Forces and Motion

Isolating objects

Physics Narrative for 14-16 Supporting Physics Teaching

Looking for interactions; performing extractions

Putting on forces spectacles was used as a metaphor for seeing the world in a new way in the SPT: Forces topic. Now, in the SPT: Force and motion topic, this idea of seeing differently is developed and made more formal.

Identify the object – here a rock resting on the bottom of a pond. Then look for clues in the environment that there are significant interactions between the selected object and the environment. The ultimate aim is to extract the object from this messy environment of the lived-in world and to get it into a different, more sterile, but more predictable environment.

Here you are taking a step towards constructing a functional model, re-describing the object in a way that enables you to predict its changes in motion.

The extraction procedure

The very first step in extracting an object from its environment and building an idealised model of the situation is to isolate it. This isolation strips away the environment, leaving a bare object, characterised only by its mass, and with only the forces acting on it as a reminder of the environment it has left behind.

indicative interactionforce acting on object
displacing fluidbuoyancy
gripping a surface grip
slipping over a surfaceslip
moving in a fluiddrag
mass attractinggravity
charge attracting or repellingelectric
magnets attracting or repellingmagnetic

In this example, a child pushes a sledge over a rough surface.

The child's hands are compressed and provide a driving force  →  compression force.

The surface is rough and provides a retarding force  →  slip force.

The object being pulled is on the Earth and has mass  →  gravity force.

As you've not added the forces together yet, you can't say what the resultant force is. In particular, you can't say whether it's zero.

More interactions leads to more forces post-extraction

Here's a more complex situation – and more complex in a significant way. That is, the object is in a messier environment than our first example. It is hanging underwater. Yet the procedure to re-imagine it in a more natural environment is exactly the same.

And the process generates a physical description: it respects the phenomenal world. Every interaction of the object-to-be-isolated with its environment in the lived-in world results in a force acting on the pure mass that represents the object in it's natural world. This natural world, far removed from our everyday world, and yet capable of somehow explaining much of it, is the world of a mass and the forces acting on it. There are no other things in this very simple world. It's the world that it required the creative and powerful imagination of Newton to create.

Later, you'll see just how fruitful it is to imagine objects in this kind of world. Because it is their natural environment, objects can express their natural behaviour, and we can predict exactly how they'll move. Comparing such predictions with their behaviour in the lived-in world results in a match (or otherwise) between the model and our measurements of the phenomena. The quality of the match is a measure of the worth of the model. It might be very useful to know what will happen, and our ability to predict this with reasonable certainty may convince you that there is a case to be made that this very simple environment is actually a good description of how the world actually is.

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