Re: [Phys-l] Inertia?

["John Clement" <clement@hal-pc.org>]

The two mass hanging from pulleys was an alternative suggested by Ron
Thornton to using dueling fans.  Ron claimed that it was not as convincing
because the students paid too much attention to the mechanism, while they
tended to accept the fans.

John M. Clement
Houston, TX


> > They hardly
> > ever target bodies at motion with balanced forces.  One can talk about
> sail
> > boats, planes, bicycles, cars... having balanced forces, but students do
> not
> > see or believe that the forces are balanced.  The only demo I know of
> that
> > really works is the dueling fans from the ILDs.
>
> I have another balanced-forces-keeping-a-constant-velocity lab that works
> pretty well. In this lab I set up a pulley with excellent bearings, like a
> Pasco Smart Pulley, on a tall lab stand. I then attach two 500 g same-
> sized masses over the pulley, like in a traditional Atwood's Machine. Then
> I run through a series of student predictions and possible models that go
> like this:
>
> 1. If you move the masses so that they are the same distance from the
> pulley and at rest, will the pulley start to turn? Explain how you decided
> on your answer.
>
> 2. If you add a small mass (a paper clip) to one mass, will the masses
> start to move? If so, will they move at a constant velocity or will they
> start to get faster and faster? Explain how you decided on your answer.
>
> 3. Remove the paper clip. If you place the masses so that they are
> different distances from the pulley, will the masses start to move? If so,
> will they move at a constant velocity or will they start to get faster and
> faster? (Yes, some students notice that the mass of string isn't the same
> on both sides, but with my pulleys there is enough friction to still hold
> the position.) Explain how you decided on your answer.
>
> 4. Place the masses so that one mass is near the top of the pulley and the
> other is hanging low. Give the top mass a short push downwards and start
> it moving. Will the masses keep moving at a constant velocity or will they
> get faster and faster or slow down? (They do slow down but not much and
> certainly not visibly.) Explain how you decided on your answer.
>
> Marc "Zeke" Kossover
> The Jewish Community High School of the Bay
>
>
>
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