discrepant event: baggie blowing up a person

concept application

air pressure keeps cars off the ground.

a 1995 Lincoln Mark VIII costs: $38,800 --- it weighs: 3,720 pounds

________ pounds = the weight supported by each tire
if the pressure in the tires is 30 # pushing on ever sq. in. - then there must be
- ________ square inches of the four tires touching the ground, and
- ________ square inches = is the area were one tire touches the ground.

the product ( "answers" ) --- vs --- the process ( how to get "the answers" )

the car weighs 3,720 pounds and there are four tires --- so . . . what process leads you to the answer of 930 pounds ?
since every square inch is pushing with a force of 30 # ---
how many square inches are need to give a total "push" of 3,720 pounds ?
That would be -- 30# + 30# + 30# . . . etc.
124 square inches are needed.
What process was used to get that answer ?
there are 124 square inches for all the tires touching the ground, so each one must have . . . . square inches touching the ground.

By the weigh --- what does this car cost per pound ? :-)

going farther
How do these variables "fit" into the concept ? --
- an 18-wheel semi-truck ?
- and a unicycle ?
- and those fat tractor tires ?
- but skinny bicycle tires ?
- a tricycle ?

other thoughts to make your brain go round and round

from: -- physics begins with an M -- mysteries, magic and myth -- p 130

6.) Why do bicycle tires need to be pumped up to higher pressure than automobile tires when they don't need to support as much weight ?

It is certainly true that bicycle tires do not need to support as much weight as automobile tires.

The difference in weight may be perhaps a factor of 20 or 30.
But, consider the difference in area of the contact region between the tire and the ground. Bicycle tires are much thinner ( less wide ) than automobile tires. In addition, there are only two tires on a bicycle and four on an automobile.

Thus, the total area of contact is much less for a bicycle than for a car, by a factor of 40 or 50.
So, for the bicycle, even though the required force is less, the force is spread out over a smaller area, which more than com pensates for the smaller force and the resulting pressure must actually be higher.

Bicycle tires will work at lower pressures, of course, but the flatter tire will have more area in contact with the ground ( more friction -- more work to move ), so that it still provides the same force.
Similarly, we could pump an automobile tire to a higher pressure. The result would be that the edges of the tire would lift off the road, resulting in a reduced area.

Leyden note:
since the area of the bike tire is 40-50 times smaller -- and the weight is 20-30 times smaller --- the total lifting effect is about 2 times less than needed. So-o-o-o-o --- the bike tire is inflated to 2 times more pressure as an auto tire in order to compensate. That is why the pressure may be 60 # / sq. in.

What Happens to the Tread That Wears Off Tires ?
The tread wears gradually off our tires. After a few years of heavy mileage, it eventually becomes bare. But we don't see bits of tread on the road ( except from premature blowouts, of course ). Highways are not discolored with blackened tread bits. Does tire tread disappear along with our socks ?

The automobile industry, the tire industry, and some independent pollution experts have long been concerned about what may seem to be a trivial problem. Two specialists in the chemistry department of the Ford Motor Company have estimated that 600,000 metric tons of tire tread are worn off American vehicles every year. The possibility was more than remote that all of this material might remain in the air, in suspendable particles, which could be dangerous to humans. So they sought a way to measure what happens to the disappearing tread.

Leyden note:
tonnes is the 'metric' spelling of the word, ton -- that is, 1,000 kilogram - 2,200 pounds. Back in the 1970's the USA agreeded to sell Russia grain, and the farmers thought the price was right. However, the Russians were quoting prices for 2,200 pounds of grain -- a tonne -- and the farmers were thinking in terms of 2,000 pound equals one ton. Oops. It wasn't such a good deal -- $$$$ -- after all. It was 10% less than they thought.

Tests to determine the presence of tire tread were held in three different sites, all of which presented some problems.
First, indoor tests were designed to simulate driving wear on a tire. Unfortunately, without ambient weather conditions, worn rubber simply tended to stick to the simulated road surface. Scientists knew this wasn't what happened under real conditions, for the
second type of tests, on real highways, indicated that virtually no rubber stayed on the road, due to wind, rain, and movement of surrounding traffic. Additionally, surface areas around highways were sometimes cleaned by maintenance crews, hindering efforts to measure long-term accumulation of tire tread.
The third type of test, in tunnels, might be thought to show the maximum possible buildup of tire tread, except that road surfaces in tunnels tend to wear tires less than surface streets, and the lack of natural wind and rain in the tunnel made any extrapolation difficult.

Still, the combined results of these experiments did provide quite a lot of information about exactly what happens to tire tread. Whereas the most common substance in exhaust fumes is dangerous lead, the most plentiful tire debris is in the form of styrene- butadiene rubber (SBR), the most common rubber hydrocarbon in treads.

Most of the tread debris is not in the form of gas, but rather in microscopic particles that are heavy enough to fall to the ground.

All road and tunnel tests seem to confirm that particle debris found along roadsides accounted for at least 50 percent of the total missing tire tread, and possibly much more. One study indicated that 2 percent of all roadside dustfall consisted of worn tread material. Another study, in Detroit, found that of the total particulate loading in the air, only 1 percent was tread dust. Even in tunnel tests, tire tread comprised only 1 to 4 percent of the total airborne particulate matter generated-a percentage far less than that of the exhaust emissions of gasoline-and diesel-powered vehicles.

All the tests concur, then, that the vast majority of worn tread in particle form falls on the ground instead of staying in the air.

What happens to the rest of the worn tread ?
Much of it is dissolved through oxidation and devulcanization ( a chemical reaction that reverses the process used to harden rubber ). One estimate speculated that devulcanization accounted for 30 percent of the disappearing SBH. Wind, water runoff, oxygen, and microbial attack all act to help degrade tread particulates, which degenerate faster than the tread rubber on tires in any case.

In fact, nobody could get very excited about the possible environmental dangers of worn tire tread. If the tread particulate were light enough to remain airborne, it could cause some harm, but the 95 percent plus that settles into the ground near the roadway poses no health hazard. K. L. Campbell, of Firestone Tire & Rubber Company, points out that "Tire tread rubber is essentially an inert material so it doesn't contribute to acid rain or soil pollution." And because worn tire-tread particles on the ground are in too small a form even to see with the naked eye, we aren't even aware that they are there. Which proves again that what you can't see can't hurt you.

Submitted by Larry Orb in, of Florissant, Missouri.
Thanks also to: Brad Miles, of Victoria, British Columbia; C. William Foster, Jr., of Tulsa, Oklahoma; and Art Lombard, of Oakland, California.

from: p72-73 --- WHY DO CLOCKS RUN CLOCKWISE? ( and other imponderables; d. feldman --- harper & row 1987 ) 10/23r/95 - 4/12f/96