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Bing Yu
11-16-2001, 03:29 AM
Dear Colleagues,

I posted the following question regarding soccer ball pressure and mass.

__________________________________________________ ________

I need some help to explain some data we obtained in one of our research

project. One of my students did a study on the impact between soccer
ball and head. She used soccer balls in 3 sizes: size 3, size 4, and
size 5. She inflated each ball to 10 PSI, 12 PSI, and 14 PSI, and
measured ball masses. Here are the results:

Size 3 ball: 10 PSI: mass = 0.19947 kg, 12 PSI: mass = 0.20339 kg, 14
PSI: mass = 0.32404 kg

Size 4 ball: 10 PSI: mass = 0.22305 kg, 12 PSI: mass = 0.22309 kg, 14
PSI: mass = 0.36728 kg

Size 5 ball: 10 PSI: mass = 0.25701 kg, 12 PSI: mass = 0.25981 kg, 14
PSI: mass = 0.42660 kg

The student and a biomedical engineering faculty repeatedly measured the

pressures and masses and confirmed these results are correct. Our
questions is: why did ball masses had significant increases when ball
presure increased from 12 PSI to 14 PSI? Thank you very much for your
help.
__________________________________________________ ___________

I have recieved some responses, but most of these responses either told
us that we put air into the ball and air has mass, or suggested us to
check our equipment.

I know I am not smart, and air has mass, but I don't believe that, for a
soccer ball, we need to pump in over 0.1 kg air to increase presure from
12 PSI to 14 PSI.

We repeatedly checked and calibrated our equipment, and did not find any
evidence that our data are due to equipment error. In addition, each
ball was measured using two different pieces of equipment with
essentially same results.

My student measured the ball again this morning, and got the same
results as I posted yesterday. Also, she found that the ball mass at 0
PSI was very similar to that at 14 PSI.

We will appreciate it if anybody can provide a convincing explanation to
our observations. Thanks lot.

Bing

--
Bing Yu, Ph.D.
Assistant Professor
Director
Center for Human Movement Science
Division of Physical Therapy
The University of North Carolina at Chapel Hill
Chapel Hill, NC 27599-7135

Tel: 919-843-8643
Fax: 919-966-3678
E-mail: byu@med.unc.edu

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