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Re: Here's one to ask your teacher... (long reply) (archive)

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Posted by Tom M. on January 15, 2000 at 10:42:29:

In Reply to: Re: Here's one to ask your teacher... posted by Zip on January 15, 2000 at 09:07:32:

: : : We are talking about pressure in a Physical Science class. We are trying to find out why a much bigger tire (for equipment, for example) wll hold less air pressure than a regular tire that is much smaller.

: : : We look forward to hearing from you.

: : : Thank you.

: : ...see if he/she knows how to determine the weight of a car with just 2 tools found in most tool boxes.

: : Let me know if you can't figure it out.

: : Tom M.

: I'll bite

OK here's how it works. If you think about the tire contact patch sitting flat on the ground, when the car is parked, the tires are in equilibrium. (They're not going up or down.) For equilibrium to exist the force on the inside of the tire pressing down must be exactly equal to the force pressing up on the outside of the tire.

You can find out the pressure inside the tire in pounds per square inch by using tool #1, a tire pressure gauge. Then use tool #2, a ruler to measure the contact patch. That gives you the number of square inches. Multiply the 2 numbers together and you have the weight supported by the tire. Repeat for the other 3 tires and sum, that's the car's weight.

This one took me a second to agree with when I first heard it but it makes sense. What happens when you decrease the pressure in a tire? The pounds per square inch goes down. But the number of pounds the tire is supporting doesn't change, so the number of square inches has to go up to keep the total force constant. Thus the tire flattens out.

There are tons of assumptions in this calculation. The biggest error comes from the fact that the air pressure isn't the only thing holding up the car. The sidewalls do a big part of it, especially in those low profile 18 inch wheel setups. (That's the principle behind the run flat tire, very stiff sidewalls.)

Anyway enough Mr. Science for now.

Tom M.

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