How can a bird fly? What makes it stay up in the air?
What makes a gun kick?
Why do you sink when you stop swimming?
Whenever anything moves, it pushes something else in an opposite
direction. When you row a boat you can notice this; you see the oars
pushing the water backward to push the boat forward. Also, when you
shoot a bullet forward you can feel the gun kick backward; or when
you pull down hard enough on a bar, your body rises up and you
chin yourself. But the law is just as true for things which are not
noticeable. When you walk, your feet push back against the earth; and
if the earth were not so enormous and you so small, and if no one else
were pushing in the opposite direction, you would see the earth spin
back a little for each step you took forward, just as the big ball
that a performing bear stands on turns backward as the bear tries to
walk forward.
[Illustration: FIG. 38. The horse goes forward by pushing backward on
the earth with his feet.]
The usual way of saying this is, "Action and reaction are equal and
opposite." If you climb a rope, the upward movement of your body is
the action; but you have to pull down on the rope to lift your body
up. This is the reaction.
Without this law of action and reaction no fish could swim, no
steamboat could push its way across the water, no bird could fly, no
train or machine of any kind could move forward or backward, no man or
animal could walk or crawl. The whole world of living things would be
utterly paralyzed.
[Illustration: FIG. 39. As he starts to toss the ball up, will he
weigh more or less?]
When _anything_ starts to move, it does so by pushing on something
else. When your arms start to move up, they do so by pushing your body
down a little. When you swim, you push the water back and down with
your arms and legs, and this pushes your body forward and up. When a
bird flies up into the air, it pushes its body up by beating the air
down with its wings. When an airplane whirs along, its propeller fans
the air backward all the time. Street-car tracks are kept shiny by the
wheels, which slip a little as they tend to shove the track backward
in making the car move forward. Automobile tires wear out in much the
same way,--they slip and are worn by friction as they move the earth
back in pushing the automobile forward. In fact, if there are loose
pebbles or mud on the road, you can see the pebbles or mud fly back,
as the wheels of the automobile begin to turn rapidly and give their
backward push to the earth beneath.
[Illustration: FIG. 40. Action and reaction are equal; when he pushes
forward on the ropes, he pushes backward with equal force on the
seat.]
Here are a couple of experiments that will show you action and
reaction more clearly:
EXPERIMENT 26. Stand on a platform scale and weigh yourself.
When the beam is exactly balanced, move your hands upward and
notice whether you weigh more or less when they _start_ up.
Now move them downward; when they _start_ down, do you weigh
more or less? Toss a ball into the air, and watch your weight
while you are tossing it. Does your body tend to go up or down
while you are making the ball go up?
EXPERIMENT 27. Go out into the yard and sit in a rope swing.
Stop the swing entirely. Keep your feet off the ground all
through the experiment. Now try to work yourself up in the
swing; that is, make it swing by moving your legs and body and
arms, but not by touching the ground. (Try to make it swing
forward and backward only; when you try to swing sidewise, the
distance between the ropes spoils the experiment.) See if you
can figure out why the swing will not move back and forth.
Notice your bodily motions; notice that when half of your body
goes forward, half goes back; when you pull back with your
hands, you push your body forward. If you watch yourself
closely, you will see that every backward motion is exactly
balanced by a forward motion of some part of your body.
_APPLICATION 22._ Explain why you push forward against the
table to shove your chair back from it; why a bird beats down
with its wings against the air to force itself up; why you
push back on the water with your oars to make a rowboat go
forward.
INFERENCE EXERCISE
Explain the following:
101. Water comes up city pipes into your kitchen.
102. When you try to push a heavy trunk, your feet slip out
from under you and slide in the opposite direction.
103. When you turn a bottle of water upside down with a small
piece of cardboard laid over its mouth, the water stays in the
bottle.
104. You can squeeze a thing very tightly in a vise.
105. There is a water game called "log rolling"; two men stand
on a log floating in the water and roll the log around
with their feet, each one trying to make the other lose his
balance. Explain why the log rolls backward when the man
apparently runs forward.
106. The oil which fills up the spaces between the parts of
a duck's feathers keeps the duck from getting wet when a hen
would be soaked.
107. Sleds run on snow more easily than wagons do.
108. In coasting down a hill, it is difficult to stop at the
bottom.
109. When you light a pinwheel, the wheel whirls around as the
powder burns, and the sparks fly off in all directions.
110. You cannot lift yourself by your own boot straps.