TO GO TO ANY OF THE PAGES LISTED BELOW, CLICK ON ITS TITLE CHAPTER 12 Force 1 12-1 What is a force? 2 Forces and Force Diagrams Enrichment Activity for Lesson 12-1 3 12-2 What is gravity? 4 Gravitational Force Enrichment Activity for Lesson 12-2 5 THE BIG IDEA Integrating Earth Science: What keeps planets and satellites in their orbits? 6 12-3 How does a spring scale work? 7 12-4 What is friction? 8 12-5 How can friction be changed? 9 12-6 What is air resistance? 10 Hypercharge Enrichment Activity for Lesson 12-6 11 12-7 What is pressure? 12 12-8 What is air pressure? 13 12-9 What is water pressure? 14 Water Pressure and Ocean Depth Enrichment Activity for Lesson 12-9 15 Chapter 12 Key Term Review 16 Chapter 12 Test 17 Chapter 12 Answer Key 20 (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 1
12-1 What is a force? Write true if the statement is true. If the statement is false, change the underlined term to make the statement true. 1. The weight of an object is a measure of the force of gravity. 2. The force of wind keeps a kite flying. 3. The force of a push is not in the direction of the push. 4. Forces equal in size and opposite in direction are called unbalanced forces. 5. Balanced forces do not cause a change of motion in an object. 6. When unbalanced forces act on a moving object, the motion of the object will stay the same. 7. An example of magnetic force is the attraction of a paper clip to a magnet. Skills: classifying, interpreting a diagram Decide whether each drawing below illustrates a force that is a push or a force that is a pull. Write push or pull to indicate your answer in the spaces provided. 1. 2. 3. 4. (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 2
Forces and Force Diagrams Enrichment Activity for Lesson 12-1 Skills: analyzing, identifying, modeling PART A Identify the type and direction of the force or forces shown in each picture. 1. 2. 3. 4. 5. 6. PART B Draw a force diagram, similar to the diagrams shown in your text, to describe each of the following situations. 1. A 15-N textbook is pushed upward with a force of 15 N. 2. A 15-N textbook is lifted up with a force of 20 N. (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 3
12-2 What is gravity? The diagram below shows the Sun and the nine planets in the solar system, including the Moon. Use the diagram to answer the following questions. 1. Which body probably has the greatest mass? 2. Which body probably has the smallest mass? 3. Based on your answers to questions 1 and 2, which body has the strongest force of gravity? 4. Based on your answers to questions 1 and 2, which body has the weakest force of gravity? 5. On which body would you weigh the most? 6. On which body would you weigh the least? Skills: calculating, comparing, interpreting a table The table shows how the distance of an object from the center of Earth changes, and how this affects the force of Earth s gravity on the object. Fill in the blanks to complete the table. The first one has been done for you. Distance (d ) Force of Gravity (1/d 2 ) 1. 3 times as far 1/9 2. 1/25 3. 64 times as far 4. 1/8,836 5. 18 times as far 6. 1/625 (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 4
Gravitational Force Enrichment Activity for Lesson 12-2 Skills: interpreting diagrams The diagrams below illustrate several important properties of gravitational force. In each picture, the sizes of the arrows are proportional to the size of the forces between the objects. Assume that the larger the size of the object, the larger its mass. Study the diagrams. Then, answer the questions. A. B. C. D. 1. Look at Figures A and B. What is the same in both pictures? 2. How do Figures A and B differ? 3. What important property of gravitational force is illustrated by Figures A and B? 4. Look at Figure C. How are the three pairs of objects different? 5. What property of gravitational force is illustrated by Figure C? 6. How do the two objects in Figure D differ from each other? 7. What does Figure D tell you about gravitational force? 8. Which diagram is most like the situation that exists when an apple falls to the ground? Why? (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 5
