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Hewitt problems 13,14,17,19,31,32,43,54, (55) 13. What is the value of the Earth’s gravitational field at the center of Earth? Zero, at the center of the Earth, gravity is pulling you in every direction equally. Also, the equation becomes undefined when the denominator becomes zero 14. If you stepped into a hole that passed completely through Earth, you’d oscillate down and up. How long would a one-way trip take? How long would a round trip take? The book says a one way trip would take 45 minutes, so a round trip would take 90 minutes 17. Why are ocean tides higher at the time of a full moon? Full moons only happen when the sun Earth and moon are all lined up, that means that the tides they cause are also all lined up. • 19. Why are black holes black? Because the gravity is so strong, even light cannot escape it. (warping of the space-time continuum) • 31. Stephan says a crumpled paper weighs more than a normal one. Gravity is the same for both pieces of paper. There is just less wind resistance on the crumpled one. • 32. Earth and the Moon are gravitationally attracted to each other. Does the more massive Earth attract the moon with a grater force, the same force, or less force than the moon attracts Earth? The force is equal between both the moon and Earth, just in opposite directions (Newton’s Third Law). Newton’s Universal Law of Gravitation provides for only one force between two objects, so they must have the same magnitude. • 43. The Sun exerts almost 200 times more force on the oceans of Earth than the Moon does. Why then, is the Moon more effective in raising tides? Because the moon is closer, the difference in gravity (Δg) between the sides due to the moons pull is much larger than change (Δg) the Sun’s pull. • 54. Isabella drops a chunk of iron of mass m from the roof of her high school and it accelerates at g. Then she ties two chunks of iron together, of mass 2m. Show that when she drops the double chink, the acceleration of fall is also g. When Isabella doubles the mass, f=ma shows that double the force would be needed to accelerate it at the same rate. So no matter what, objects on Earth accelerate at -9.8 m/s2. • 55. 1 Newton= 1kgm/s2 , so N/Kg = kgm/s2/kg = m/s2