Message-Id: <l03102801b281861b6a66@[209.142.19.162]>
In-Reply-To: <199811150812.AAA05660@mail.inreach.com>
Date: Wed, 25 Nov 1998 02:03:29 -0800
To: pinhole@exploratorium.edu
From: Ronald Wong <ronwong@inreach.com>
Subject: friction, wooden blocks, flying, and ropes
Sarah Wise asked a number of questions a while ago:
>question #1
>
>Can someone explain why friction force is independent of the area of
>contact between the two surfaces? Does anyone know of demonstrations that
>can show this?
Well, the simplest explanation is that it IS independent of the area of
contact between the two surfaces. That is, when scientist study the
frictional force generated as one object slides over another, they find
that the frictional force does not depend on the surface area - all other
factors remaining the same.
Now if the question was "What is friction?" than that would be another
kettle of fish (actually "a can of worms" would be more appropriate).
>It seems that if this is true, it should take an equal amount of force to
>pull a wooden block at constant velocity across a table on its widest face,
>as it does to pull it on its narrowest face. Students who tested this,
>however,
>found that differing amounts of force were needed to counteract sliding
>friction.
The key point is "... all other factors remaining the same." The block of
wood comes from a tree. Think of the tree trunk as a series of cylinders
nested one inside the other. The physical properties of these cylinders
varies considerably from one to another. The lumber from which the blocks
of wood came was made by cuts running parallel to the axis of symmetry of
the cylinders. One of the consequences is that each of the faces of the
block is different in nature from the others and generates different
frictional forces when sliding over a given surface.
That you had the students test this statement was an important thing to
have done. Scientists do this all the time. The fact that your students
found their results in disagreement with an accepted idea (published in a
science book, no less) was an important lesson. Such disagreements happen
frequently in science.
Creating situations where students can pursue an activity similar to that
found in science is an important component of a good science program.
Students frequently are exposed to circumstances in the classroom where
their experience differs from what they are being asked to believe. It's a
valuable experience for them and it gives us an opportunity to see to what
degree our science program is giving them the skills that will allow them
to resolve such differences to everyone's satisfaction.
It would be interesting to hear what hypotheses your students offered in
light of their results and whether or not the differences were resolved
satisfactorily through further investigations on their part.
>
>question #2
>
>When an airplane experiences lift, what type of a force is this "lift
>force"? Is it an example of Newton's 3rd law, where air molecules' react
>with an upwards force to the weight of an airplane? This seems too
>simplified an explanation.
>
>Looking at the Bernoulli effect, the greater velocity of air molecules
>above the wing produces an area of low pressure above the wing, so the
>force exerted by the air molecules colliding with the underpart of the wing
>is higher than the force of air molecules colliding on the upper surface of
>the wing.
>
>With all of this in mind, is lift force an example of a larger category of
>forces -- a normal force, for example, or even more broadly electroweak
>forces?
>
In the past, the use of the Bernoulli effect to explain the presence of a
lifting force on an airplane's wing involved work-energy or
conservation-of-energy principles. Your explanation is one that is
frequently heard. Maybe others can give you a more detailed analysis based
on Bernoulli's Principle.
This tradtional approach is under question currently.
An examination of the actual flow of air over the surface of a wing shows
that it is directed downwards as a result of its movement over the wing. As
a result, the simple application of Newton's third law would account for
the lifting force. As the wing pushes the air down the wing is pushed up.
There are problems using Bernoulli's Principle to explain the lifting force
on an airplane's wing. It runs into difficulties when you try to explain
how it is that the same wing experiences lift even when the plane is flying
upside down. Another point is that the traditional explanation always
involves wings with asymmetrical cross-sections. The fact is, there are
airplanes that have wings with symmetrical cross-sections and they fly just
fine despite the fact that there should be no pressure differences between
the top and bottom of the wing due to the symmetry. So, Mr. Bernoulli,
where does the lift come from?
>question #3
>
>If a rope is said to break under x amount of weight force, does this really
>mean that it breaks under 2x amount of force?
>
>=46or example, if Harry the 250N mountain climber dangles from a rope which
>will break when >300N of weight is applied (as advertised) and is secured
>to a tree at the top of a cliff, isn't the rope actually experiencing 500N
>of force: 250N from Harry dangling and 250N from the reaction of the tree
>to the pull of the rope?
Take a closer look at the forces acting on BOTH ends of this rope when it
is being pulled on.
To understand what is really going on when this rope breaks we must start
with one end of the rope securely attached to something. Let's securely
attach it to a hook anchored into the side of a building.
When you pull on one end of the rope with a force < or = to 300 N the rope
pulls on the hook with the same size force and in the same direction as
your force. Newton's 3rd law says that an opposite and equal force must
therefore act on that end of the rope (rope pulls on the hook, so hook
pulls on the rope). So, whenever you pull on this rope, an identical size
force acts on the other end of this rope in the opposite direction.
When someone said this rope would break when a force > 300 N acts on it, it
was under similar circumstances. When he/she applied a force greater than
300 N on this rope it broke. Thus, anytime there are a PAIR of forces
acting in opposite directions on the end of the rope that are > 300 N, the
rope breaks.
When Harry dangles from the rope, he pulls one end of the rope down with a
force of 250 N. The rope pulls down on the tree with a force of 250 N and
the tree responds by pulling up on the rope with a force of 250 N. So each
end of the rope is experiencing a pair of forces acting in opposite
directions that is less than the 300 N. So, it doesn't break.
Of course, we're ignoring the mass of the rope in this analysis but now
that you know what is involved you can throw it in if you want. Depending
on the weight of the rope, Harry may or may not be safe.
By the way, the two opposing forces must BOTH be larger than 300 N before
the rope will break. If someone pulled on one end of the rope with a 10 000
N force while someone pulled on the other with only 300 N, the rope
wouldn't break! It'll come close to breaking, but it (theoretically)
wouldn't break.
>"Not everything that can be counted counts,
>and not everything that counts can be counted."
>--Albert Einstein
Leave it to Einstein to go around killing pairs of birds with one stone (IQ
and intelligence, as one example).
Cheers - ron