Section 2: What Is a Force? (p. 115) 34 HOLT SCIENCE AND TECHNOLOGY

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1 20. Scientifically speaking, how do you know the cyclist in Figure 4, on page 113, is accelerating? 21. Another name for acceleration in which velocity increases is acceleration. 22. Negative acceleration, or acceleration in which velocity decreases, is also called. 23. What kind of acceleration is occurring in Figure 5, on page 114? 24. When you are standing completely still at the equator, you are accelerating. True or False? (Circle one.) 25. How can you recognize acceleration on a graph? a. The graph shows distance versus time. b. The graph shows time versus distance. c. The graph shows velocity changing as time passes. d. The graph is a straight line. Review (p. 114) Now that you ve finished Section 1, review what you learned by Section 2: What Is a Force? (p. 115) Mark the following statements True or False. 1. All forces have size and direction. 2. A force is a push or a pull. 3. Forces are expressed in liters. 34 HOLT SCIENCE AND TECHNOLOGY

2 Forces Act on Objects (p. 115) 4. You can exert a push without there being an object to receive the push. True or False? (Circle one.) 5. Name three examples of objects that you exert forces on when you are doing your schoolwork. 6. In which of the following situations is a force being exerted? (Circle all that apply.) a. A woman pushes the elevator button. b. A pile of soil sits on the ground. c. Socks like the ones in Figure 7, on page 116, cling together when they have just come out of the dryer. d. Magnets stick to the refrigerator. Forces in Combination (p. 116) 7. In Figure 8, on page 116, how does the net force help the students move the piano? 8. Suppose the dog on the left in Figure 9, on page 117, increased its force to 13 N. Which dog would win the tug-of-war? Explain. DIRECTED READING WORKSHEETS 35 CHAPTER 5

3 Unbalanced and Balanced Forces (p. 117) 9. Why is it useful to know the net force? 10. Forces are unbalanced when the net force is not equal to. 11. To start or change the motion of an object, you need a(n) force. (balanced or unbalanced) 12. Forces are balanced when the net force applied to an object is zero. (less than, greater than, or equal to) 13. Are the forces on the cards in Figure 10 balanced? How do you know? Review (p. 118) Now that you ve finished Section 2, review what you learned by Section 3: Friction: A Force that Opposes Motion (p. 119) 1. What force is responsible for the painful difference between sliding on grass and sliding on pavement? The Source of Friction (p. 119) 2. Friction occurs when the hills and valleys of two surfaces stick together. True or False? (Circle one.) 3. Pavement creates more friction than grass. Why is that? 36 HOLT SCIENCE AND TECHNOLOGY

4 20. Scientifically speaking, how do you know the cyclist in Figure 4, on page 113, is accelerating? Sample answer: A change in velocity means acceleration. Because the cyclist s velocity increased from 1 m/s south to 5 m/s south, I know that the cyclist is accelerating. 21. Another name for acceleration in which velocity increases is positive acceleration. 22. Negative acceleration, or acceleration in which velocity decreases, is also called deceleration. 23. What kind of acceleration is occurring in Figure 5, on page 114? Centripetal acceleration is occurring. 24. When you are standing completely still at the equator, you are accelerating. True or False? (Circle one.) 25. How can you recognize acceleration on a graph? a. The graph shows distance versus time. b. The graph shows time versus distance. c. The graph shows velocity changing as time passes. d. The graph is a straight line. Review (p. 114) Now that you ve finished Section 1, review what you learned by Section 2: What Is a Force? (p. 115) Mark the following statements True or False. 1. True All forces have size and direction. 2. True A force is a push or a pull. 3. False Forces are expressed in liters. 34 HOLT SCIENCE AND TECHNOLOGY

5 Forces Act on Objects (p. 115) 4. You can exert a push without there being an object to receive the push. True or False? (Circle one.) 5. Name three examples of objects that you exert forces on when you are doing your schoolwork. Accept any reasonable answer. Sample answer: I exert a force on a book when I open it, I exert a force on the keys of a keyboard when I type, and I exert a force on a chair when I sit on it. 6. In which of the following situations is a force being exerted? (Circle all that apply.) a. A woman pushes the elevator button. b. A pile of soil sits on the ground. c. Socks like the ones in Figure 7, on page 116, cling together when they have just come out of the dryer. d. Magnets stick to the refrigerator. Forces in Combination (p. 116) 7. In Figure 8, on page 116, how does the net force help the students move the piano? When the students apply force in the same direction, their forces are added together to produce a combined net force. 8. Suppose the dog on the left in Figure 9, on page 117, increased its force to 13 N. Which dog would win the tug-of-war? Explain. Sample answer: The dog on the left would win the tug-of-war because the net force would be 1 N in favor of the dog on the left. DIRECTED READING WORKSHEETS 35 CHAPTER 5

6 Unbalanced and Balanced Forces (p. 117) 9. Why is it useful to know the net force? It will help you determine the effect of the force on the motion of an object. 10. Forces are unbalanced when the net force is not equal to zero. 11. To start or change the motion of an object, you need a(n) unbalanced force. (balanced or unbalanced) 12. Forces are balanced when the net force applied to an object is equal to zero. (less than, greater than, or equal to) 13. Are the forces on the cards in Figure 10 balanced? How do you know? Yes; if they were unbalanced, the pile of cards would be collapsing. Review (p. 118) Now that you ve finished Section 2, review what you learned by Section 3: Friction: A Force that Opposes Motion (p. 119) 1. What force is responsible for the painful difference between sliding on grass and sliding on pavement? The force of friction is responsible. The Source of Friction (p. 119) 2. Friction occurs when the hills and valleys of two surfaces stick together. True or False? (Circle one.) 3. Pavement creates more friction than grass. Why is that? Pavement is rougher than grass. 36 HOLT SCIENCE AND TECHNOLOGY