Identifying forces
Physics Narrative
for 14-16
Three kinds of forces: identifying forces to replace interactions
As you isolate an object from its environment – in order to place the object in a completely natural environment – so you'll find the need to categorise the kinds of interactions with the environment, in order to identify the forces. These interactions can be of two kinds: interactions with the local environment and interactions with the non-local environment. In the object's new stripped-down environment, these are replaced by, respectively, contact and non-contact forces.
Forces (in the SPT: Forces topic) were first introduced as acting on objects to support those objects – so performing the same function as a human agent. This kind of agency on the part of the environment is as a result of interactions between the object and the environment that can be replaced by a force exerted by the environment. That force can support an object, and it's at right angles to the surface of the object – a normal force. These normal forces can also accelerate or retard objects, so perhaps, now that you know a little more about normals (perhaps from the SPT: Light topic), we can comfortably call this set normal forces
.
The contact forces are of two further kinds: those that are normal to the surface of the object in contact with the environment and those that retard. In the SPT: Forces topic these were called forces that can support and frictional forces. Now we'd suggest using somewhat more apposite and precise collective terms: normal forces
and retarding forces
(these forces always act so as to reduce motion).
Then there are the three forces that act without contact: gravity, electrical and magnetic.
So there are three by three – that is, nine different kinds of forces that might appear. Here is a table of the forces and the interactions.
| interaction of object with environment | replaced by force acting on object |
|---|---|
| solid environment stretched | tension |
| solid environment compressed | compression |
| fluid environment displaced | buoyancy |
| not moving past a solid part of the environment | grip |
| moving past a solid part of the environment | slip |
| moving through a fluid part of the environment | drag |
| massive object in an environment with mass | gravity |
| charged object in a charged environment | electric |
| magnetic object in a magnetic environment | magnetic |
And here are the nine forces, in their three groups:
| normal | retarding | non-contact |
|---|---|---|
| tension | grip | gravity |
| compression | slip | electric |
| buoyancy | drag | magnetic |
Transforming a description: from interaction to force
As you move from from the messy complex world to the simple natural world for un-skinned objects, focus on particular facets of the environment to check if these are such that the interactions require a force arrow to be added in the new world in order to reproduce the effect of the interaction.
In moving from the lived-in world to the object's natural environment, you really are re-imagining the world – you're creating a parallel universe where the rules are different.
