Message-Id: <199811150049.QAA21055@noontide.lick.pvt.k12.ca.us>
Date: Sat, 14 Nov 1998 17:55:40 -0800
To: "Pinhole Listserv" <pinhole@exploratorium.edu>
From: swise@lick.pvt.k12.ca.us (bliss)
Subject: Re: Pinhole Digest #88 - 11/14/98
My physics students have come up with three questions that we can't come to
a consensus to among ourselves. Perhaps you all can help -- thanks!
Sarah Wise
Lick-Wilmerding H.S.
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?
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.
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?
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?
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Sarah Wise
Lick-Wilmerding H.S.
755 Ocean Avenue
San Francisco, CA 94112
swise@lick.pvt.k12.ca.us
"Not everything that can be counted counts,
and not everything that counts can be counted."
--Albert Einstein