THE Big IDEA Integrating Earth Science Chapter 12 What keeps planets and satellites in their orbits? Look at pages 250 and 251 of your text to answer the following questions. 1. What is a thought experiment? 2. Why did Einstein do thought experiments? 3. If Earth had no atmosphere there would be no to slow down a thrown ball. 4. What shape does the path of a thrown ball follow? 5. The harder you throw a ball the 6. If Earth had no air resistance, what would happen if you were to throw a ball hard enough? Skill: inferring Complete the following. 1. An object goes into orbit when the of an object is great enough to offset the force of pulling on it. 2. Refer to Figure 12-12. What would happen if the ball were to be thrown harder than the one shown in Path C? 3. What happens to an artificial satellite when its velocity is slowed? Science Log Writing Activity Complete the Science Log on a separate sheet of paper. To complete the Big Idea Online, go to www.conceptsandchallenges.com. Follow the online instructions. (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 6
12-3 How does a spring scale work? PART A Label the diagram below using the terms hook, pointer, and scale. PART B Complete the following. 1. The basic unit of distance is the. 2. The basic unit of mass is the. 3. The basic unit of force is the. Skill: inferring Read the following situation. Write two inferences that may explain the situation. You have two bathroom scales at home. You decide to weigh yourself on both scales. You notice that you weigh slightly more on one scale than on the other scale. (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 7
12-4 What is friction? PART A Complete the following. 1. What is sliding friction? 2. What is air resistance? 3. What is rolling friction? PART B Decide which kind of friction has the greatest effect in each situation in the table below. Place a check mark in the correct column. Situation Sliding Friction Air Resistance Rolling Friction 1. Riding a skateboard 2. A raindrop falling 3. A leaf falling 4. Pulling a wagon 5. A wheelchair moving 6. Pushing a book off a shelf 7. Writing with a pencil Skills: organizing, classifying List five examples of friction that affect you in everyday life. Classify each type of friction as sliding friction, air resistance, or rolling friction. Organize your information in a table. 1. 2. 3. 4. 5. (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 8
12-5 How can friction be changed? Circle the term or phrase in parentheses that best completes each statement. 1. The oil in a car s engine (increases / decreases) friction. 2. Rolling friction is (greater / less) than sliding friction. 3. There is (more / less) resistance when walking on an icy road than on a dry road. 4. Shoes with spiked soles have (more / less) friction with the ground than normal shoes do. 5. Oil (is / is not) an example of a lubricant. 6. Sand is often put on icy driveways to (increase / decrease) the amount of friction. 7. (Increasing / Reducing) friction makes it easier to move an object. 8. The size of a frictional force (depends / does not depend) on the types of surfaces in contact with each other. Skills: analyzing, identifying, diagramming Complete the following. You need to move a box full of books across a tile floor. Identify three ways in which you could reduce the amount of friction in order to move the box easily. Diagram each method. Method 1 Method 2 Method 3 (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 9
12-6 What is air resistance? Use the diagram below to answer the following questions. 1. Which object has more air resistance? Explain your answer. 2. Which object will hit the ground first? 3. When these objects reach their terminal velocity, how will the upward force of air resistance compare to the downward pull of gravity? 4. When will these objects have a velocity of zero? 5. What condition would have to exist for these two objects to hit the ground at the same time? 6. What will happen to each object s acceleration as it falls? Skill: researching Use reference materials to find out at what rate an object accelerates as it falls toward Earth. Write the results of your research below. (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 10
