From: Ronald Wong (ronwong@inreach.com)
Date: Thu Nov 06 2003 - 00:50:06 PST
Message-Id: <l03102804bbcfa1ced240@[209.209.18.112]> Date: Thu, 6 Nov 2003 00:50:06 -0800 From: Ronald Wong <ronwong@inreach.com> Subject: Re: pinhole Re: forces/friction
Dave brought up the following:
>Let's say that you are driving a car and turn off the engine
>so that there is no forward force due to the engine, and the only force at
>work is
>the force of friction which acts in the backwards direction.
>
>Now... what if this "coasting" car hit a brick wall. Certainly we would
>describe a
>force being applied to the brick wall by the car, yet we just said that
>there is no
>force in the forward direction.
Most people are familiar with the fact that if one body pushes against
another, that body exerts a force on the other. If you want to push a table
across the floor you bring your hands up to the edge of the table and push.
Assuming you push hard enough, the table moves across the floor in the
direction of your push. Good old common-sense, Aristotelian physics.
So far, so good.
-------------------------------
When I introduced my students to Newton's three laws, one of the activities
I would do began with
A. having them extend their index finger in front of their face and examine
it's tip.
B. asking them to examine it's shape very carefully (getting students to
become better at
observing the world around them is one of the objectives of this course).
C. pointing out to them that this was the shape of their finger tip when no
force was acting on it
I then directed them to use their index finger to exert a downward force on
the top of their desks.
While pushing down on the top of their desks I asked them to examine the
tip of their finger and take note of any changes that may have taken place
(for those with scrawny index fingers, I suggested they use a thumb
instead).
Although some took longer than others, all of them finally came to
recognize that the tip of their finger was flattened as a result of pushing
down on the top of their desks. On occasion, those who used their thumbs
were able to point out to the others that the greater the push, the more
the tip of their thumbs became flattened.
To make clear what it was that was flattening their finger tip, I would
then tell them to use their left hand index finger tip to exert a force on
the tip of their right hand index finger.
They quickly saw that the force acting on the right finger tip flattened it
(i.e. the flattening of a finger tip is the result of a force acting on it).
They also saw that even as they flattened the finger tip of the right hand,
the finger tip of their left hand was flattened as well.
At the very same time that one body exerted a force on another, a force
acted simultaneously on the first body but in the opposite direction (how's
that for a hands-on learning experience!).
We then hauled out the spring balances and started making measurements.
In the end, they learned that whenever two bodies interact with one another
each exerts an opposite but equal force on the other.
At a more subtle level: A. You need two bodies (you can't create a force
with just one).
B. Forces ALWAYS appear in the form of opposite and
equal pairs.
C. Each of the pair of forces acts on only one of
the two bodies.
(There would be no change in the universe if it
was otherwise)
-------------------------------
As long as the car was coasting along, there was only the frictional force
acting on the car, opposing it's motion, and slowing it down. As you said,
there was "...no force in the forward direction".
As soon as the car struck the wall - exerting a horizontal force on the
wall in the "forward" direction - an equal force acted horizontally on the
car in the opposite direction. It is this force that slows the car down,
brings it to rest, and causes all the car's damage due to it's collision
with the wall.
ron
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