Chapter 3 Acceleration and free fall - Light and Matter

126 Chapter 4 Force and motion
it seem that the phenomena might have more in common than had
been apparent. In the Newtonian description, there is only one cause
for a change in motion, which we call force. Forces may be of different
types, but they all produce changes in motion according to the
same rules. Any acceleration that can be produced by a magnetic
force can equally well be produced by an appropriately controlled
stream of water. We can speak of two forces as being equal if they
produce the same change in motion when applied in the same situation,
which means that they pushed or pulled equally hard in the
same direction.
The idea of a numerical scale of force and the newton unit were
introduced in chapter 0. To recapitulate briefly, a force is when a
pair of objects push or pull on each other, and one newton is the
force required to accelerate a 1-kg object from rest to a speed of 1
m/s in 1 second.
More than one force on an object
As if we hadn’t kicked poor Aristotle around sufficiently, his
theory has another important flaw, which is important to discuss
because it corresponds to an extremely common student misconception.
Aristotle conceived of forced motion as a relationship in which
one object was the boss and the other “followed orders.” It therefore
would only make sense for an object to experience one force at
a time, because an object couldn’t follow orders from two sources at
once. In the Newtonian theory, forces are numbers, not orders, and
if more than one force acts on an object at once, the result is found
by adding up all the forces. It is unfortunate that the use of the
English word “force” has become standard, because to many people
it suggests that you are “forcing” an object to do something. The
force of the earth’s gravity cannot “force” a boat to sink, because
there are other forces acting on the boat. Adding them up gives a
total of zero, so the boat accelerates neither up nor down.
Objects can exert forces on each other at a distance.
Aristotle declared that forces could only act between objects that
were touching, probably because he wished to avoid the type of occult
speculation that attributed physical phenomena to the influence
of a distant and invisible pantheon of gods. He was wrong, however,
as you can observe when a magnet leaps onto your refrigerator or
when the planet earth exerts gravitational forces on objects that are
in the air. Some types of forces, such as friction, only operate between
objects in contact, and are called contact forces. Magnetism,
on the other hand, is an example of a noncontact force. Although
the magnetic force gets stronger when the magnet is closer to your
refrigerator, touching is not required.