100 THE UNIVERSE A VAST SYSTEM OF PARTS Similar equipment is used in sugar refineries and many other industries to remove crystals from the liquids in which they are formed. The Babcock milk-tester and cream-separator separates the cream from the denser portions of the milk because the denser portions, with greater inertia, have a greater centrifugal tendency and resistance to centripetal acceleration. This tendency is also applied in centrifugal water pumps, the governors of engines, and the cowboy's lariat. And finally, it is this same centrifugal resistance to centripetal acceleration that causes the earth to bulge at the equator. STUDY QUESTIONS 1. State Newton's laws of motion and illustrate each by a concrete example not given in the text. 2. Upon what two things does the measure of attraction between two bodies depend? 3. State Newton's law of universal gravitation. 4. Give evidence that Newton's laws of motion are inv'ariable. 5. Differentiate between mass and weight. 6. Why does the earth not travel in a straight line through space relative to the position of the sun? 7. Give an illustration of an application of centrifugal force in the home. 8. Give five illustrations of the application of centrifugal force. 9. \\ hat is centrifugal force? 10. What keeps the earth in its orbit? 11. Would a planet of large mass have to move more or less rapidly than a planet of smaller mass to enable its centrifugal force to balance the force of attraction of the sun? 12. Pluto is a small planet like the earth, but it is much farther from the sun than the earth. Does it move more or less rapidly than the earth? 13. W'hy is it advisable to slow down on turns? 14. Why is it thought that the companion of Sirius is composed of very dense matter? 15. State and illustrate Newton's first law of motion, i.e., the law of inertia. 16. State and illustrate Newton's second law of motion, i.e., the law of force. 17. State and illustrate Newton's third law of motion, i.e., the law of action and reaction. 18. What is weight, and how is it measured? 19. W'hat is mass, and how is it measured? 20. What is force, and how is it measured? 21. Use the terms force, velocity, and acceleration in describing the motion of an automobile. 22. Explain the tides. 23. Explain the precession of the earth. 24. Describe an exp)eriment which seems to confirm Einstein's suggestion that space is curved.
UNIT II SECTION 7 THE EARTH IS BUT ONE OF THE SUN'S PLANETS WHOSE MOTION FOLLOWS UNIVERSAL LAWS Introduction. Around the sun revolve bodies or planets of varying size. The earth is but one of these planets. Some of them revolve in orbits relatively close to the sun, while others are so far distant that their existence was never even suspected by astronomers until recent times. The ancient Babylonians, Egyptians, and Greeks recognized five "wanderers" ; Pluto