PURPOSE: To show that tire pressure does not change when the tire is loaded within its normal operating limits.
DESCRIPTION: A gauge reads the air pressure in the tire. The tire can be loaded by someone sitting on the platform, causing the tire to flatten on the bottom because of the strain. Q: After the tire is loaded, will the pressure in the tire be (a) greater than, (b) less than, or (c) the same as, the pressure before being loaded. A: The pressure will stay the same. The flattening at the bottom, causing an apparent decrease in volume and increase in pressure, is compensated by slight bulging at all points around the tire.
Click your mouse on the photograph above to see what happens when someone actually sits on the stool, putting weight on the tire. To reinforce that answer, recall that you put 35 PSI in your spare tire when it is lying in the trunk and that remains the same when you mount the tire.
SUGGESTIONS: Ask your students to guess the result after you do the experiment but do not show them the gauge. Point out that you can put air into your spare tire up to its operating pressure while the tire is still in your trunk.
See Question of the Week #81 for information on using this demonstration to enhance class involvement.
REFERENCES: (PIRA unknown.)
EQUIPMENT: Tire with gauge.
SETUP TIME: 10 minutes.