Message-Id: <3.0.32.19981119075317.007a2700@netmail.home.com>
Date: Thu, 19 Nov 1998 07:53:18 -0700
To: swise@lick.pvt.k12.ca.us
From: The Lahrs <JohnJan@lahr.org>
Subject: Physics Questions
>From: swise@lick.pvt.k12.ca.us (bliss)
>Date: Sat, 14 Nov 1998 17:55:40 -0800
>question #1
>Can someone explain why friction force is independent of the area of
>contact between the two surfaces?
Friction depends in a linear way on the force per unit area between two
objects. Consider a block of wood weighing 20 N that has one side
with an area of 10 sq cm and another with an area of 5 sq cm.
When resting on the smaller side there is 20/5 = 4 N per sq cm pressing down
on the table. If the coefficient of friction is .25, then the required
sliding force is .25 x 4 = 1 N per sq cm. Multiply this by the total
number of square centimeters (1 x 5) to find that the total force required
for sliding is 5 N.
When resting on the larger side there is 20/10 = 2 N per sq cm pressing
down on the table. Again the required sliding force for each of these
square centimeters is .25 x 2 = .5 N per sq cm. Multiply this by the total
number of square centimeters (.5 x 10) to find that the total force
required for sliding is 5 N, which is the same as for the smaller side.
>Does anyone know of demonstrations that can show this?
There are some good experiments on the Center of Excellence for Science and
Mathematics Educations at the University of Tennessee at Martin web site.
( http://cesme.utm.edu/resources/math/MAG/6-8MAGActivities.pdf/6-8.html )
Here is one on static and sliding friction:
http://192.239.146.18/resources/Science/PSAM/psam20.pdf
>
>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.
I need to do this myself to see how close to the "ideal" case one can come
with block of wood. As with most things, the first approximation, in this
case that
the coefficient of friction is a constant, should be reasonably good.
However, there will be some random fluctuations as well.
I'm working an experiment that is aimed at explaining how earthquakes,
which are a rapid process, can result from the very slow motion of the
Earth's tectonic plates. In this "table-top seismology" demonstration,
"earthquakes" are start when static friction is exceeded and sliding stops
when sliding friction is no longer exceeded. Although one might think that
each "earthquake" created by this apparatus would be identical, there are
large variations. Just as in the Earth, some events are very small and
some quite large. You can get a draft of this experiment at:
( http://giseis.alaska.edu/lahr/tabletop ) If you try it out, I would
really appreciate feedback on how it worked and suggestions for improvements.
>question #2
>When an airplane experiences lift,...
There is an article in The Physics Teacher, Vol. 36, Nov. 1998, on flight.
It boils down to the airplane deflecting air downward. Bernoulli's theorem
is not used. You'll have to see the article for the details.
>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?
>
>For 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?
No. Consider a horizontal boundary through the rope. There are rope
molecules above the boundary that are chemically bound to rope molecules
below the boundary. The molecules below the boundary are pulling down on
the molecules above with a force of 250N. The molecules above the boundary
are pulling up on the molecules below with a force of 250N. The total
force that the molecules (and rope) is subjected to is just 250N. The rope
will not break unless it is subjected to a force greater than 300N.
Hope this helps.
John
* John C. and Jan H. Lahr *
* JohnJan@lahr.org *
* 1925 Foothills Road *
* Golden, Colorado 80401-1718 *
* (303) 215-9913 *
* http://lahr.org/john-jan *
* AOL version of homepage: *
* http://lahr.org/john-jan/homepage.html *
* http://giseis.alaska.edu/lahr *