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IPS_06_TE_U3_C2.qxd 12/12/06 1:53 AM Page 239 Activity 1 What Causes Gravity? 3 GRAVITATIONAL INTERACTIONS Activity 1: What Causes Gravity? In Activity 1, your class came up with several ideas about what causes gravity. You and your class probably thought that these might be causes of gravity: a) the Earth’s magnetic field b) the rotation of the Earth c) air pressure from the Earth’s atmosphere Pick one of these ideas, and design a quick experiment to test whether it could be the cause of gravity. 1. Write a paragraph describing your experiment. Your paragraph must be at least five complete sentences. 2. Draw and label a sketch of your experiment. 3. What experimental result would show that you might have found the cause of gravity? Activity 2: Testing Ideas about Gravity Suppose you were visiting a third-grade class, and some students were talking about their ideas on what causes gravity. 1. One of the students, Jacob, asks you, “Since we learned that the Earth is a big magnet, isn’t that what causes gravity?” Write how you would simply demonstrate to Jacob why the Earth’s magnetic field is not the cause of gravity. Include any drawings or diagrams on your answer sheet that you would make to help Jacob understand. 2. Danielle asks, “I read that the Earth spins around on its axis. Wouldn’t its spinning cause gravity?” Write how you would simply demonstrate to Danielle why the Earth’s rotation is not the cause of gravity. Include any drawings or diagrams that you would make to help Danielle understand. 3. Leticia asks, “Doesn’t the air around us push down on us, causing gravity?” Write how you would simply demonstrate to Leticia why air pressure from the Earth’s atmosphere is not the cause of gravity. Include any drawings or diagrams that you would make to help Leticia understand. Activity 3: More on Gravitational Interactions © It’s About Time 303 Activity 1 Practice Activity 2 Practice 1. Students will have various responses. Check to see if the experiment tests the hypothesis the student chose, and follows the rules on “How To Evaluate an Experimental Design for a Fair Test.” 1. (Example 1) I would show Jacob that objects like pens and pennies are not attracted to a magnet, but both fall toward the ground, so gravity cannot be caused by the Earth's magnetism. 2. Students will have various responses. 3. Students will have various responses. 3. (Example) If I could, I would show Leticia the video that showed the mass placed on the mass scale. The video showed that the reading on the mass scale did not change when the air pressure around the mass and scale was greatly reduced. If air pressure was the cause of gravity, the reading on the scale would have reduced as the air was taken out of the bell jar. Activity 3 Practice (Questions 1-3) In the video of the Cavendish experiment, a meter stick is suspended by a fine thread. Bottles are attached to each end of the meter stick. To test Newton’s idea about what causes gravity, heavy boxes of sand are brought close to the bottles, but the bottles and boxes do not touch each other. There is an observable gravitational interaction between each box and its bottle. Unit 3 • Chapter 2 2. (Example) I would attach a string to a plastic toy soldier, and tape the string to a rotating globe, so that the soldier is standing up on the Earth in the northern hemisphere (but not at the North Pole). I would ask Danielle, “Which way does the Earth's gravity pull on the soldier?” She might say it holds the soldier on Earth, but after spinning the globe, Danielle would see the toy soldier will fly off the Earth. So, because the spinning tends to have objects on the outside surface fly off, the Earth's rotation cannot be the cause of gravity. (Example 2) I would have two magnets. I would show Jacob the two magnets, the Earth being like one magnet. If I turn around the other magnet, it is repelled from the first magnet. However, either way the second magnet is turned, it still is attracted to the Earth when you let it fall. Thus, the Earth's magnetism is not the cause of gravity. 1. If the boxes of sand had much more sand in them, we would observe a greater rotation of the meter stick. The meter stick would rotate more because the gravitational interaction between the bottles and the boxes of sand would be greater. As the mass increases the interaction strength increases. 2. If the boxes of sand were not brought so close to the bottles, the meter stick would not have rotated as much. The meter stick would not have rotated as much because the gravitational interaction between the boxes and bottles would have been less. The gravitational interaction strength decreases as the distance between the interacting objects increases. 3. If the bottles were replaced by bigger bottles with more water in them, we would observe a greater rotation of the meter stick. The InterActions in Physical Science 239 PRACTICES—ANSWERS INTERACTIONS AND FORCES IPS_06_TE_U3_C2.qxd 12/12/06 1:53 AM Page 240 CHAPTER 2 GRAVITATIONAL INTERACTIONS meter stick would rotate more because the gravitational interaction between the bottles and the boxes would be greater. As the mass increases the interaction strength increases. thread meter stick bottles box of sand Activity 4 Practice box of sand 1. What would you observe if the boxes of sand had much more sand in them? Why? Answer this question by writing at least two complete sentences. 