Message-Id: <l03102800b2552eadc350@[209.142.17.241]>
In-Reply-To: <199810210712.AAA27967@mail.inreach.com>
Date: Thu, 22 Oct 1998 12:28:34 -0800
To: "Pinhole Listserv" <pinhole@exploratorium.edu>
From: Ronald Wong <ronwong@inreach.com>
Subject: Re: Steel balls and a steel plate
Deborah Hunt said:
>
>I am trying to locate the 1987, 1st Student Edition, of the Handbook of
>Chemistry and Physics. I specifically need, page F-51 which has info on
>the coefficient of restitution for steel balls against a steel plate.
>
>Does anyone have a copy?
>
Deborah:
Question: How is it possible for you to know that it is on page F-51 of a
handbook published 11 years ago and not have the information you seek? Did
someone actually tear that page out of your 1987 handbook? - I'm sorry,
that's two questions.
Is the 1987 handbook the only one CRP published that has this information?
- Uh, three questions.
With all the steel balls and steel plates that are probably lying around at
the Exploratorium I would suggest you do the following:
Find an electronic stopwatch (there must be plenty of these lying around
the Exploratorium as well). Most of these - even the cheapest ones - will
have a lap function. Find out how it works. In the days of the mechanical
stopwatches, there were models with split second hands that performed the
same function.
Drop a steel ball onto a steel plate and press the "start" button every
time you hear the sound of the ball striking the steel plate. Depending on
which type of watch you have, you will have a record of the time the ball
was in the air between each collision with the steel plate (My stopwatch
for instance will record up to 100 consecutive collisions).
Assuming the earth's mass is so much greater than the mass of the ball that
its motion can be neglected, the coefficient of restitution is nothing more
than the final speed of the ball after a collision divided by the initial
speed of the ball before.
The final speed of the ball determines the time it will be in the air
before it hits the plate again and is proportional to this time. In a
similar fashion, the initial speed of the ball before the collision is
proportional to the time it was in the air since the previous collision.
By dividing the last reading of the elapsed time by the previous one you
will have the coefficient of restitution. Repeating this over and over
again you will have a series of values from which you can determine the
coefficient of restitution and it's uncertainty for your steel ball and
steel plate - ten lapses giving you nine values for instance.
Of course, this method neglects the effects of air friction but you'll
probably get a value that is good enough for any work that would be done in
a classroom.
I'm quite sure it is better than that achieved by the traditional method
which involved measuring the successive heights to which a ball rose as it
bounced along.
The whole process will take you less time than reading this message. It may
even take you less time than it takes for you to figure out how to use the
lap function of your stopwatch.
Have fun. ron