Hypercharge Enrichment Activity for Lesson 12-6 Skills: inferring, relating concepts When you drop an object, the force of gravity pulls it to the ground. Did you know that another force may be opposing gravity and pushing the object up? This mysterious new force is what scientists call hypercharge. The following passage describes hypercharge. Read the passage and answer the questions that follow. Some scientists think they have found a new force. This force, called hypercharge, is a very weak force. It opposes gravity and affects objects no farther apart than 200 meters. This force is also believed to vary in strength from element to element. It is strongest in iron. Consider the experiment of dropping a ball and a feather at the same time in a vacuum. Normally, you would expect both objects to hit the ground at the same time. If hypercharge exists, however, the feather would hit the ground first. The iron ball s hypercharge would oppose gravity more than the feather s hypercharge would. This means that the ball would fall more slowly. Apollo astronauts tried to test hypercharge by dropping a hammer and a feather on the Moon. Both objects fell at the same rate and landed at the same time. Does this disprove the hypothesis that hypercharge exists? Further research and experimentation is needed. If hypercharge is a very weak force, then its effect may be too small to measure without very precise equipment. 1. In the space below, draw a picture to show how scientists think hypercharge acts on a falling object. Also, show the force of gravity in your drawing. 2. If the hypercharge hypothesis is correct, why might you expect different objects to fall at different rates? 3. What information given in the passage might lead you to suspect that hypercharge is related to magnetic force? 4. Do you think hypercharge exists? Why or why not? What kind of experiments would you do if you were a scientist who wanted to find out more about the possible existence of hypercharge? (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 11
12-7 What is pressure? Use the formulas below to complete the table. Round any decimals to the nearest thousandth. pressure = force area force = pressure area area = force pressure Pressure (N/Area) Force (N) Area (Unit 2 ) 1. 15 N 200 cm 2 3 N/ cm 2 2. 39 cm 2 3. 24 N 36 cm 2 3 N/ m 2 4. 10 m 2 20 N/ m 2 5. 82 m 2 0.25 N/ m 2 0.5 N 6. 12 N/ m 2 300 N 7. 8. 520 N 65 m 2 Skills: measuring, calculating Use a metric ruler to find the height and width of the sides of each rectangle shown below. Round each of your measurements to the nearest centimeter. Find the area. The pressure on each is given. Then, calculate the force acting on each rectangle. 1. height width Area = cm 2 Figure A The pressure on Figure A is 9 N/cm 2. What is the force? 2. height width Area = cm 2 Figure B The pressure on Figure B is 3.5 N/cm 2. What is the force? (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 12
12-8 What is air pressure? Write true if the statement is true. If the statement is false, change the underlined term to make the statement true. 1. Air molecules are not in constant motion. 2. Air pressure increases with altitude. 3. Bernoulli s principle states that as the speed of a fluid increases, the pressure it exerts decreases. 4. In an airplane wing, the top is less curved than the bottom. 5. Air pressure is measured with a nanometer. 6. Normal air pressure at sea level will support a column of mercury 760 cm high. 7. Pressure caused by the weight of Earth s atmosphere is called gravitational pressure. Skills: inferring, identifying The following statements describe the forces acting on an airplane. Decide which force is being described. Write gravity, lift, thrust, or drag in the space provided. 1. This is the upward force on the bottom of the wings. 2. This is the force pushing the plane forward. 3. This is the frictional force that slows down an airplane. 4. This force is equal to the weight of the plane. 5. A plane s engine provides this extra force. 6. Streamlining a plane can reduce this. 7. A plane uses too much fuel if there is too much of this. 8. This force pushes the wings up. (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 13
12-9 What is water pressure? Circle the term or phrase in parentheses that best completes each statement. 1. Air and water are both (fluids / liquids). 2. If water comes from a well, a (pump / water tank) creates water pressure. 3. Water pressure (increases / decreases) with depth. 4. In central water systems, the water tank is usually located (above / below) the homes supplied with water. 5. Submarines are built to withstand (more / less) pressure than other ships. 6. Air pressure on the surface of a pool is (greater / less) than the water pressure at the bottom of a pool. 7. (Hydraulic / Air) systems use the properties of fluids to transfer force. 8. An object submerged in water receives pressure from (all directions / one direction). Skills: explaining, analyzing Analyze the following situation. Then, write an explanation for it. When dams are constructed, they need to be made thicker at the base than at the top. Why is this so? (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 14
