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There are a number of strange particles, a neutron and a proton are examples,
which are called baryons. In any reaction whatever in nature, if we count how
many baryons are coming into a process, the number of baryons# which come
out will be exactly the same. 'Phere is another law, the conseruation oƒ leptons.
W© can say that the group of particles called leptons are: electron, mu meson,
and neutrino. 'Phere is an antielectron which is a positron, that is, a —1 lepton.
Counting the total number of leptons in a reaction reveals that the number in
and out never changes, at least so far as we know at present.
'These are the six conservation laws, three of them subtle, involving space and
time, and three of them simple, in the sense of counting something.
With regard to the conservation of energy, we should note that auailable
energy is another matter—there is a lot of jiggling around in the atoms of the
water of the sea, because the sea has a certain temperature, but it is impossible
to get them herded into a deñnite motion without taking energy from somewhere
else. That is, although we know for a fact that energy is conserved, the energy
avajlable for human utility is not conserved so easily. The laws which govern how
much energy is available are called the laus oƒ thermodWnœmics and involve a
concept called entropy for irreversible thermodynamic processes.
Finally, we remark on the question oŸ where we can get our supplies oŸ energy
today. Our supplies of energy are from the sun, rain, coal, uranium, and hydrogen.
The sun makes the rain, and the coal also, so that all these are from the sun.
Although energy is conserved, nature does not seem to be interested ïn it; she
liberates a lot of energy from the sun, but only one part in two billion falls on the
earth. Nature has conservation of energy, but does not really care; she spends a
lot of it in all directions. We have already obtained energy from uranium; we can
also get energy from hydrogen, but at present only in an explosive and dangerous
condition. Tf it can be controlled in thermonuclear reactions, it turns out that
the energy that can be obtained from 10 quarts of water per second is equal to
all of the electrical power generated in the United States. With 150 gallons of
running water a minute, you have enough fuel to supply all the energy which is
* Counting antibaryons as —1 baryon.
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used in the United States today! "Therefore it is up to the physicist to figure out
how to liberate us from the need for having energy. It can be done.
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Tĩn+© (ra3eổl ÍÌsÉcrrtc©
5-1 Motion
In this chapter we shall consider some aspects of the concepts of #ne and
đistance. It has been emphasized earlier that physics, as do all the sciences,
depends on øbseruøiion. One might also say that the development of the physical
sciences to their present form has depended to a large extent on the emphasis
which has been placed on the making of quøaniitati»e observations. Only with
quantitative observations can one arrive at quantitative relationships, which are
the heart of physics.
Many people would like to place the beginnings of physics with the work done
350 years ago by Galileo, and to call him the first physicist. Ủntil that time, the
study of motion had been a philosophical one based on arguments that could be
thought up in one”s head. Most of the arguments had been presented by Aristotle
and other Greek philosophers, and were taken as “proven.” Galileo was skeptical,
and did an experiment on motion which was essentially this: He allowed a ball
to roll down an inclined trough and observed the motion. He did not, however,
Jjust look; he measured hou ƒar the ball went in hou long a từme.
'The way to measure a distance was well known long before Galileo, but there
wWere no accurate ways of measuring time, particularly short times. Although
he later devised more satisfactory clocks (though not like the ones we know),
Galileo”s first experiments on motion were done by using his pulse to count off
cequal intervals of time. Let us do the same.
'We may count of beats of a pulse as the ball rolls down the track: “one...
make a small mark at the location of the ball at each count; we can then measure
the đZstance the ball travelled from the point of release in one, or two, or three,
etc., equal intervals of time. Galileo expressed the result of 52s observations in
this way: If the location of the ball is marked at 1, 2, 3, 4,... units of time
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“STARTE -'ONE" Dœt
Lm ` c~'THREE”
Fig. 5-1. A ball rolls down an inclined track.
from the instant of its release, those marks are distant from the starting point in
proportion to the numbers 1, 4, 9, 16,... Today we would say the distance 1s
proportional to the square of the time:
'The study of motion, which is basic to all of physics, treats with the questions:
where? and when?
5-2 Time
Let us consider first what we mean by me. What ¡s time? It would be nice
1ƒ we could fnd a good defnition of time. Webster defines “a time” as “a period,”
and the latter as “a time,” which doesnt seem to be very useful. Perhaps we
should say: ““Dime is what happens when nothing else happens.” Which also
doesn't get us very far. Maybe it is just as well if we face the fact that tỉme is
one oŸ the things we probably cannot define (in the dictionary sense), and just
say that it is what we already know it to be: it is how long we waitl
'What really matters anyway is not how we đeƒfne time, but how we measure
it. One way of measuring time is to utilize something which happens over and
over again in a regular fashion—something which is periodic. For example, a day.
A day seems to happen over and over again. But when you begin to think about
1%, you might well ask: “Are days periodic; are they regular? Are all days the
same length?” One certainly has the impression that days in summer are longer
than days in winter. Of course, some of the days in winter seem to get awfully
long 1ƒ one is very bored. You have certainly heard someone say, “My, but this
has been a long day!”
Tt does seem, however, that days are about the same length ơn the œuerage.
ls there any way we can test whether the days are the same length—either from
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one day to the next, or at least on the average? One way is to make a comparison
with some other periodic phenomenon. Let us see how such a comparison might
be made with an hour glass. With an hour glass, we can “create” a periodic
Occurrence ¡iŸ we have someone standing by it day and night to turn it over
whenever the last grain of sand runs out.
We could then count the turnings oŸ the glass from each morning to the next.
We would fnd, this time, that the number of “hours” (¡.e., turnings of the glass)
was not the same each “day.” We should distrust the sun, or the glass, or both.
After some thoupht, ¡it might occur to us to count the “hours” from noon to noon.