2. What would you observe if the boxes of sand were not brought so close to the bottles? Why? Answer this question by writing at least two complete sentences. 1. (See forces drawn on diagram below.) 3. What would happen if the bottles were replaced by bigger bottles with more water in them? The apple would weigh the most on Jupiter. Activity 4: Weight 2. The Moon is less massive than the Earth and hence has the weaker gravitational interaction. The Moon wouldn't pull you down as hard as the Earth. The result of this is that you could jump higher on the Moon because it has a weaker gravitational interaction on you than the Earth does. 1. In the diagram below, astronauts on different planets are dropping apples. force exerted by Mercury on apple Mercury force exerted by Venus on apple Venus force exerted by Earth on apple Earth force exerted by Jupiter on apple Jupiter The planets appear in order of their gravitational pull. Mercury is the least massive planet and has the weakest gravitational pull. Jupiter is the most massive 4 and has the strongest gravitational pull. Mercury’s gravity = 10 of Earth’s 9 gravity. Venus’ gravity = 10 of Earth’s gravity, and Jupiter’s gravity = 2 12 times Earth’s gravity. a) Copy the drawings, then draw and label force arrows to show the force exerted by each planet on its apple. Pay attention to the lengths of your force arrows! One is done for you. b) On which planet would the apple weigh the most? 2. Mercury, a planet in our Solar System, and Callisto, a moon of the planet Jupiter, have about the same volume. However, Mercury’s mass is about 3 times greater than Callisto’s mass. Why could a future astronaut (perhaps you!) jump higher on Callisto than on Mercury? Use gravitational interaction ideas to answer this question. Write at least three complete sentences. Callisto InterActions in Physical Science © It’s About Time 304 Mercury 240 UNIT 3: INTERACTIONS AND FORCES IPS_06_TE_U3_C2.qxd 12/12/06 1:53 AM Page 241 Activity 5 Putting Together Gravitational Interaction Ideas GRAVITATIONAL INTERACTIONS 3. Copy and complete the table below. person slows down as she moves upward person’s speed increases as she falls Other than drag, does an interaction affect the person‘s motion? If so, what kind of interaction(s) affect the person‘s motion? If so, what are the interacting objects? Is a force (or forces) being exerted on the person? If so, what is your evidence? On the pictures in the top row, label force arrows to represent any forces on the person. Label force arrows on diagram. 3 Label force arrows on diagram. Activity 5: Putting Together Gravitational Interaction Ideas Analyze and explain the following situations using How To Write an Analysis and Explanation. You will need to copy the diagrams onto your answer sheet. All of the analyses should include the interacting objects and their interaction type and labeled force arrows showing the forces being exerted on the object named in the task. You will not need to draw energy diagrams. Be sure that you would get a good evaluation using How To Evaluate an Analysis and Explanation. Activity 5 Practice 1. (Why does the softball slow down as it moves upward away from the bat?) Analysis: There is a gravitational interaction between the ball and the Earth. Explanation: The ball slows down because there is an unbalanced force acting on the ball, opposite to the ball's direction of motion. The force acting on the ball is exerted by the Earth and is due to the gravitational interaction between the Earth and the ball. This force points toward the Earth, which is opposite to the direction of motion of the ball. What goes up… 1. A softball player hits a pop fly straight up into the air. After the softball leaves the bat, why does the softball slow down as it moves upward away from the bat? (You may ignore the drag interaction, since it is very small.) Write an analysis and explanation. Unit 3 • Chapter 2 305 3. Table: Density and Buoyancy Other than drag, does an interaction affect the person’s motion? © It’s About Time If so, what kind of interaction(s) affect the person’s motion? If so, what are the interacting objects? Is a force or forces being exerted on the person? If so, what is your evidence? Yes Yes Gravitational Gravitational The earth and the person The earth and the person The jumper’s motion changes, which means there is an unbalanced force. Since the jumper slows down, the force must be opposite to her motion. The jumper’s motion changes, which means there is an unbalanced force acting on her. Since she is speeding up, the force is in the direction of her motion. On the pictures in the top row, label force arrows to represent any forces on the person. InterActions in Physical Science 241 PRACTICES—ANSWERS INTERACTIONS AND FORCES IPS_06_TE_U3_C2.qxd 12/12/06 1:53 AM Page 242 CHAPTER 2 GRAVITATIONAL INTERACTIONS 2. (Why does the softball speed up as it moves downward toward the ground?) Analysis: … must come down! There is a gravitational interaction between the ball and the Earth. 