Water Pressure and Ocean Depth Enrichment Activity for Lesson 12-9 Skills: calculating, interpreting graphs Water pressure increases with depth. Scientists have found that water pressure in the ocean increases 1 atmosphere for every 10 meters of depth below the surface. (An atmosphere is a unit of pressure that is approximately equal to 10 N/cm 2.) The following graph shows various levels of ocean depth from sea level to 6,000 meters below sea level. Complete the graph by calculating the water pressure for each depth. Then, answer the questions. 1. How much greater is the pressure at 6,000 meters compared with the pressure at 3,000 meters? 2. What would be the water pressure 100 meters below the surface of the ocean? 1,000 meters below the surface? 3. What must be true about animals that live far beneath the surface of the ocean? 4. Normal air pressure is about 1 atmosphere. How far could you dive below the surface of the ocean without feeling more pressure than you are normally used to? 5. If you were to graph depth vs. pressure, with depth along the horizontal axis and pressure along the vertical axis, what would the shape of the graph look like? Why? (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 15
Chapter 12 Key Term Review In the spaces provided, write the term that best fits each description. Unscramble the letters in brackets [ ] to spell a topic that is discussed in this chapter. Write the topic in the space provided at the bottom of the page. Then, explain the meaning of the topic in one or two sentences. 1. Force of attraction between all objects in the universe [ ] [ ] 2. Empty space [ ] [ ] 3. Force per unit area [ ] 4. Materials that reduce friction [ ] [ ] 5. A push or pull that always acts in a certain direction [ ] 6. Instrument used to measure air pressure [ ] 7. States that as the speed of a fluid increases, its pressure decreases [ ] 8. SI unit of force [ ] [ ] 9. Force that opposes the motion of an object [ ] 10. Forces equal in size but opposite in direction [ ] 11. Pressure caused by the force exerted by Earth s atmosphere [ ] [ ] Topic: Explanation: (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 16
Chapter 12 Test Interpreting Diagrams Match each diagram with the correct description. Write your answers in the spaces provided. A. B. C. D. E. 1. An example of air resistance 2. Examples of sliding friction 4. An example of rolling friction 5. A force that is a pull 3. A force that is a push Multiple Choice Write the letter of the term or phrase that best completes the statement or answers the question. 1. The air pressure at the top of a mountain is higher than the air pressure a. at sea level. b. in a canyon. c. in a valley. d. acting on an orbiting satellite. 2. All of the following are forces except a. gravity. b. mass. c. magnetism. d. friction. 3. Universal gravitation was discovered by a. Bernoulli. b. Einstein. c. Newton. d. Galileo. (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 17
Chapter 12 Test (continued) 4. In order to find the amount of pressure exerted on an object, you must divide a. pressure by area. b. area by force. c. weight by pressure. d. force by area. 5. The force that opposes the downward motion of a falling object is called a. pressure. b. terminal velocity. c. air resistance. d. friction. 6. Two objects are placed on a spring scale. The pointer moves to 4 for object A and to 6 for object B. Object A weighs a. 3/2 as much as B. b. 2/3 as much as B. c. twice as much as B. d. three times as much as B. 7. According to Bernoulli s principle, as the speed of a fluid increases, its pressure a. increases. b. remains constant. c. decreases. d. may increase or decrease. 8. Hail falls to the ground because of the force of a. magnetism. b. friction. c. gravity. d. electricity. 9. Streamlining an airplane reduces a. thrust. b. drag. c. lift. d. weight. 10. Sand is sometimes spread on an icy surface in order to a. decrease friction. b. increase air resistance. c. decrease air resistance. d. increase friction. 11. A chair has a weight of 7 N. As you push the chair, friction equal to a force of 5 N pushes in the opposite direction. How will the total force needed to push the chair compare to the force of friction? a. The comparison depends on the type of friction. b. The total force will be less. c. The total force will be equal to the force of friction. d. The total force will be greater. 