Why is it harder to keep your balance on stilts than on your
feet?
Why does a rowboat tip over more easily if you stand up in it?
In Pisa, Italy, there is a beautiful marble bell tower which leans
over as if it were just about to fall to the ground. Yet it has stood
in this position for hundreds of years and has never given a sign of
toppling. The foundations on which it rested sank down into the ground
on one side while the tower was being built (it took over 200 years
to build it), and this made it tip. But the men who were building
it evidently felt sure that it would not fall over in spite of its
tipping. They knew the law of stability.
[Illustration: FIG. 13. The Leaning Tower of Pisa.]
All architects and engineers and builders have to take this law into
consideration or the structures they put up would topple over. And
your body learned the law when you were a little over a year old, or
you never could have walked. It is worth while for your brain to know
it, too, because it is a very practical law that you can use in your
everyday life.
If you wish to understand why the Leaning Tower of Pisa does not fall
over, why it is hard to walk on stilts, why a boat tips when a person
stands up in it, why blocks fall when you build too high with them,
and how to keep things from tipping over, do the following experiment
and read the explanation that follows it:
EXPERIMENT 12.[2] Unscrew the bell from a doorbell or a
telephone. You will not harm it at all, and you can put it
back after the experiment. Cut a sheet of heavy wrapping paper
or light-weight cardboard about 5 x 9 inches. Roll this so as
to make a cylinder about 5 inches high and as big around as
the bell. Hold it in shape by pasting it or putting a couple
of rubber bands around it. Cut two strips of paper about an
inch wide and 8 inches long; lay these crosswise; lay the
bell, round side down, on the center of the cross. Push a
paper fastener through the hole in the bell (the kind shown in
Figure 14) and through the crossed pieces of paper, spreading
the fastener out so as to fasten the paper cross to the
rounded side of the bell. Bend the arms of the cross up around
the bell and paste them to the sides of the paper cylinder so
that the bell makes a curved bottom to the cylinder, as shown
in Figure 15.
[Illustration: FIG. 14.]
[Footnote 2: TO THE TEACHER. If you have a laboratory, it is well to
have this cylinder already made for the use of all classes.]
[Illustration: FIG. 15. In this cylinder the center of weight is so
high that it is not over the bottom if the cylinder is tipped to any
extent. So the cylinder falls over easily and lies quietly on its
side.]
[Illustration: FIG. 16. But in this one the center of weight is so low
that it is over the base, no matter what position the cylinder is in.]
[Illustration: FIG. 17. So even if the cylinder is laid on its side it
immediately comes to an upright position again.]
Try to tip the cylinder over. Now stuff some crumpled paper loosely
into the cylinder, filling it to the top. Tip the cylinder again. Will
it stay on its side now? Force all the crumpled paper to the bottom
of the cylinder. Now will it stay on its side? Take out the crumpled
paper and lay a flat stone in the bottom of the bell, holding it
in place by stuffing some crumpled paper in on top of it. Will the
cylinder tip over now? Take the stone out, put the crumpled paper in
the bottom of the cylinder, put the stone on top of the paper, and
again try to tip the cylinder over. Will it fall?
* * * * *
The center of the cylinder was always in one place, of course. But the
_center of the weight_ in that cylinder was usually near the bottom,
because the bell weighed so much more than the paper. When you raised
the center of weight by putting the stone up high or filling the
cylinder with crumpled paper, just a little tipping moved the center
of weight so that it was not directly over the bell on which the
cylinder was resting. Whenever the center of weight is not over the
base of support (the bottom on which the thing is standing), an object
will topple over. Moving the center of weight up (Figs. 15 and 16)
makes an object less stable.
The two main points to remember about stability are these: the wider
the base of an object, the harder it is to tip over; and the lower the
center of the weight is, the harder it is to tip over.
If you were out in a rowboat in a storm, would it be better to sit up
straight in the seat or to lie in the bottom of the boat?
Why is a flat-bottomed boat safer than a canoe?
[Illustration: FIG. 18. Which vase would be the hardest to upset?]
Where do you suppose the center of weight of the Leaning Tower of Pisa
is,--near the bottom or near the top?
_APPLICATION 8._ If you had a large flower to put into a vase
and you did not want it to tip over easily, which of the three
vases shown in Figure 18 would you choose?
_APPLICATION 9._ Some boys made themselves a little sail-boat
and went sailing in it. A storm came up. The boat rocked badly
and was in danger of tipping over. "Throw out all the heavy
things, quick!" shouted one. "No, no, don't for the life of
you do it!" called another. "Chop down the mast--here, give me
the hatchet!" another one said. "Crouch way down--lie on the
bottom." "No, keep moving over to the side that is tipped up!"
"Hold the things in the bottom of the boat still, so they'll
not keep rolling from side to side." "Jump out and swim!"
Every one was shouting at once. Which parts of the advice
should you have followed if you had been on board?
INFERENCE EXERCISE
Explain the following:
21. A ship when it goes to sea always carries ballast (weight)
in its bottom.
22. If the ship springs a leak below the water line, the water
rushes in.
23. The ship's pumps suck the water up out of the bottom of
the ship.
24. The water pours back into the sea from the mouths of the
pumps.
25. As the sailors move back and forth on the ship during a
storm, they walk with their legs spread far apart.
26. Although the ship tips far from side to side, it rights
itself.
27. However far the ship tips, the surface of the water in the
bottom stays almost horizontal.
28. While the ship is in danger, the people put on life
preservers, which are filled with cork.
29. When the ship rocks violently, people who are standing up
are thrown to the floor, but those who are sitting down do not
fall over.
30. If the ship fills with water faster than the engines can
pump it out, the ship sinks.
CHAPTER TWO
MOLECULAR ATTRACTION