2. Chapter Menu Lesson 1: Combining Forces Lesson 2: Types of Force
Lesson 3: Unbalanced Forces and Acceleration
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3. 2.1 Combining Forces force contact force noncontact force net force
unbalanced forces
balanced forces
Newton’s first law of motion

4. 2.1 Combining Forces
Force
When more than one force acts on an object, the combined effect is caused by the sum of all applied forces.
A push or a pull on an object is a force.

5. Contact and Noncontact Forces
2.1 Combining Forces
Contact and Noncontact Forces
A contact force is exerted only when two objects are touching.
A noncontact force is exerted when two objects are not touching.
Magnets exert a noncontact force on each other.

6. Force is a Vector Vectors have magnitude and direction.
2.1 Combining Forces
Force is a Vector
Vectors have magnitude and direction.
Force has a direction and a magnitude.
Force is measured in Newtons.

7. 2.1 Combining Forces
Combining Forces
When more than one force acts on an object, the forces combine.
The combination of all the forces acting on an object is called the net force.

8. Combining Forces in the Same Direction
Forces are vectors and require a reference direction.
When forces in the same direction combine, the net force is in the same direction and the size of the force is the sum of the forces.

9. Combining Forces in the Same Direction (cont.)
If two people push the dresser to the right, the forces are added together.

10. Combining Forces in Opposite Directions
When forces in opposite directions combine, the net force is in the direction of the larger force.
The size of the net force is the difference between the two forces.

11. Combining Forces in Opposite Directions (cont.)
Two people exerting a force in opposite directions show how vectors combine to create net force.

12. Unbalanced and Balanced Forces
2.1 Combining Forces
Unbalanced and Balanced Forces
If the net force on an object is not zero, it is an unbalanced force.
If the net force on an object does equal zero, it is a balanced force.

13. 2.1 Combining Forces
Balanced Forces

14. Balanced Forces and Motion
2.1 Combining Forces
Balanced Forces and Motion
If two people push with the same force, the door does not move.
The forces have the same magnitude.
The forces are in opposite directions.
The net force on the door is zero and the forces are balanced.
The motion of the door does not change.

15. Unbalanced Forces and Motion
2.1 Combining Forces
Unbalanced Forces and Motion
Unbalanced forces change motion.
If two people push with different force, the door moves.
The forces have a different magnitude and are in opposite directions.
The net force on the door is not zero and the forces are unbalanced.
The door moves in the direction of the larger force.

16. Newton’s First Law of Motion
2.1 Combining Forces
Newton’s First Law of Motion
If the net force on an object is zero, the velocity of the object does not change.
If the net force is zero and the object is at rest, it remains at rest.
If the net force is zero and the object is moving, it continues to move in a straight line with constant speed.
Newton’s Laws of Motion

17. Inertia and Mass Mass is the amount of matter in an object.
2.1 Combining Forces
Inertia and Mass
Mass is the amount of matter in an object.
The size of the net force needed to cause a certain change in motion depends on the object’s mass.
A bicycle is easier to push than a car because it has less mass.

18. 2.1 Combining Forces A B C D
Which of the following changes occur when an unbalanced force acts on an object?
A motion
B mass
C inertia
D weight
Lesson 1 Review

19. 2.1 Combining Forces A B C D
What is the net force acting on a door if one person pushes to the right with a force of 2000 N and a second person pushes with a force of 1500 N to the right?
A 3500 N to the right
B 3500 N to the left
C 500 N to the left
D 500 N to the right
Lesson 1 Review

20. B You turn at constant speed. C You coast to slow down.
2.1 Combining Forces A B C D
You are riding a bike. In which of the following situations are the forces acting on the bike balanced?
A You pedal to speed up.
B You turn at constant speed.
C You coast to slow down.
D You pedal at constant speed.
Lesson 1 Review

21. End of Lesson 1

22. 2.2 Types of Forces gravity law of universal gravitation weight
friction
elastic force
tension force
compression force
normal force

23. Gravity Many types of forces act on objects.
Gravity is an attractive force between all objects that have mass.

24. 2.2 Types of Forces
Gravity (cont.)
The ball does not travel in a straight line because of the unbalanced force of gravity acting on it.

25. The Law of Universal Gravitation
2.2 Types of Forces
The Law of Universal Gravitation
The Law of Universal Gravitation states that all objects with mass are attracted to each other.
The magnitude of attraction depends on the mass of each object and the distance between them.

26. The Law of Universal Gravitation (cont.)
2.2 Types of Forces
The Law of Universal Gravitation (cont.)
The gravitational force becomes stronger as either or both objects increase in mass.
The gravitational force becomes weaker as the distance between the objects increases.

27. 2.2 Types of Forces
Weight and Mass
Mass is the amount of matter in an object and does not change with location.
Weight is the gravitational force on a object and changes with location.
Weight is a force and a vector.
Weight changes with height above Earth.

