Forces and Motion

Identifying forces

Physics Narrative for 14-16 Supporting Physics Teaching

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 environmentreplaced by force acting on object
solid environment stretchedtension
solid environment compressedcompression
fluid environment displacedbuoyancy
not moving past a solid part of the environment grip
moving past a solid part of the environmentslip
moving through a fluid part of the environmentdrag
massive object in an environment with massgravity
charged object in a charged environmentelectric
magnetic object in a magnetic environmentmagnetic

And here are the nine forces, in their three groups:


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.

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