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(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.)=20
There are two forces acting on the block: the gravitational force
exe=
rted 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
"nor=
mal"
force? (I've also seen references to it as the reaction force -not
a=
s often
now as forty or fifty years ago, but that is another story, perhaps
f=
or
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
o=
f the
force, telling us its direction, not its reason-d'=EAtre . For
consis=
tency
here, perhaps we should call the gravitational force the vertical
for=
ce.
It could be argued that calling this force the normal force helps
rem=
ind us
of its direction when the block is resting on an inclined surface.
B=
ut do
we need this reminder? Wouldn't it be better if students understood
=
that
the contact force associated with the surface deformation resulting
f=
rom 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
(ex=
cept
for the non-existent zero friction situation). A common approach is
=
to
write (and speak) as if there were two separate forces: the normal
a=
nd the
frictional (less often called the tangential). But really we have
on=
ly the
one contact force. One of the "laws" of friction relates two
compone=
nts 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.
Amo=
ng
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
tal=
king
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
situ=
ation
there is a contact force between the hand and rope and another
distin=
ct
contact force between the rope and the object but no contact force
be=
tween
hand and object. I have read references to the 'indirect" force; I
b=
elieve
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