Re: The Rise and Fall of Simple Machines

[Richard Tarara <rtarara@SAINTMARYS.EDU>]

----- Original Message -----
From: "Ludwik Kowalski" <KowalskiL@MAIL.MONTCLAIR.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, November 23, 1999 2:08 PM
Subject: Re: The Rise and Fall of Simple Machines


> In the spirit of a Carnot diagram (a circle with arrows) a
> simple machine has only two arrows: work in and work out.
> The number of joules going in is slightly smaller than the
> number of joules coming out. No joules would be lost in
> an ideal machine (negligible mass and friction). This is
> not very different from what we have in calorimetry,
> except that here joules represent "heat" and the amount of
> work done is negligible. Thus "conservation of heat" and
> "conservation of work" before the conservation of energy.

I think you mean slightly more in than out.

I find using the simple machine (the inclined plane in my instance) is a
good lab experience to introduce the concept of efficiency.  Almost everyone
comes into class with the idea that a machine is something 'that does work'
for us.  By measuring the work input and the useful work output on the
incline, we can easily show that it takes more input than we get in output.
Then why use the machine?  This opens the door for the idea of Mechanical
Advantage.

For the more advanced students the experiment with the incline produces an
interesting question.  {The inclines used have a pulley attached to one end
and the applied force is due to the weight of a mass hung over the edge of
the incline used, with the pulley, to pull a loaded, wheeled cart, up the
ramp, at constant speed.}  The efficiency of the machine seems to increase
slightly with increasing angle (or at least stabilize at a value greater
than 90%), but the amount of energy 'lost' seems to increase with angle.
Why?  Friction between the cart and the incline is actually decreasing.
Only the sharpest students see (sometimes hear) the source of the problem.

Rick

*******************************************************
Richard W. Tarara
Department of Chemistry & Physics
Notre Dame, IN 46556
219-284-4664
rtarara@saintmarys.edu

FREE Physics Educational Software
Available for Download

see:  www.saintmarys.edu/~rtarara/ for details

New Win9.x and PoweMac packages available.

New versions/new content/'fixes'  posted frequently.
*******************************************************