Inference Exercise
Explain the following:
91. As you tighten a screw it becomes harder to turn.
92. There is a process for partly drying food by whirling it rapidly in a perforated cylinder.
93. It is easier to climb mountains in hobnailed shoes than in smooth-soled ones.
94. When you bore a hole with a brace and bit, the hand that turns the brace goes around a circle many times as large as the hole that is being bored.
95. The hands of some persons become red and slightly swollen if they swing them while taking a long walk.
96. A flywheel keeps an engine going between the strokes of the piston.
97. In dry parts of the country farmers break up the surface of the soil frequently, as less water comes up to the surface through pulverized soil than would come through the fine pores of caked earth.
98. After you have apparently cleaned a grease spot out of a suit it often reappears when you have worn the suit a few days.
99. Mud flies up from the back wheel of a boy's bicycle when he rides along a wet street.
100. A typewriter key goes down less than an inch, yet the type bar goes up nearly 5 inches.
Section 13. Action and reaction.
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.
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.