The world seen through forces spectacles can be very complex. There are many more forces acting in the situation than those shown. (Think of the forces exerted by the muscles in the strained forearm and in the aching fingers.) To help you make sense of the complex world of forces, there is a simple strategy: focus on an object and its interactions with its environment–both local and remote. So first isolate one object from its environment. Here, let's choose the hand. Then identify the forces acting on it by considering the interactions of that object with the environment.
Here's what we do:
Making a model
Now consider the forces acting on the bag.
The process of simplifying a complex situation by concentrating on one part is an example of scientific modelling. An even simpler sketch of the situation might reduce the bag and its contents to a
point, as shown in the additional step here.
Let's summarise the three stages in this modelling process:
forces spectacles, enabling the identification of forces by considering interactions between object and environment.
The key idea is that we're dealing with the world one object at a time–no more.