2. After the softball reaches the highest point of its path, it falls back to the field. Why does the softball speed up as it moves downward toward the ground? (You may ignore the drag interaction, since it is very small.) Explanation: Write an analysis and explanation. Force exerted by Earth on ball The ball speeds up because there is an unbalanced force acting on the ball in the ball's direction of motion. The force acting on the ball is exerted by the Earth and is due to the gravitational interaction between the Earth and the ball. This force point points toward the Earth, which is in the same direction that the ball is moving. 3. Two planets have the same size, but Planet A is more massive than Planet B. How does the weight of a book on Planet A compare with the weight of the same book on Planet B? Write an analysis and explanation. Activity 6: Orbital Motion (Questions 1-3) Analyze and explain the situations by following How To Write an Analysis and Explanation. You will need to copy the diagrams below onto your answer sheet. All of the analyses should include the interacting objects and their interaction type, and labeled force arrows in the diagrams showing the forces being exerted on the object named in the task. You will not draw energy diagrams. Be sure that you would get a good evaluation using How To Evaluate an Analysis and Explanation. 1. As a rotating space station simulates the feeling of gravity, the men and women inside the space station also move around in a circle. Analyze and explain why the people move around in a circle. 3. (How does the weight of a book on Planet A compare with Planet B?) 2. A satellite is an object that is launched into space to orbit the Earth. Some satellites are used to reflect radio waves from distant points on Earth that cannot send and receive the radio waves directly. These radio waves are used for radio and TV, cell phones, and computer navigation systems like you might find in your car. Someday, satellites may be used to collect solar energy and transmit energy to Earth. Analysis: There is a gravitational interaction between Planet A and the book, and between Planet B and the book. Analyze and explain why a satellite orbits the Earth rather than moves off into outer space. 3. Another satellite that orbits the Earth (though not a man-made one!) is the Moon. See diagram below. Analyze and explain how the Moon is kept in its orbit. Explanation: Since the planets are the same size, but Planet A is more massive than 306 force exerted by Planet A on book Planet B InterActions in Physical Science force exerted by Planet B on book Planet B, there is a stronger gravitational interaction between the book and Planet A than between the book and Planet B. A stronger gravitational interaction means greater force (or weight) exerted on the book from Planet A than from Planet B. Activity 6 Practice 1. (Why do the people on a rotating space station move around in a circle?) Analysis: There are applied interactions between the floor and each person. (See force arrows on diagram.) Explanation: There is an Force unbalanced, exerted by constant floor on inward force person exerted by the floor on each person associated with applied interactions. This constant inward force exerted by the floor on the people causes them to continuously change their direction of motion and move around in a circle. 2. (Analyze and explain why a satellite orbits the Earth Force exerted by Earth rather than on satellite moving off into outer space.) Analysis: There is a gravitational interaction between the Earth and the satellite. (See force arrow on diagram.) (Note: From another view, the force arrow diagram may be drawn as such:) Explanation: There is an 242 UNIT 3: INTERACTIONS AND FORCES Force exerted by Earth on satellite © It’s About Time Planet A IPS_06_TE_U3_C2.qxd 12/12/06 1:53 AM Page 243 Activity 7 Terminal Speed 3 GRAVITATIONAL INTERACTIONS Activity 7: Terminal Speed The Parachute Inflates! (Analyze and Explain) (Questions 1-2) In the activity, you analyzed and explained how a sky diver reaches terminal speed. As the sky diver approaches the ground, she will, of course, need to open her parachute! What do you think happens to the speed of the sky diver when her parachute inflates in the air? Immediately after the sky diver’s parachute catches the air and inflates, she and the parachute will slow down until they reach another slower terminal speed, allowing the sky diver to make a gentle landing on the ground. 1. Analyze and explain why the sky diver slows down immediately after her parachute inflates. In your analysis, include the interacting objects and their interaction type, and a diagram with labeled force arrow(s) showing the force(s) being exerted on the sky diver. You do not need to draw energy diagrams. 