12. The idea that all objects are attracted to one another is called a. Bernoulli s principle. b. the law of force. c. Newton s law. d. the law of gravity. 13. Lubricants are used to a. decrease friction. b. increase friction. c. decrease gravity. d. increase gravity. 14. If the area remains the same, you can increase the amount of pressure by a. increasing the force. b. decreasing the pressure. c. increasing the mass. d. decreasing the mass. 15. The weight of an object tells you a. how large the object is. b. how heavy the object is. c. how tall the object is. d. the shape of the object. 16. What does a spring scale measure? a. volume b. temperature c. weight d. distance (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 18
Chapter 12 Test (continued) 17. An astronaut on the Moon dropped a 2-N rock and 7-N rock from the same height at the same time. Which will hit the surface of the Moon first? a. 2-N rock b. 7-N rock c. Neither will hit the Moon. d. They will hit the surface at the same time. 18. The forward force that helps a plane take off is called a. thrust. b. drag. c. lift. d. air resistance. 19. A common lubricant in a car s engine is a. air. b. oil. c. water. d. soap. 20. As you increase the depth of an object underwater, the water pressure on the object a. increases. b. decreases. c. stays the same. d. levels off. Written Response Answer the following questions in complete sentences. 21. CONTRAST AND COMPARE: What are the differences between air pressure and water pressure? What are the similarities? 22. EXPLAIN: If two groups of people are in a tug of war, what must happen for the rope to stay in place? For it to move in a certain direction? Explain your answers. (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 19
Answer Key CHAPTER 12: FORCE 12-1 What is force? 1. true 2. true 3. is 4. balanced 5. true 6. change 7. true 1. push 2. pull 3. push 4. pull 12-1 What is force? Enrichment Activity: Forces and Force Diagrams PART A 1. pull upward 2. push forward and upward 3. magnetic force pulling toward magnet 4. force of gravity pulling the leaf downward 5. pull toward each person 6. force of gravity pulling downward, force of wind pushing the pot off the sill PART B 1. a 15-N arrow pointing up from the book with another 15-N arrow pointing down from it 2. a 20-N arrow pointing up from the book with a 15-N arrow pointing down from it 12-2 What is gravity? 1. Sun 2. Moon 3. Sun 4. Moon 5. Sun 6. Moon 2. 5 times as far 3. 1/4,096 4. 94 times as far 5. 1/324 6. 25 times as far 12-2 What is gravity? Enrichment Activity: Gravitational Force 1. the size and mass of the objects 2. The distances between the objects and the sizes of the forces are different. 3. The size of the gravitational force between objects decreases as the distance between the objects increases. 4. They are different sizes and masses, and the gravitational forces between each pair of objects are different. 5. The gravitational force between objects decreases as the mass of the objects decreases. 6. One object is much larger, and has more mass, than the other. 7. Between objects with different masses, the object with greater mass exerts a greater gravitational force than does the object with less mass. 8. Figure D; Earth has much more mass, and therefore exerts a much greater gravitational force than the apple. The Big Idea 1. thinking about what would happen in a given situation without actually performing the experiment 2. He liked to try to work out new ideas in his imagination before beginning mathematical proofs or calculations. 3. air resistance 4. a curve 5. longer the curve 6. The shape of the curve would be long enough to follow the curve of Earth s surface, and the ball would be in orbit around Earth. 1. velocity, gravity 2. The ball will break away from the orbit of Earth and continue in a straight line away from Earth. 3. If it slows enough, the satellite will eventually fall to Earth. 12-3 How does a spring scale work? PART A A. pointer B. scale C. hook PART B 1. meter 2. kilogram 3. newton Possible answers: The spring on one scale has been stretched beyond its elastic limit and no longer stops the pointer at the correct point on the scale. The pointer on one scale was not on zero. One scale was not level. 12-4 What is friction? PART A 1. friction that occurs when objects slide over each other 2. a type of friction that pushes on an object that is moving through air 3. friction that occurs when one object rolls over a surface PART B 1. Rolling Friction 2. Air Resistance 3. Air Resistance 4. Rolling Friction 5. Rolling Friction 6. Sliding Friction 7. Sliding Friction Possible answers: riding in a car: rolling friction; walking: sliding friction; raindrops falling: air resistance; flying a kite: air resistance; painting a wall: sliding friction; riding a bicycle: rolling friction (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 20
