From: Ronald Wong (ronwong@inreach.com)
Date: Mon Nov 20 2000 - 11:59:30 PST
Message-Id: <l03102801b63e9fb28535@[209.209.20.42]> Date: Mon, 20 Nov 2000 11:59:30 -0800 From: Ronald Wong <ronwong@inreach.com> Subject: re: acceleration lab ideas
Mike said:
>I am trying to develop a simple acceleration lab where kids can observe
>somthing that accelerates slowly, mark its changing positions, and make an
>acceleration graph.
>... Anyone have an idea?
Galileo had the same problem. He needed to slow down the rate at which
objects fell to the earth. His solution to the problem was the inclined
plane.
Get some lengths of 2X4's and cut a grove down their length using a router,
table saw, milling machine, whatever. Prop one end up and roll a ball down
the inclined plane. By varying the height of the elevated end, you can
adjust the size of the acceleration to suit your needs.
Basically, the students release a ball from a given position and, at equal
intervals of time, mark it's position on the 2X4 using a piece of chalk.
There are a couple of ways to establish equal intervals of time. They can
have someone with a stop watch call out at the end of equal intervals of
time or they could do what Galileo is said to have done - sing a simple
melody and use the beat of the music as a time interval. Your students
could sing a tune like "Mary had a Little Lamb" and, at the first beat of a
bar (or whatever interval they have chosen), make a mark on the 2X4 where
ever the ball happens to be at that moment.
You didn't say in what way they would analyze their data but I would like
to suggest that you consider having them measure and plot the distance
traveled DURING each time interval if you haven't already done so. Since
the time intervals are all equal, the distance traveled is proportional to
the average velocity. This speeds up the analysis and has added benefits.
In fact, if you consider the time interval used to be the unit of time, the
distances ARE the average velocities and the difference in the distances
from one time interval to the next represents not only the change in the
velocity during that time interval but the acceleration as well.
In the "good old days" there was an excellent way to get across the
connection between velocity, acceleration and distance. If you still have
timing tape lying around (these came in the form of spools of very narrow
paper tape), you can hand the spools out to the students and have them lay
a strip of the tape on the 2X4 and mark the position of the ball on the
tape as the ball rolls down the inclined plane. The tape is then cut at the
points where the marks where made and laid out lengthwise side by side in
the order of the elapsed time with one end of each of the tapes butted up
against a common reference line (this could be a piece of masking tape
stuck to the lab table).
In doing this, the students will have quickly constructed a simple
velocity-time graph where the unit of time is the time interval they used
in their lab.
At a glance they can see that the velocity changed by equal amounts in
equal intervals of time (if all went well) - proof that the acceleration
was uniform (in fact, the difference in the lengths IS the acceleration in
terms of their time interval). This difference is nothing more than the
"slope" of the graph where the "run" is equal to their interval of time.
They can also see that over any period of time, the sum of the pieces of
tape is nothing more than the distance traveled during that time period. In
other words, the "area" under the curve of the velocity-time graph is the
distance traveled for a given period of time.
Using the paper tape approach, they can quickly do a second run with a
greater angle of inclination and set up a second "velocity-time" graph.
Comparing this one with the former one, they can immediately see that the
acceleration is still uniform and that the only effect of increasing the
angle is to increase the rate of change of the velocity - the acceleration.
Of course, you don't need a grooved 2X4 and a ball. An ordinary 2X4 and
some "frictionless" toy cars will work just fine. You don't even need a 2X4
or timing tape, other things can be used as a substitue for these as well.
You get the idea.
Lots of luck/fun. - ron
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