Two students are discussing their physics homework prior to class. They
are discussing an object that is being acted upon by two individual forces
(both in a vertical direction); the free-body diagram for the particular object
is shown at the right. During the discussion, Anna Litical suggests
to Noah Formula that the object under discussion could be moving. In fact, Anna
suggests that if friction and air resistance could be ignored (because of their
negligible size), the object could be moving in a horizontal direction.
According to Anna, an object experiencing forces as described at the right
could be experiencing a horizontal motion as described below.
Noah Formula objects, arguing that the object could not have
any horizontal motion if there are only vertical forces acting upon it. Noah
claims that the object must be at rest, perhaps on a table or floor. After all,
says Noah, an object experiencing a balance of forces will be at rest. Who do
you agree with?
Remember last winter when you went sledding down the hill and
across the level surface at the local park? (Apologies are extended to those
who live in warmer winter climates.)
Imagine a the moment that there was no friction along the level surface
from point B to point C and that there was no air resistance to impede your
motion. How far would your sled travel? And what would its motion be like? Most
students I've talked to quickly answer: the sled would travel forever at
constant speed. Without friction or air resistance to slow it down, the sled
would continue in motion with the same speed and in the same direction. The
forces acting upon the sled from point B to point C would be the normal
force (the snow pushes up on the sled) and the gravity
force (see diagram at right). These forces are
balanced and since the sled is already in motion at point B it will continue in
motion with the same speed and direction. So, as in the case of the sled and as
in the case of the object
that Noah and Anna are discussing, an object can be moving to the
right even if the only forces acting upon the object are vertical forces.
Forces do not cause motion; forces cause accelerations.
Newton's first law of motion declares that a force is not needed to
keep an object in motion. Slide a book across a table and watch it slide to a
rest position. The book in motion on the table top does not come to a rest
position because of the absence of a
force; rather it is the presence of a force
- that force being the force of friction - that brings the book to a rest
position. In the absence of a force of friction, the book would continue in
motion with the same speed and direction - forever (or at least to the end of
the table top)! A force is not required to keep a moving book in motion; and a
force is not required to keep a moving sled in motion; and a force is not
required to keep any object horizontally moving object in motion. To read more
about this misconception, return to an earlier
lesson.