A topic in Learners' conceptions and thinking
Newton's first law is a common source of learning difficulties and alternative conceptions.
(Read about common alternative conceptions.)
Newton-1: The law
Newton's first law (N1) is concerned with inertia.
It describes how a body (object) remains in its current state of motion, unless acted upon by a force.
So a stationary object remains stationary – unless acted upon by a force.
A moving body continues to move in the same direction at the same speed – unless acted upon by a force.
(Newton's second law concerns what happens if such a force is applied.)
The counter-intuitive nature of the first law
Most people have no trouble accepting that something that is stationary will remain stationary until acted upon by a force – as that seems to reflect common experience.
However, from the time we are children we experience moving objects seeming to come to a stop by themselves unless they have an internal source of motive force. No one has ever thrown a ball and found that it continues to move in the same direction at the same speed indefinitely!
It is not that N1 is wrong, just that there are usually resistive forces such as friction with the ground or air resistance acting – but there may not be apparent form what we observe.
Because we all acquire an intuitive model of what really happens to objects we set in motion (that they soon come to a stop, and, if in the air, they fall to the ground) very young, this continually confirmed intuitive model tends to dominate our thinking. Not only do most people expect moving objects to slow down and stop before they learn school physics, they usually continue to have those expectations after studying the topic in school physics. (Which is fair enough, as in terms of everyday experience, they are right!)
Some students learn the physics model and can use it in school exercises and tests (being able to compartmentalise two different forms of knowledge), but others find it difficult to overcome their strong intuitive sense of how the world is.
[These kinds of intuitive notions that we acquire early in life, before we have language operate automatically in our pre-conscious thinking. That is, we do not need to deliberate and reflect – we just automatically expect objects to behave in this way. Scientists have to learn to discipline themselves to spot when they are thinking in this way and over-ride it with their formal knowledge.
Such 'knowledge' that operates well below conscious awareness has been explored in some detail, and modelled as what are called 'p-prims' ('phenomenological primitives'.)]
Different 'ontologies' of motion
The physicist considers a stationary object and an object moving at a constant velocity to largely similar. (After all, whether something is moving or not depends on the viewer. An object on your table on a moving train is stationary form your perspective, but not to someone on a platform. An object sitting 'stationary' on your table at home is rotating around the earth's axis and orbiting the sun, and moving with the rest of the solar system through the galaxy, and is moving with the rest of the galaxy as seen form another galaxy!
To the physicist, the more important distinction is between constant motion, and accelerated motion.
Yet our visual system has evolved to especially notice motion – so our experience of the world tends to primarily distinguish moving form not moving.
So, here are two different ways of dividing things up.
If something is moving, can we infer no force is acting?
The corollary of 'a body (object) remains in its current state of motion, unless acted upon by a force' would seem to be 'a body (object) changes its current state of motion, if acted upon by a force'. This is true as long as the force is not balanced out by another force acting on the body. If two forces of equal magnitude (size) act on the body but in opposite senses, then they will 'balance' and can cancel.
(Strictly, the two forces also have to act along the same line or they comprise what is called a couple and they will cause the body to rotate {if initially not rotating – or change its rotation otherwise}.)
So,
A stationary object remains stationary – unless acted upon by a net force.
A stationary body will start to move if acted upon by a net force.
A moving body continues to move in the same direction at the same speed – unless acted upon by net force.
A moving body's velocity will change if it is acted on by a net force.
Velocity refers to the direction of motion as well as the magnitude (speed). So,
A moving body's velocity (its direction or speed or both) will change if it is acted on by a net force
Alternative conceptions relating to Newton-1: examples
A science professor writes about "the concept of 'a balance of forces' keeping the moon circling the earth and the earth in orbit around the sun"
A science professor writes "there was a balance of forces: while the geological strata were being formed and mountains were raised up, at the same time the land was constantly being eroded"
The book Student Thinking and Learning in Science: Perspectives on the Nature and Development of Learners' Ideas gives an account of the nature of learners' conceptions, and how they develop, and how teachers can plan teaching accordingly.
It includes many examples of student alternative conceptions in science topics.
