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374 Unit 3: Motion and Forces

Forces change the motion of objects in predictable ways.

KEY CONCEPTS SUMMARY

VOCABULARYforce p. 345net force p. 347Newton’s first law p. 349

inertia p. 350

VOCABULARYNewton’s second law p. 354

centripetal force p. 358

VOCABULARYNewton’s third lawp. 361

VOCABULARYmomentum p. 368collision p. 370conservation of momentum p. 371

Forces change motion.1

2

4

Chapter Review

Newton’s first law Objects at rest remain at rest, and objects in motionremain in motion with the same velocity, unless actedupon by an unbalanced force.

Newton’s second lawThe acceleration of an object increases withincreased force and decreases with increased mass,and is in the same direction as the force.

Newton’s third law When one object exerts aforce on another object, thesecond object exerts anequal and opposite force onthe first object. reaction

forceactionforce

Force and mass determine acceleration.

3 Forces act in pairs.

Forces transfer momentum.• Momentum is a property of a moving object.• Forces in collisions are equal and opposite.• Momentum is conserved in collisions.

unbalancedforce

object inmotion

object at rest unbalancedforce

small mass larger mass

larger mass, same force = decreased acceleration

small force larger force

same mass, larger force = increased acceleration

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Chapter 11: Forces 375

14. An action force and its reaction force are

a. equal in size and direction

b. equal in size and opposite in direction

c. different in size but in the same direction

d. different in size and in direction

15. John pulls a box with a force of 4 N, and Jasonpulls the box from the opposite side with aforce of 3 N. Ignore friction. Which of the following statements is true?

a. The box moves toward John.

b. The box moves toward Jason.

c. The box does not move.

d. There is not enough information to determine if the box moves.

16. A more massive marble collides with a lessmassive one that is not moving. The totalmomentum after the collision is equal to

a. zero

b. the original momentum of the more massive marble

c. the original momentum of the less massivemarble

d. twice the original momentum of the more massive marble

Short Answer Write a short answer to each question.

17. List the following objects in order, from theobject with the least inertia to the object withthe most inertia: feather, large rock, pencil,book. Explain your reasoning.

18. During a race, you double your velocity. How does that change your momentum?

19. Explain how an object can have forces actingon it but not be accelerating.

20. A sea scallop moves by shooting jets of waterout of its shell. Explain how this works.

Reviewing Vocabulary

Copy and complete the chart below. If the left column is blank, give the correct term. If the right column is blank, give an examplefrom real life.

Reviewing Key Concepts

Multiple Choice Choose the letter of the best answer.

11. Newton’s second law states that to increaseacceleration, you

a. increase force c. increase mass

b. decrease force d. increase inertia

12. What units are used to measure force?

a. kilograms c. newtons

b. meters d. seconds

13. A wagon is pulled down a hill with a constantvelocity. All the forces on the wagon are

a. balanced c. increasing

b. unbalanced d. decreasing

Term Example from Real Life

1. acceleration

2. centripetal force

3. The pull of a handle on a wagon

4. inertia

5. mass

6. net force

7. Newton’s first law

8. Newton’ssecond law

9. When you’re walking, youpush backward on the ground, and the ground pushes youforward with equal force.

10. momentum

376 Unit 3: Motion and Forces

Thinking Critically

Use the information in the photographs belowto answer the next four questions.

The photographs above show a toy calledNewton’s Cradle. In the first picture (1), ball 1 is lifted and is being held in place.

21. Are the forces on ball 1 balanced? How doyou know?

22. Draw a diagram showing the forces acting on ball 2. Are these forces balanced?

In the second picture (2), ball 1 has been let go.

23. Ball 1 swung down, hit ball 2, and stopped.Use Newton’s laws to explain why ball 1stopped.

24. Use the principle of conservation of momen-tum to explain why ball 5 swung into the air.

Copy the chart below. Write what will happento the object in each case.

28. INFER A baseball is three times more massivethan a tennis ball. If the baseball and the tennis ball are accelerating equally, what canyou determine about the net force on each?

1 2 3 4 5 51 2 3 4

Cause

25. Balanced forces act

on an object.

26. Unbalanced forces

act on an object.

27. No force acts on

an object.

Effect

1 2

Using Math Skills in Science

Complete the following calculations.

29. What force should Lori apply to a 5 kg box togive it an acceleration of 2 m/s2?

30. If a 10 N force accelerates an object 5 m/s2,how massive is the object?

31. Ravi applies a force of 5 N to a wagon witha mass of 10 kg. What is the wagon’s acceleration?

32. Use the informationin the photographon the right to calculate themomentum of theshopping cart.

33. PREDICT Look again at the tug of war picturedon pages 342–343. Describe what informationyou need to know to predict the outcome ofthe game. How would you use that informationand Newton’s laws to make your prediction?

34. WRITE Pick an activity you enjoy, such as running or riding a scooter, and describe how Newton’s laws apply to that activity.

35. SYNTHESIZE Think of a question you haveabout Newton’s laws that is still unanswered.What information do you need in order toanswer the question? How might you find theinformation?

If you need to do an experiment for your unit project, gather the materials. Be sure to allowenough time to observe results before the projectis due.

velocity = 0.5 m/s

mass = 40 kg

Chapter 11: Forces 377

Analyzing DataTo test Newton’s second law, Jodie accelerates blocks of ice across a smooth,flat surface. The table shows her results. (For this experiment, you can ignorethe effects of friction.)

Study the data table and then answer the questions that follow.

1. The data show that as mass becomes greater, acceleration

a. increases

b. decreases

c. stays the same

d. cannot be predicted

2. From the data, you can tell that Jodie wasapplying a force of

a. 1 N c. 3 N

b. 2 N d. 4 N

3. If Jodie applied less force to the ice blocks, theaccelerations would be

a. greater c. the same

b. less d. inconsistent

4. If Jodie applied a force of 6 N to the 2 kg block of ice, the acceleration would be

a. 2 m/s2 c. 3 m/s2

b. 4 m/s2 d. 5 m/s2

5. The average mass of the ice blocks she pushed was

a. 1.5 kg c. 3 kg

b. 2.5 kg d. 4 kg

6. If Jodie used a 3.25 kg block in her experiment,the force would accelerate the block somewherebetween

a. 1.0 and 1.1 m/s2

b. 1.1 and 1.3 m/s2

c. 1.3 and 1.6 m/s2

d. 1.6 and 2.0 m/s2

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Extended ResponseAnswer the two questions in detail. Include some ofthe terms shown in the word box. Underline eachterm you use in your answer.

7. Tracy ties a ball to a string and starts to swing theball around her head. What forces are acting onthe ball? What happens if the string breaks?

8. Luis is trying to pull a wagon loaded with rocks. What can he do to increase the wagon’s acceleration?

Newton’s second law velocitymass inertiagravity balanced forcescentripetal force unbalanced forces

Accelerating Blocks of Ice

Mass (kg) 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Acceleration (m/s2) 4.0 2.7 2.0 1.6 1.3 1.1 1.0