Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Nomenclature: Normal forces



(Arising from statements made in the Weight and Mass thread).

Take a block sitting on a horizontal table top (No, don't take it; I want to
talk about it.)
There are two forces acting on the block: the gravitational force exerted by
the earth and the contact force exerted by the table surface.

My question is: why do so many of us call this contact force the "normal"
force? (I've also seen references to it as the reaction force -not as often
now as forty or fifty years ago, but that is another story, perhaps for
another day.)

I'm not disputing that when the block is "sitting" the contact force is
directed perpendicular to the surface. Even so, this is a property of the
force, telling us its direction, not its reason-d'être . For consistency
here, perhaps we should call the gravitational force the vertical force.

It could be argued that calling this force the normal force helps remind us
of its direction when the block is resting on an inclined surface. But do
we need this reminder? Wouldn't it be better if students understood that
the contact force associated with the surface deformation resulting from two
surfaces pressing together and not moving relative to each other must be
perpendicular to the surfaces.

When the surfaces are not stationary with respect to each other the
direction of the contact force is no longer normal to the surface (except
for the non-existent zero friction situation). A common approach is to
write (and speak) as if there were two separate forces: the normal and the
frictional (less often called the tangential). But really we have only the
one contact force. One of the "laws" of friction relates two components of
that contact force.

I argue for more use of the term contact force to describe all those forces
resulting from contact of objects and the resulting deformation. Among
other things this would help students overcome the misconception that tables
do not exert forces on objects sitting on them: they just get in the way of
the gravitational force.

Additional emphasis on the contact nature of all these
"non-action-at-a-distance" forces could prevent bad calls such as talking
about a force exerted by a hand on an object when they are separated by a
rope attached to the object and pulled on by the hand. In this situation
there is a contact force between the hand and rope and another distinct
contact force between the rope and the object but no contact force between
hand and object. I have read references to the 'indirect" force; I believe
this is bad physics and certainly no help to students trying to draw
free-body diagrams and write Newton second law equations.

As a final observation, I wonder how many of our students think that
"normal" force means the "usual" or "ordinary" force.

Brian McInnes