2. Does the mass of the sky diver increase, decrease, or stay the same after the parachute inflates? Explain your answer. Multiple Choice 3. Which of these variables affects the strength of a gravitational interaction? a) how fast the Earth rotates b) the atmospheric (air) pressure c) the distance between the objects d) the volume of the objects e) the strength of the Earth’s magnetic field 4. Which of these statements is not true? a) There is a gravitational interaction between all objects in the universe. b) Gravitational interactions can happen between objects that are not touching. c) Gravitational interactions are very difficult to observe unless one of the objects is very massive, like a planet. d) The weight of an object is the force exerted by a planet on the object. e) Increasing the mass of an object has no effect on its weight. 5. A rocket drops off a camera to take pictures of the Earth. Which arrow (a), (b), (c), (d), or (e) best represents the direction of the force exerted by the Earth on the camera? 4 (Questions 6-7) Mercury has a weaker gravitational pull than Earth (about 10 as much). 6. The mass of an apple a) will be the same on both planets. b) will be less on Mercury than on Earth. A c) will be greater on Mercury than on Earth. d) cannot be compared because Mercury has a very thin atmosphere around it compared to Earth. B C camera D E © It’s About Time Unit 3 • Chapter 2 unbalanced, constant inward force acting on the orbiting satellite toward the Earth. The force exerted by the Earth on the satellite is associated with the gravitational interaction. This constant inward force exerted by the Earth on the satellite causes the satellite to continuously change its direction of motion and circle the Earth. Analysis: There is a gravitational interaction between the Earth and the moon. (See force arrow on diagram.) (Note: From another view, the force arrow diagram may be drawn as such:) Force exerted by Earth on Moon Explanation: 3. Force exerted by Earth on Moon Earth Moon 307 There is an unbalanced, constant inward force acting on the orbiting moon toward the Earth. The force exerted by the Earth on the moon is associated with the gravitational interaction. This constant inward force exerted by the Earth on the moon causes it to continuously change its direction of motion and orbit the Earth. Activity 7 Practice 1. Analysis: There is a gravitational interaction between the ball and the Earth. There is a drag interaction between the parachutist (with parachute) and the air. Explanation: Force exerted by air on parachutist Force exerted by Eartth on parachutist The parachutist slows down immediately after the parachute inflates because the upward force exerted by the air on the parachutist (drag) is greater than the downward force exerted by the Earth on the parachutist (gravitational). This means there is an unbalanced force on the parachutist in the direction opposite her motion. This unbalanced upward force causes her to slow down as she falls toward the ground below. 2. The mass of the parachutist (including her parachute) stays the same, because both she and the parachute have the same amount of material whether the parachute is inflated or not. (There is a very slight increase in the weight (not the mass) of the parachutist as she gets closer to the ground because she gets closer to the center of Earth. However, this increase is negligible, because the distance that she falls (maybe 4000 meters) is very small compared to the radius of Earth (about 6,400,000 meters).) 3. (c) the distance between the objects 4. (e) (not true) Increasing the mass of an object has no effect on its weight. 5. (b) (The direction of gravitational force exerted by Earth on camera is toward the center of Earth.) 6. (a) will be the same on both planets. InterActions in Physical Science 243 PRACTICES—ANSWERS INTERACTIONS AND FORCES IPS_06_TE_U3_C2.qxd 12/12/06 1:53 AM Page 244 CHAPTER 2 GRAVITATIONAL INTERACTIONS 7. (b) will be less on Mercury than on Earth. 8. (d) (The direction of gravitational force exerted by Saturn on the piece of ice is toward the center of Saturn.) 7. The weight of an apple a) will be the same on both planets. Weight b) will be less on Mercury than on Earth. ? Weight c) will be greater on Mercury than on Earth. d) cannot be determined because Mercury has a very thin atmosphere around it compared to Earth. Activity 8 Practice Earth Mercury 8. In Activity 6, you read about the rings around Saturn consisting of pieces of ice. Another view of Saturn is shown below. At the position shown, which force arrow (a), (b), (c), (d), or (e) best shows the direction of the force exerted by Saturn on the piece of ice? 1. The Newton, named after Sir Isaac Newton, is the standard unit of force strength. 2. 20 N to the right A E 3. 0, no direction (The forces are balanced.) D B C Activity 8: Unbalanced and Balanced Forces 1. What is the standard unit of force strength, and who is it named after? 2. Suppose a bicycle experiences forces like those shown by the force arrows with the force strengths in the picture below. What is the strength and direction of the unbalanced force on the bicycle? 30 N 70 N 20 N 3. Later, the forces on the bicycle are like those shown by the force arrows in the picture below. What is the strength and direction of the unbalanced force on the bicycle?) 