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 by considering the interactions of that object with the environment.
Here's what we do:
In moving between steps 2 and 3 we're building a model of the situation. How do we know what to include in the model? By paying attention to the situation.
Here two elements of the local environment are both stretched — the forearm and the bag. They're warped, and we'll see that these kinds of distortion lead to a tension or compression force.
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 world one object at a time — no more.