the tension in a rope for example: on a microscopic level, a rope is made of many fibers tightly twisted together. The atoms and molecules in each fiber interact with the others around it, and the overall effect is that you can pull on the rope. In principle you could do calculations based on the electromagnetic force between each individual atom, but this would be a prohibitively complex calculation, even for our best supercomputers. So instead we use the classical model to describe the macroscopic effect.

We don’t directly notice the strong and weak nuclear forces; these are interactions between subatomic particles. (The term *nuclear* refers to the nucleus of an atom.)