In the absence of a force: A body at rest will remain at rest.
A body in motion will remain in uniform motion.
The above is Newton's First Law. It was revolutionary when Newton first
proposed it, because it seems so clearly wrong! Most of the things we
deal with on a day-to-day basis don't move uniformly unless we "force
them to." Indeed for more than 1000 years before Newton is was considered
heresy to suggest that things would or could move without being forced
to do so. Aristotle had taught that everything had its place and would
go there and stop unless forced to do otherwise. Thus air is in the sky,
because that is its natural place, similarly for the moon, sun and stars.
Other objects belong on the surface of the earth so they fall there unless
held up.
I am sure that you are smiling at the quaint views of these ancient philosophers.
But in truth this view is a very natural one and I think there is some
level in our subconscious where we all feel that Aristotle was right.
For example pilots find it completely natural to talk about pulling the
nose up - and then holding it up. But according to Newton the nose should
stay up once you have pulled it up - WOW! Did you ever stop to think about
that before?
There are hundreds of questions about why the airplane does what
it does in flight that most of us never think to even ask, because
our ancient understanding of physics causes us to incorrectly think
that forces are required when in truth none are required. The example
of the airplane's nose given above is just one example. It does
not usually occur to us to wonder why the nose has to be held up
- because almost everything else we encounter in life has to be
held up.
The red ball inside the square to the left obeys Newton's first
law perfectly. I.E. there is no friction and no gravity. You can
move the ball or throw it with your mouse. You can throw it very
slowly, or quickly. If you throw it quickly it is hard to catch
it again, hence the stop button. Try it and see if Newton's first
law makes more sense to you.
Note: once you release the ball it exhibits UNIFORM MOTION until
it bounces off the walls without any loss of energy, it then undergoes
uniform motion again until the next bounce. The process repeats
forever with no loss of energy.
Newton's first law can be restated in a negative form:
In the presence of a force: a body will not remain at rest. A
body will not exhibit uniform motion.
While you don't usually see Newton's first law stated in the negative,
it might actually be more instructive to us as pilots to think of it this
way. This formulation reminds us more clearly that when we see uniform
motion there is no force. So, if there appears to be a force, then there
must be an "offsetting" force that we should start looking for.
It is often in identifying the unanticipated offsetting force that we
will gain our greatest understanding of aerodynamics.
The NET Force Concept
Everyone knows that the green block on the table to the left
applies a force equal to its weight downward and that the table
applies an equal but opposite force upward on the block. The upward
force applied by the table offsets the weight of the block so that
no net force acts on the block (and the floor applies an upward
force on the table equal to the weight of the table plus the block
so that no net force acts on the table.) The result is that there
is NO NET FORCE acts on the green block and it remains motionless.
It is also clear that if we increase the weight of the box, say
by reaching over and putting something into it, then the table will
automatically push up harder, keeping the net force acting on the
box equal to zero.
When we talk about "holding the nose up," most of us
realize that in an airplane with elevators at the back we are really
holding the tail down. Still what we "visualize" is the
picture shown to the left.
But this can't be right.
Because the nose is staying steady and the airplane is moving with
uniform motion we know from the negative formulation of Newton's
first law that there is no net force.
There MUST be an offsetting force (it is shown in orange to the
left) that exactly balances the red force.
NOTE: the red force is what you literally "feel" on the
control column. In other words you really do have to "pull"
back with a force on the control column.
It is because it does not easily occur to us to think that the
orange force must exist that I have said that we naturally think
more like Aristotle than Newton. Given that Aristotle was one of
the most intelligent people to ever live we should feel no shame
in this. It just means that a subject as esoteric as Aerodynamics
is not going to come naturally to us. We need to train ourselves
to look for offsetting forces.
By the way, the offsetting force in this case is formed because the angle
of attack of the tail is changed. We will discuss that in more detail
in the chapter called Control.