30 N 50 N 20 N InterActions in Physical Science © It’s About Time 308 244 UNIT 3: INTERACTIONS AND FORCES IPS_06_TE_U3_C2.qxd 12/12/06 1:53 AM Page 245 Activity 9 Buoyancy GRAVITATIONAL INTERACTIONS 4. In the Yukon, a team of four dogs heading west across a snow-covered plain exerts a force of strength 2000 N on a loaded sled. As the sled is moving, there is a wind blowing toward the east that exerts a force of strength 400 N on the sled. In addition, there is a friction force of strength 700 N resisting the motion of the sled. What is the strength and direction of the unbalanced force on the sled? 5. Consider a situation where you are pushing to the left with a force of strength 10 N against a heavy chair, but the chair is not moving. Because the chair is not moving, the multiple forces acting on it must be balanced. Assume that a friction force is preventing you from moving the chair. Find the strength and direction of the friction force. 6. Three boys and two girls engage in a tug of war. Simon, Antonio and Chan exert forces of 30 N, 45 N, and 20 N on the rope, all directed to the left. Rosalie and Luisa exert forces of 20 N and 30 N on the rope, both directed to the right. a) What is the strength and direction of the unbalanced force on the rope? b) Elizabeth joins the girls in the tug of war and balances the forces on the rope. What is the strength and direction of the force Elizabeth exerts on the rope? Activity 9: Buoyancy bag of water To answer some of the problems below, you will need to refer to these density values: • aluminum – 2.7 g/cm3 • brass – 8.0 g/cm3 • steel – 7.6 g/cm3 • gold – 19.3 g/cm3 • mercury – 13.0 g/mL • water – 1.0 g/mL 3 • oxygen – 0.0013 g/cm • air – 0.0012 g/cm3 • helium – 0.00017 g/cm3 3 ? tank of water 1. As you saw in the activity, a plastic bag filled with water immersed in a tank of water remains motionless in the tank after being let go. If the weight of the bag is 5 N, is the buoyant force greater than 5 N, less than 5 N, or equal to 5 N? Explain your answer. 2. A plastic bag filled with vegetable oil weighing 5 N is immersed in a tank of water and let go. The oil-filled bag rises to the top and floats. a) When the bag is let go, is the buoyant force greater than 5 N, less than 5 N, or equal to 5 N? Explain your answer. b Is the density of the vegetable oil greater than, less than, or equal to the density of water? Explain your answer. buoyant force exerted on balloon by surrounding air = 200 units 3. For her friend Maya’s birthday party, Sofia wants to inflate a few dozen balloons so that they rise up toward the ceiling. a) Why should Sofia use helium gas to fill up the balloons rather than other gases like oxygen or air? b) Look at the force arrow diagram beside the balloon. There are 50 units of force from the combined weight of the helium gas and balloon, and 200 units of upward buoyant force exerted on the balloon. How many units of force are needed from a downward pull of the string to hold the balloon in place? Explain your answer. weight of balloon & helium gas = 50 units force exerted by string on balloon = ? units © It’s About Time Unit 3 • Chapter 2 309 4. Choose east as positive direction. Force exerted by dogs = -2000 N Force exerted by wind = + 400 N Force exerted by friction = + 700 N Total unbalanced force = -2000 N + 400 N + 700 N = -900 N The unbalanced force on the sled has a strength of 900 N and points to the west. direction Forces exerted by boys = - 30 N 45 N - 20 N = -95 N Forces exerted by girls = + 20 N + 30 N = 50 N Total unbalanced force = -95 N + 50 N = -45 N The unbalanced force on the rope has a strength of 45 N and points to the left. 5. Forces on chair to right = Forces on chair to left Friction force = Push force on chair = 10 N The strength of the friction force is 10 N, and it is pointing to the right. B. Since the unbalanced force points to the left, to balance the forces, Elizabeth must exert a force to the right. Force exerted by Elizabeth to right = Unbalanced force to left Force exerted by Elizabeth to right = 45 N 6. A. Choose right as positive direction, left as the negative The strength of the force Elizabeth exerts on the rope is 45 N, and the force points to the right. Activity 9 Practice 1. The upward buoyant force is equal to 5 N because it must balance the downward weight force for the baggie to remain motionless. 2. A. The upward buoyant force must be greater than 5 N for the oilfilled baggie to rise to the top from rest. B. The density of the vegetable oil must be less than the density of water because the oil-filled baggie rises to the top from rest. (Knowing that the oil-filled baggie floats means that the weight of the oil in the baggie must be less than the water it displaces. Since the volume of the oil-filled baggie and the water it displaces are the same, the density of the oil-filled baggie must be less than the density of the water.) 