Onions don't come from the market with arrows attached labelled:
I'm being pulled down by gravity with a force of about 3 newton
I weigh 3 newton.
The fact that forces aren't visible, labelled and ready to be described offers a challenge to the imagination. You must learn to recognise where forces are acting. So change your perspective: moving from the physical (green panes) to a theoretical (blue panes) description.
If you look at things you'd find in the kitchen, you can spot lots of different objects. Each of these has many different facets that you can describe: the materials from which objects are made; the ways in which they move around; the forces acting on these objects; the colours of the objects. In this topic, for one object at a time, we'll be describing the forces–and only the forces.
An everyday scene
So let's take an everyday scene.
This might look like a person carrying some shopping and indeed that is just what it is.
But view the scene through
forces spectacles, shifting from the physical to the theoretical, to find out how a physicist might
see the situation when looking at this scene through forces spectacles.
The left hand re-description is too complex–there are still several objects: the shopper's head: the bag; the rest of the shopper. Simpler is better–so always focus on just one object. And even then we've only shown a few of the forces acting on the objects. The idea is to avoid hedgehog-like diagrams, with pointy bits heading off in all directions. Like rolled up hedgehogs, they're hard to handle.
The second is better for just this reason, as we have reduced the shopper and his bag to a single object. Why do we say
better? Simply because the model (and it is a model) is something that will allow us to make predictions.
An alternative way to make it simpler is by concentrating on some parts of the situation. The rule is always, always to focus on a single object at a time. Just one.
Get those spectacles on