28. 2.2 Types of Forces
Friction
Friction is a force that opposes the movement between two surfaces in contact.
A book pushed across a table slows down because of friction.
Friction is an unbalanced force acting on the book to slow it down.

29. 2.2 Types of Forces
Static Friction
Static friction is the force between two surfaces in contact that keeps them from sliding when a force is applied.
A force is applied to a heavy box, but the box doesn’t move.
The forces are balanced, the force pushing the box equals the force of static friction pushing in the opposite direction.

30. 2.2 Types of Forces
Sliding Friction
When the force pushing on the box is larger than the maximum static friction force, the box begins to slide.
The frictional force that acts on the sliding box is called sliding friction.

31. Sliding Friction (cont.)
2.2 Types of Forces
Sliding Friction (cont.)
The book pushed across the table slows down because of sliding friction.
Without sliding friction, the book would continue moving without a force being applied.

32. Sliding Friction (cont.)
2.2 Types of Forces
Sliding Friction (cont.)
Usually friction is present and an unbalancing force must be applied to keep an object moving.
When friction is greatly reduced, objects move with nearly constant velocity without an applied force.

33. 2.2 Types of Forces
Elastic Forces
An elastic force occurs when a material is stretched or compressed.
A diving board exerts an upward elastic force on the diver when it is bent downward.

34. 2.2 Types of Forces
Tension
A tension force is a pulling force applied by an object when it is stretched, such as a rubber band.

35. 2.2 Types of Forces
Compression
A compression force is a squeezing force applied to an object that can make an object shrink.
The size of the compression force exerted by a material is equal to the size of the force that compresses the material.

36. 2.2 Types of Forces
Normal Forces
A normal force is the force exerted by an object that is perpendicular to the surface of the object.

37. 2.2 Types of Forces
Normal Forces (cont.)
The normal force exerted by the table is an elastic force.
The weight of the glass pushing down on the table is a compression force.

38. 2.2 Types of Forces
Normal Forces (cont.)

39. 2.2 Types of Forces
Normal Forces (cont.)

40. Forces in the Horizontal Direction
2.2 Types of Forces
Forces in the Horizontal Direction
Friction balances forces applied in a horizontal direction.
Friction equals the horizontal force on an object that is not changing motion.

41. Forces in the Vertical Direction
2.2 Types of Forces
Forces in the Vertical Direction
Upward normal force balances the downward force of gravity on an object that is not moving vertically.

42. Forces in the Vertical Direction (cont.)
2.2 Types of Forces
Forces in the Vertical Direction (cont.)

43. 2.2 Types of Forces A B C D
A(n) ____ force is the force exerted by an object that is perpendicular to the surface of the object.
A compression
B elastic
C normal
D tension
Lesson 2 Review

44. Which force causes a rolling ball to slow down? A sliding friction
2.2 Types of Forces A B C D
Which force causes a rolling ball to slow down?
A sliding friction
B static friction
C normal force
D gravity
Lesson 2 Review

45. The gravitational force between two objects ____ as ____ increases.
2.2 Types of Forces A B C D
The gravitational force between two objects ____ as ____ increases.
A increases; distance
B decreases; mass
C increases; velocity
D decreases; distance
Lesson 2 Review

46. End of Lesson 2

47. 2.3 Unbalanced Forces and Acceleration
centripetal force
Newton’s second law of motion
Newton’s third law of motion

48. Unbalanced Forces and Velocity
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity
Unbalanced forces cause accelerations.
Without the upward normal force exerted by the table, the ball accelerates downward because of the unbalanced force of gravity.

49. Unbalanced Forces and Velocity (cont.)
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity (cont.)
An unbalanced force applied to a moving object in the same direction as the motion speeds the object up.

50. Unbalanced Forces and Velocity (cont.)
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity (cont.)
An unbalanced force applied to a moving object in the opposite direction as the motion slows the object down.
Friction is applied in the opposite direction and slows the sled down.

51. Unbalanced Forces and Velocity (cont.)
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity (cont.)
Unbalanced forces can also change the direction of motion.
A thrown ball curves downward because of the unbalanced force of gravity.

52. Unbalanced Forces and Velocity (cont.)
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity (cont.)
A ball tied to a string and swung is a horizontal circle has circular motion.
The velocity of the ball changes because the direction of motion changes.
The string exerts an unbalanced force on the ball called centripetal force.

53. Unbalanced Forces and Velocity (cont.)
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity (cont.)
Centripetal force is the force perpendicular to the velocity and pulls toward the center of the circle.
The string keeps the ball moving in a circle.
Unbalanced forces cause an object to speed up, slow down, or change direction.

54. Newton’s Second Law of Motion
2.3 Unbalanced Forces and Acceleration
Newton’s Second Law of Motion
The acceleration of an object equals the net force divided by the object’s mass.
Unbalanced forces cause an object to accelerate.
What is Newton’s second law of motion?

