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[Phys-L] Re: Energy & Projectile Lab

It does indeed sound like your "error" is due to the (significant) ro=
tational energy of the ball. As in many experiments, the discrepancie=
s arise because of real physics that hasn't been accounted for, not b=
ecause of poor experimental technique.=20
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.
How about bouncing some balls instead? The students can measure the =
height of the first bounce and compare it to the initial drop height.=
If you have photogate timers, you might be able to directly measure=
the velocity of the ball right before it hits the floor, to calcluat=
e a kinetic energy. They can measure the coefficient of restitution (=
bounce height/initial height) of various types of balls and the amoun=
t of energy lost per bounce. They could use the information to predi=
ct the height of, say, the third bounce. If you want to do projectile=
-motion parabolas, try dropping the balls vertically onto a slightly =
tilted board in order to get a launch angle. I haven't done this, but=
it has a chance of working.=20
Or maybe there's something you can do with those suction-cup "popper"=
toys in which the energy is stored in a spring and, after some delay=
, the toy pops into the air. These can be placed on a tilted surface=
, too.
Just an idea or two...if you try 'em, I'd be curious to know how they=
work out.
Vickie Frohne


=46rom: Forum for Physics Educators on behalf of Dwight K. Souder
Sent: Thu 3/17/2005 2:51 PM
Subject: Energy & Projectile Lab

Greetings everyone. I'm looking for suggestions/advice for a lab. W=
e are
currently going over potential/kinetic energy and recently covered
projectiles. I would like for them to do a lab that consists of both=
. The
idea I have (that I'm certain others had done) was to roll a marble d=
plastic tube. The marble then would roll across and off of the lab t=
and hit the floor at a certain spot. Based upon knowing how high the=
is from the table, they can calculate the speed of the marble as it e=
the tube and rolls across the table. Knowing the speed of the marble=
height of the lab table, they should be able to figure the range of t=
marble. To add some fun to it, I also planned on having them place
bulls-eye of where they think it'll land...each ring was worth some p=

When I've tested the setup, I'm not getting very good results. My se=
tup was
the following: top of the plastic tube was 60cm from the top of the =
table, 90.07cm from the lab table to the floor, and the marble's rang=
e was
97cm. I figured that it should've landed 147cm from the edge of the =
table (about 34% error). I've tried different tube heights and my er=
would fluctuate (the best being 24% error).

I'm curious if others had done something similar and suggestions for
improving my setup.

Thank you,
Dwight Souder
Ashland, OH
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