3. A. Luisa should use helium gas to fill the balloons so that they will rise up to the ceiling because the density of helium is less than the density of air. (Since the inflated balloon and the volume of air displaced are equal, the lesser density of helium than air means the helium-filled balloon weighs less than air and will float in air.) B. 150 units. In order to hold a helium-filled balloon in place, the forces must be balanced. Thus, an additional downward pull of 150 units along with the 50 units weight force will balance an upward buoyant force of 200 units. 4. (d) All three cubes experience the same buoyant force (because they displace the same volume (and therefore weight) of water). 5. (d) gold 6. (b) The buoyant force strength is equal to the block's weight. (Note: Students may find this problem a bit tricky. The reason the answer is InterActions in Physical Science 245 PRACTICES—ANSWERS INTERACTIONS AND FORCES IPS_06_TE_U3_C2.qxd 12/12/06 1:53 AM Page 246 CHAPTER 2 GRAVITATIONAL INTERACTIONS (b), and not (c), is because when an object is floating, it is at rest (or more precisely, it isn't moving in the vertical direction up or down). Therefore, the forces are balanced. For those who answer (c), point out that the block is not completely submerged. Unless the object is completely submerged, comparing densities isn't enough to determine whether the buoyant force is greater than the weight.) Activity 10: Potential Energy 1. Julie stands on a diving board 1 meter above a diving pool. Her twin sister Diana, who weighs the same as Julie, stands on a high-dive platform 10 meters above the pool. a) Which system has more gravitational potential energy, the Julie-and-Earth system or the Diana-and-Earth system? Explain your answer. b) Diana dives off of the 10-meter platform. As she falls toward the pool, does her gravitational potential energy increase, decrease, or stay the same? As she falls toward the pool, does her kinetic energy increase, decrease, or stay the same? c) Later, Julie’s big brother Bill, who weighs more than Julie, joins her standing on the 1-meter diving board. Which system has more gravitational potential energy, the Julie-and-Earth system or the Bill-and-Earth system? Explain your answer. Multiple Choice Part 2 (In Practice Book) 2. The bookcase in Evelyn’s bedroom has four shelves. Suppose the book with a mass of 0.5 kg fell off a shelf. From which shelf would the fall of the book convert the most potential energy into kinetic energy? a) bottom shelf (located 0.1 m above the floor) b) second shelf (located 0.5 m above the floor) c) third shelf (located 0.9 m above the floor) d) top shelf (located 1.3 m above the floor) 1. See Table Below. Activity 10 Practice 3. André’s teacher gives him an aluminum spring that has a mass of 800 g. After a lesson about potential and kinetic energy, André’s teacher asks him how he would go about increasing the potential energy of the spring. André considers the four ideas listed below. Which response does not increase either the spring’s potential energy or the potential energy of the spring-and-Earth system? a) compressing the spring b) putting the spring on the floor c) stretching the spring d) tossing the spring up into the air 1. A. The Diana-and-Earth system has more gravitational potential energy than the Julie-and-Earth system. Even though they both weigh the same, Diana is higher above the pool along the Earth's surface. 4. Choose the Earth-and-bird system with the most gravitational potential energy. a) Earth and an eagle with a mass of 4.5 kg flying at a height of 1000 m b) Earth and a gull with a mass of 0.5 kg flying at a height of 100 m c) Earth and a falcon with a mass of 1.0 kg flying at a height of 1000 m d) Earth and a raven with a mass of 1.0 kg flying at a height of 100 m B. As Diana falls toward the pool, her gravitational potential energy decreases (because she is getting closer to the Earth), and her kinetic energy increases (because of the simple relationship between gravitational potential energy and kinetic energy). 2. (d) Top shelf (located 1.3 m above the floor) 3. (b) putting the spring on the floor (does not increase spring's potential energy) 4. (c) Earth and a falcon with a mass of 1.0 kg flying at a height of 1000 m 5. (d) Ledge located 53 m below the top of the waterfall 310 InterActions in Physical Science Table: The Buoyancy of Objects Object Mass Volume Density Water: Sink or Float? Salt Water: Sink or Float? Baseball 145 g 232 cm3 0.63 g/cm3 Float Float Can of Cola (12 oz) 384 g 379 cm3 1.01 g/cm3 Sink Float Can of Diet Cola (12 oz) 371 g 379 cm3 .98 g/cm3 Float Float Milky Way™ Bar 60 g 50 cm3 1.20 g/cm3 Sink Neither Toy Wagon 55 g 63 cm3 0.87 g/cm3 Float Float (includes open part) Part 2 (In Practice Book) 1. (b) (greatest elastic potential energy) 2. (d) (greatest kinetic energy after spring released) 246 UNIT 3: INTERACTIONS AND FORCES © It’s About Time C. The Bill-and-Earth system has more gravitational potential energy than the Julie-and-Earth system. Even though they are both the same height above the pool along the Earth's surface, Bill weighs more than Julie. 