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Transcript
CHAPTER 3 Laws of Motion Understanding Our Universe FIRST EDITION Palen | Kay | Smith | Blumenthal © 2012 by W. W. Norton & Company 1 2 3 4 5 You read in the paper that a new planet was found. The article states that when it is closest to its star, it moves at 31 km/s. When it is farthest from its star, it moves at 35 km/s. This story has an error because: a. Kepler’s 2rd law says the planet has to sweep out equal areas in equal times, so the speed of the planet cannot change. b. Planets stay at a constant distance from their stars, they don’t move closer or farther away c. Kepler’s 3rd law says the planet must move fastest when it’s closest, not when it’s farther away. 6 The figure shows a planet’s orbit that obeys Kepler’s laws. However, not all of the shaded regions have equal areas. During which part of the planet’s orbit (A, B, C, or D) would the planet move with the greatest speed? 7 8 Photo: humbletree on Flickr 9 10 An unbalanced force must be acting when: A. An object accelerates B. An object changes direction, but not speed C. An object changes speed, but not direction D. An object changes speed and direction E. All of the above 11 Your weight equals the force between you and Earth. Suppose you weigh 60 kg. Then the force on Earth is: A. 60 kg. B. Much smaller than 60 kg because your mass is much less than Earth’s. C. Exactly zero, since only massive objects have gravity. 12 13 Imagine you are standing at the top of a tall tower. You drop 3 objects shaped like a ball made out of different materials. Neglecting air resistance, in what order do they hit the ground? a. Styrofoam, pumpkin, lead b. Lead, pumpkin, styrofoam c. Pumpkin, lead, styrofoam d. All hit at same time 14 15 Newton’s Universal Law of Gravitation Fgrav = G × m1 × m2 r2 Suppose you are transported to a planet with twice the mass of Earth, but the same radius. Your weight would? a. Increase b. Decrease c. Stay the same 16 Earth and the Moon have a gravitational force between them. The mass of the Moon is 1.2% of that of the Earth. Which statement is incorrect? A. The force on the Moon is much larger than that on Earth. B. The forces are equal size, even though the masses are different. C. The Moon has a larger acceleration than Earth. 17 18 The Space Shuttle is in a circular orbit at a constant speed of 28,000 km/hr. Is gravity acting on the spacecraft? A. The spacecraft is not accelerating since the speed is constant. No force = no gravity. B. The direction of the motion is constantly changing. This counts as an acceleration. Forces produce gravity. C. The astronauts inside are weightless. No weight equals no gravity. 19 20 21 Summary: Science at Work! • Newton expanded Galileo’s observations and results into 3 Laws. • Newton derived Kepler’s Laws from his law of gravity. Newton, Kepler, Galileo Griffith Observatory. L.A., CA • Newton’s Laws were tested by Kepler’s observations, showing his law of gravity to be correct. 22