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On 03/17/05 18:48, Frohne, Vickie wrote:_______________________________________________
In my experience, students are often confused by projectile motion, a=
nd confused again by rotational motion. The confusion is so great tha=
t perhaps it's better not to mix these two difficult concepts in the =
same lab. For example, when I do projectile motion, we use a photoga=
te to measure the ball's velocity at the edge of the table. I don't =
have the students calculate it based on the height and angle of the r=
amp, because they're not ready for that complication yet. Besides, fr=
iction in the system can lead to significant energy losses even when =
rotational motion is taken into account, so that any projectile calcu=
lation based on the intial potential energy is bound to fail.
There's a lot of good common sense in there.
Here's an alternative that aims for some of the same
points, and is perhaps simpler in some ways, and more
complicated in others.
We agree that the frictional and rotational complications
are a nightmare for students at this level.
The photogate is a good solution, but not every classroom
has an abundance of such things.
Here is an idea for an "energy and projectiles" lab that
doesn't have (to first order) friction and rotation
problems, yet doesn't require any of that mysterious
1) Start with the ordinary "Newton's Cradle" demo. This
is to establish that steel balls can swing through the air
and whack into each other with only modest dissipation.
2) Let the students build a "Newton's Golf" setup as sketched
One key physics idea is that at the moment of contact,
the motion is colinear with the line of centers, so no
rotational motion is imparted. This requires sighting
along the string and using some sort of right-angle
Another key idea is that the tee is rather shallow, and
you give the projectile enough loft that it departs the
tee cleanly -- no rolling. This is not terribly hard to
arrange. There is a huge window in parameter space.
You can calculate the KE from the delta h as shown.
I recommend initially having the two balls be matched
in mass, in which case nearly 100% of the KE will be
transferred to the projectile. (You can generalize this
Then it's just nice clean projectile motion from there.