5. A person living in the wilderness near a waterfall 60 m high wants to utilize the waterfall to generate electricity for his small wooden cabin. He has a turbine and generator. There are four ledges in the cliff behind the waterfall where he could place the turbine and generator he has built for this purpose. On which ledge would the turbine and generator produce the most electrical energy? a) ledge located 15 m below the top of the waterfall b) ledge located 23 m below the top of the waterfall c) ledge located 36 m below the top of the waterfall d) ledge located 53 m below the top of the waterfall IPS_06_TE_U3_C2.qxd 12/12/06 1:53 AM Page 247 Activity 12 Distance in Space 3 GRAVITATIONAL INTERACTIONS Activity 11: The Solar System 1. a) Make a close sketch of (or trace over) the picture of the Sun and planets in our Solar System in Activity 11. Label each planet and the Sun. b) On the sketch, indicate the terrestrial planets and the gas giants. c) On the sketch, label where the Asteroid Belt and the Kuiper Belt are located. 2. a) Which planet is the largest in size (excluding rings)? Which planet is the smallest? b) Which planet has the greatest mass? Which planet has the least mass? c) Which planet has the greatest density? Which planet has the least density? 3. a) Which planet do some people think could have supported life at one time? b) What are some reasons they have for this idea? 4. a) How is a planet different from a dwarf planet, other than in size? b) How is a moon different form a planet or a dwarf planet? Activity 12: Distances in Space a) astronomical unit b) light-year 2. Identify the following terms as either a unit of distance, time, or speed. b) year c) light-year d) mph e) astronomical unit 3. Rank the following distances from least to greatest: astronomical unit, light-year, 100,000,000 kilometers. (least distance) _____________________________________________ (most distance) Unit 3 • Chapter 2 Jupiter Terrestrial Planets Activity 11 Practice 1. (A through C) 2. A. Jupiter is the largest planet; Mercury is the smallest planet. B. Jupiter has the greatest mass; Mercury has the least mass. C. Earth has the greatest density; Saturn has the least density. 311 Neptune Uranus Pluto Kuiper Belt Asteroid Belt Sun © It’s About Time Saturn Mars Gas Giant Planets 6. (a) ice and rock Activity 12 Practice 3. (least distance) 100,000,000 kilometers, astronomical unit, lightyear (greatest distance) 4. Planets and other Solar System bodies have elliptical orbits. How are elliptical orbits different than circular orbits? Include a sketch of these orbits with your answer. Earth 5. (b) Venus 2. A. kilometers per hour: (unit of) speed B. year: time C. light-year: distance D. mph: speed E. astronomical unit: distance a) kilometers per hour Venus 4. A. A planet dominates its orbital neighborhood, while a dwarf planet does not. B. Planets and dwarf planets orbit the Sun, while moons orbit planets and dwarf planets. 1. An astronomical unit is the average distance between the Earth and Sun, about 150,000,000 km (150 million kilometers). 1. Define the following terms: Mercury have proposed that the layered rock outcrops that have been interpreted as signs of past water could have been left by explosive volcanic ash or an ancient meteorite impact.) KBO 3. A. Mars B. Like the Earth, Mars has relatively mild temperatures (between -100°C and 27°C), a day nearly the same as Earth's, and an atmosphere (though thinner and of different composition than Earth's). Dry channels similar to the Earth's river systems invite speculation that they once carried water, a substance critical for supporting life on Earth. (However, recent studies 4. In a planet's elliptical (oval or “eggshaped”) orbit, the planet's distance from the Sun varies depending on where it is in its orbit. In a circular orbit, an object keeps the same distance (called the radius) from a central point. Part 2 (In Practice Book) 1. (d) 660 (= 165 yr/ 0.25 yr) (number of Mercury orbits around Sun for every one of Neptune's orbits) 2. (c) 12,000 (= 149,600 km/ 12.756 km) (Earth diameters fit into 1 AU) 3. (e) 500,000 AU (= (8.6 ly) x (63,240 AU/ly) (distance of Sirius from Sun) 4. (a) Betelgeuse (At 100 km/hr, it takes 11,000,000 years to travel 1 ly distance. Over 4.5 billion years, the distance is 4,500,000,000/ 11,000,000 = 409 ly, closest to Betelgeuse (427 ly)) InterActions in Physical Science 247 PRACTICES—ANSWERS INTERACTIONS AND FORCES IPS_06_TE_U3_C2.qxd 12/12/06 1:53 AM Page 248 CHAPTER 2 GRAVITATIONAL INTERACTIONS Activity 13 Practice 1. (b) in the center of the disc where a star forms Activity 13: Gravity, Stars, and Planets 2. (c) the part of the disc far from the star is much colder, so some gases can freeze. 1. When a large cloud pulls together to form a solar system, it flattens out into a disc. Most of the cloud’s mass ends up Multiple Choice a) far out in the disc where gas giants form. b) in the center of the disc where a star forms. c) on the edge of the disc where icy objects like KBOs form. 3. (d) cores of gas giants are more massive than terrestrial planets, which means they have stronger gravity. d) close to the center of the disc where terrestrial planets form. 2. Ice is more likely to form on bodies far from the star in the center of a solar system than on bodies close to the star because: a) bodies far from the star are more massive and have stronger gravity. b) bodies far from the star exert stronger magnetic forces. 