We all have plenty of experience with how things move. Unfortunately,
all of our observations involve objects that are being pulled on by gravity.
This means that most of the time any object is in contact with something else,
interacting with it and sharing energy with it.
As a result, there seems to be a law of motion "Moving things always come
to a stop," but this is ignoring all those interactions.
A moving object slows down because it shares its energy with other things. We can
simplify the system by removing the interactions between the object under study and the
rest of the world, by using smooth surfaces, lubrication, bearings, or by rolling instead
of sliding.
Observations on these simpler systems lead to a better understanding
of what is going on.
One of Newton's laws is The Law of Inertia: "In the absence of interactions with
other things, an object will maintain constant speed and direction of motion"
(this is usually called Newton's First Law).
In the real
world, the Law of Inertia may seem a bit of an abstraction: a moving grocery cart will coast
only a short distance after you stop pushing it (and surely not in a straight line!).
This happens because the grocery cart is interacting with the rest of
the world through its squeaky wheels. However, a ball rolling on a
smooth level track gives a good example of the First Law.
Significantly, what we have done is to remove ways for the ball to interact with other
things. The point is that slowing down is not a built-in property of a moving
object: it slows down because something else has interfered with it.
We will understand why the moving object slows down only when we study its
interactions with other things; but the starting point is to understand that
left to itself the object wouldn't slow down at all.
Another way to state this law is "An isolated object will maintain constant velocity."
"Isolated" means that we have decreased the object's interactions with the rest of the world
to the point that we can ignore them. The concept "velocity" specifies both the speed and
the direction of motion.
For the present activity, in which the rolling ball is constrained
to stay on its track, the direction is fixed, and "speed" and "velocity"
are hardly different.
According to the Law of Inertia, moving with constant velocity is
"doing nothing": no force is required to accomplish this.
The law also says that when something is moving with constant
velocity, we can conclude that
that there is no force acting, or that the forces acting
have cancelled each other out.
We observed earlier that when an object is not
moving, the forces on it must balance.
This is a special case of the Law of Inertia, since an object that is standing
still has constant speed. Please note that the statement "when there is no
net force on an object, it must be at rest" is not correct; the speed must be
constant but doesn't have to be zero.
We all have plenty of experience with how things move. Unfortunately,
all of our observations involve objects that are being pulled on by gravity.
This means that most of the time any object is in contact with something else,
interacting with it and sharing energy with it.
As a result, there seems to be a law of motion "Moving things always come
to a stop," but this is ignoring all those interactions.
A moving object slows down because it shares its energy with other things. We can
simplify the system by removing the interactions between the object under study and the
rest of the world, by using smooth surfaces, lubrication, bearings, or by rolling instead
of sliding.
Observations on these simpler systems lead to a better understanding
of what is going on.