Answer Key 12-5 How can friction be changed? 1. decreases 2. less 3. less 4. more 5. is 6. increase 7. Reducing 8. depends Students diagrams should model examples of using rolling friction or the use of lubricants to reduce sliding friction. 12-6 What is air resistance? 1. A; As each object falls, air pushes up against the surfaces. Because of the parachute, object A has a larger surface for air to push against than does object B. 2. B 3. They will be equal. 4. when they stop falling, or before they begin falling 5. They would have to be falling in a vacuum. 6. It will increase until terminal velocity is reached. Any object falling near the surface of Earth will accelerate at a rate of 9.8 meters/second/second. 12-6 What is air resistance? Enrichment Activity: Hypercharge 1. Accept all drawings in which gravity is pulling the object down and hypercharge is exerting a force in the opposite direction. 2. Different materials are supposed to have different-sized hypercharge forces, and different objects are made up of different elements. The stronger the hypercharge force is on an object, the slower it will fall. 3. Hypercharge is believed to be strongest in iron, which is also strongly affected by magnetic force. 4. A possible experiment might be to suspend two iron balls on strings very close to each other. As a control, have two plastic balls very close to each other also. Observe to see if the two iron balls tend to hang farther away from each other than the plastic balls do. 12-7 What is pressure? 1. 0.075 N/cm 2 2. 117 N 3. 0.667 N/cm 2 4. 30 N 5. 1,640 N 6. 2 m 2 7. 25 m 2 8. 8 N/m 2 1. 5 cm, 6 cm, 30 cm 2, 270 N 2. 4 cm, 3 cm, 12 cm 2, 42 N 12-8 What is air pressure? 1. are 2. decreases 3. true 4. more 5. barometer 6. mm 7. air 1. lift 2. thrust 3. drag 4. gravity 5. thrust 6. drag 7. drag 8. lift 12-9 What is water pressure? 1. fluids 2. pump 3. increases 4. above 5. more 6. less 7. Hydraulic 8. all directions Since pressure exerted by liquids increases with depth, it is important that the base be thick enough to withstand the extra pressure. 12-9 What is water pressure? Enrichment Activity: Water Pressure and Ocean Depth Water pressure (atm) from top to bottom: 30, 60, 90, 120, 150, 210, 300, 390, 450, 510, 600 1. twice as great 2. 10 atm; 100 atm 3. They must be able to withstand great pressure. 4. up to 10 meters 5. a straight line sloping upward; Pressure increases at a constant rate as depth increases. CHAPTER 12: FORCE Key Term Review 1. gravity 2. vacuum 3. pressure 4. lubricants 5. force 6. barometer 7. Bernoulli s principle 8. newton 9. friction 10. balanced forces 11. air pressure Topic: terminal velocity Explanation: Terminal velocity is the speed at which air resistance and gravity acting on a falling object are equal. (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 21
Answer Key CHAPTER 12: FORCE Chapter Test Interpreting Diagrams 1. C 2. B, E 3. A, B, E 4. A 5. B, C, D Multiple Choice l. d 2. b 3. c 4. d 5. c 6. b 7. c 8. c 9. b 10. d 11. d 12. d 13. a 14. a 15. b 16. c 17. d 18. a 19. b 20. a Written Response 21. Differences: Air pressure is caused by the motion of air particles, while water pressure is caused by the motion of water particles. Air pressure is measured with a barometer, while water pressure is not. Similarities: Both air pressure and water pressure increase with the amount of air or water pressing down on an object, and both air and water are subject to Bernoulli s principle. 22. Both groups of people exert a force on the rope, but each of the forces is exerted in an opposite direction. As long as the size of both forces is the same, the forces are balanced. They do not cause the rope to move. On the other hand, if one of the forces is greater than the other, the rope will move in the direction of the greater force. (c) by Pearson Education, Inc./Globe Fearon/Pearson Learning Group. All rights reserved. Force: CHAPTER 12, page 22