4. (c) they are too small. c) the part of the disc far from the star is much colder, so some gases can freeze. d) the part of the disc far from the star just has more ice. 5. (a) the force produced by hot gas and nuclear radiation balances gravity. 3. Gas-giant planets have thicker atmospheres than terrestrial planets because a) gas giants have lower densities than terrestrial planets. b) gas giants are cooler than terrestrial planets. 6. You never see the “far side of the Moon” because the Moon had become locked into position so that one side of the Moon always faces Earth, and one side always faces away. Its rotation period is exactly equal to the period of its orbit around earth. c) gas giants exert stronger magnetic forces on gases than terrestrial planets. d) cores of gas giants are more massive than terrestrial planets, which means they have stronger gravity. 4. Gravity doesn’t pull some bodies in space into round shapes because a) they are made of rock and metal. b) they are made of ice and rock. c) they are too small. d) they are too cold. 5. A star has a round shape because a) the force produced by hot gas and nuclear radiation balances gravity. 7. (d) Pluto (always keeps its same face toward its moon Charon.) b) its magnetic forces balance gravity. c) the solid interior of the star stops gravity from making it contract (get smaller and more dense). d) nuclear reactions can only occur if the star is spherical. InterActions in Physical Science © It’s About Time 312 248 UNIT 3: INTERACTIONS AND FORCES IPS_06_TE_U3_C2.qxd 12/12/06 1:53 AM Page 249 SCIENTISTS’ CONSENSUS IDEAS—ANSWERS Scientists’ Consensus Ideas Unit 3 Chapter 2 Answer Keys (Unit 3 Chapter 2, Activities 2–6) 9. The Gravitational Interaction General Description of the Gravitational Interaction 1. The video we saw of Cavendish’s experiment. There was a meter stick suspended by a string. On each end of the meter stick was an attached bottle. There was also a mirror attached to the meter stick and a beam of light reflected off the mirror onto a wall. A large box of sand was brought near each end of the meter stick, but on opposite sides. We observed that the light beam on the wall moved. This is because the mirror attached to the meter stick rotated slightly. The only thing that seemed to cause the meter stick to rotate was the attraction between the boxes of sand and the bottles. 3. Two small magnets placed on a table can be observed to interact (as in Unit 1), overcoming the friction interaction. However, two non-magnetic objects (assume same size, shape, and mass) are not observed to interact because the gravitational interaction is weaker than the friction interaction. 4. An object will change its motion if there is an unbalanced force acting on it. If the unbalanced force is in the direction of motion, the object will speed up. The evidence for the gravitational force is that the apple’s motion changes – it speeds up toward Earth. 6. In the experiment we watched, an object was placed on a scale in a bell jar. The air was then evacuated out of the bell jar. The object did not begin to float and its scale measurement did not change. Because there was no change observed, the gravitational interaction cannot be due to air pressure. 7. Example Response: In the experiment we did, the string straightened up (pointed outward) when the bucket rotated. Gravity is not caused by Earth’s rotation because if it were the string would not have moved outward, pointing away from the bucket, when the bucket was rotated. Rather, it should have moved inward, toward the bucket and rested itself on the bucket. 8. Example Response: Gravity always attracts objects (objects are pulled toward each other); magnetism sometimes repels objects. Also, there is a magnetic attraction between a magnet and some metals, but there is a gravitational attraction between Earth and ALL objects. (Unit 3 Chapter 2, Activities 8–10) 10. Other Ideas about Forces, Motion, and Energy–Part 2 Buoyancy © It’s About Time 5. In the video, when the force exerted on the floating object by the scale was subtracted from the weight of the object, the result was the buoyant force on the object, which was 1.4 N. The weight of the water displaced by the object was also 1.4 N, equal to the buoyant force. InterActions in Physical Science 249 IPS_06_TE_U3_C2.qxd 12/12/06 1:53 AM Page 250 Sinking and Floating 9. A baggie of salt water, which is denser than water, sinks when submerged in a container of water. 10. A baggie of alcohol, which is less dense than water, rises to the surface and then floats when submerged in a container of water. 11. A baggie of water neither rises nor sinks when submerged in a larger container of water. (Unit 3 Chapter 2, Activities 11–13) 11. Earth and Space Science–Part 1 The Solar System 4. Eclipses of the Moon, which occur when the Moon passes through Earth’s shadow. Earth blocks sunlight from reaching all or part of the Moon’s surface, and the Moon doesn’t shine. 250 UNIT 3: INTERACTIONS AND FORCES IPS_06_TE_U3_C2.qxd 12/12/06 1:53 AM Page 251 Scientists’ Concensus Ideas – Answers NOTES 3 © It’s About Time CHAPTER 2 InterActions in Physical Science 251