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Re: [Phys-l] [PTSOS] anvil and sledgehammer inertia demo

A brand new (or in very good shape) pulley like a Super Pulley from Pasco as an
Atwood's machine with a 500 gram mass on each side will rotate with only a very
small change in velocity. In fact it is quite startling for the students to see
that you have to grab the mass to prevent it from smashing into the pulley.


I run through a series of activities that go like this.

Suspend the pulley from a tall lab stand. Run a piece of string over the pulley
and attach a 500 gram mass to the right side. The string should be long enough
so that one mass can just rest on the ground without another mass on the other
side hitting the pulley.

1. I start with only a mass on the left side. The string is looped over the
pulley and I am holding it. "The mass on left side pulls down with a force of
4.9 N because of the force of gravity on it. Why doesn't it fall?" Pause. "Oh,
the string is pulling up on it? How much force is that?" Let students write down
an answer. In fact I make them summarize each scenario, with a picture, and then
answer before we try the experiment.  "4.9 N? We can check that with a scale."
We check with a spring scale.

2. After adding another 500 gram mass to the right side but still holding the
string, I say, "I've added a 500 gram mass to the other side. How hard is it
pulling on the first mass?" Most students say "4.9 N." I ask "What will happen
when I let the string go?" They answer that the masses won't move.

3. "That wasn't really the question I wanted to ask. I'm going to place this
0.01 N weight on the top of the left mass. What will happen next?" Most will
predict that the masses will start to move. "The question I wanted to ask was
this: will the speed of the left mass as it hits the ground be higher than when
the left mass is halfway down?" Some say yes and some say no. "How about three
quarters of the way down?" "How about 7/8 down?" I release the string, and the
masses will smoothly accelerate all the way to the bottom. It is clear just from
looking that the masses are always speeding up. Many students find that
surprising.

4. The left mass is sitting on the table with the little weight on top of it.
"I'm going to lift the left mass and weight up and release them gently upward so
that they will leave my hand moving upward. Is there a net force on the left
mass?" Students write down answers. "Which direction is it?" More writing. "What
will that net force cause to happen? Please be specific. Perhaps a graph of the
speed or velocity would be helpful." Patience. Most students correctly predict
that the left mass will slow down and then switch directions.


5. Now I remove the extra weight and do the same maneuver as in 4. "I'm going to
lift the left mass up and release it upward so that it will leave my hand moving
upward. Is there a net force on the left mass?" Students write down
answers. "Which direction is it?" More writing. "What will that net force cause
to happen? Please be specific. Perhaps a graph of the speed or velocity would be
helpful." Patience. Many students are surprised that I have to catch the left
mass before it smashes into the pulley.

Marc "Zeke" Kossover

----- Original Message ----
From: Stefan Jeglinski <jeglin@4pi.com>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Wed, September 29, 2010 12:15:23 PM
Subject: Re: [Phys-l] [PTSOS] anvil and sledgehammer inertia demo

The really important demo is to show that a moving object
with balanced forces remains in constant motion.

I substantially agree. What are your favorite demos of this, suitable
for a classroom?


Stefan Jeglinski

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