55. Balanced Forces, Unbalanced Forces, and Newton’s Second Law
2.3 Unbalanced Forces and Acceleration
Balanced Forces, Unbalanced Forces, and Newton’s Second Law
If the net force is equal to zero, the forces are balanced.
The acceleration is zero.
The object’s motion does not change.

56. Balanced Forces, Unbalanced Forces, and Newton’s Second Law (cont.)
2.3 Unbalanced Forces and Acceleration
Balanced Forces, Unbalanced Forces, and Newton’s Second Law (cont.)
If the net force is not equal to zero, the forces are unbalanced.
The acceleration is not zero.
The object changes motion.

57. Newton’s Second Law and Planetary Motion
2.3 Unbalanced Forces and Acceleration
Newton’s Second Law and Planetary Motion
According the Newton’s second law, there must be an unbalanced force acting on Earth and the other planets.
The Sun’s gravity is the centripetal force that keeps Earth and the planets moving in a circular path around the Sun.

58. Newton’s Third Law of Motion
2.3 Unbalanced Forces and Acceleration
Newton’s Third Law of Motion
When one object exerts a force on another, the second object exerts an equal force in the opposite direction.
The feet exert a force on the ground and the ground exerts an equal force on the feet of the jumper, causing the jumper to accelerate upward.

59. 2.3 Unbalanced Forces and Acceleration

60. 2.3 Unbalanced Forces and Acceleration

61. 2.3 Unbalanced Forces and Acceleration
Force Pairs
The forces that two objects exert on each other are called force pairs.
The pairs are opposite in direction.
The pairs are equal in magnitude.
The forces do not cancel out because they act on different objects.

62. 2.3 Unbalanced Forces and Acceleration
Action and Reaction
One force in a force pair is called the action force.
The second force in a force pair is called the reaction force.
For every action force there is an opposite and equal reaction force.

63. B opposite to the object’s motion C toward the center of the circle
2.3 Unbalanced Forces and Acceleration A B C D
In what direction does the net force of an object moving with constant speed and in a circular motion occur?
A downward
B opposite to the object’s motion
C toward the center of the circle
D in the direction of the object’s velocity
Lesson 3 Review

64. Acceleration depends on both the force applied and ____.
2.3 Unbalanced Forces and Acceleration A B C D
Acceleration depends on both the force applied and ____.
A distance from the object
B speed of the object
C mass of the object
D normal force of the object
Lesson 3 Review

65. A The motion does not change. B The object speeds up.
2.3 Unbalanced Forces and Acceleration A B C D
How does the motion of an object traveling in a straight line at 5 m/s change if a balanced force is applied?
A The motion does not change.
B The object speeds up.
C The object slows down.
D The object travels in a horizontal circle.
Lesson 3 Review

66. End of Lesson 3

67. Chapter Resources Menu
Chapter Assessment
California Standards Practice
Concepts in Motion
Image Bank
Science Online
Interactive Table
Virtual Lab
BrainPOP
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68. A B C D
If a student pushes a book across a table with a force of 10 N and the force of friction is 2 N, what is the net force on the book?
A 0 N
B 2 N
C 8 N
D 12 N
Chapter Assessment 1

69. What would cause an object to have a larger acceleration?
A increasing the net force applied
B decreasing the net force applied
C increasing the mass and keeping the net force constant
D none of the above
Chapter Assessment 2

70. What causes a change in motion on an object? A zero net force D
What causes a change in motion on an object?
A zero net force
B unbalanced force
C weight
D balanced force
Chapter Assessment 3

71. A A balanced force keeps it moving. B The car is moving in a circle.
A car is driving at a constant velocity. Which of the following statements is true?
A A balanced force keeps it moving.
B The car is moving in a circle.
C An unbalanced force keeps it moving.
D Friction keeps the car moving.
Chapter Assessment 4

72. For every action force, there is a reaction force that is ____. B C D
For every action force, there is a reaction force that is ____.
A in the same direction
B equal to the two objects
C equal and in the opposite direction
D unequal and in the direction of the object with less mass
Chapter Assessment 5

73. Which of the following best describes why force is a vector?
SCI 2.a A B C D
Which of the following best describes why force is a vector?
A speed and size
B magnitude
C direction and friction
D magnitude and direction
CA Standards Practice 1

74. SCI 2.b A B C D
Two people are pushing in opposite directions on a box. One person is pushing to the left with a force of 250 N, the other is pushing to the right with a force of 400 N. What is the resulting force vector?
A 150 N to the right
B 650 N to the right
C 150 N to the left
D 400 N to the right
CA Standards Practice 2

75. SCI 2.e, 2.f A B C D
You throw a 0.9 kg ball with a force of 17 N. What is the ball’s acceleration?
A 0.05 N
B 15.3 N
C 13.3 m/s2
D 18.9 m/s2
CA Standards Practice 3

76. SCI 2.f A B C D
A tennis ball hit by a racket has an acceleration of 2000 m/s2. If the net force on the tennis ball is 125 N, what is the tennis ball’s mass?
A 16 kg
B 0.25 kg
C 0.06 kg
D 0.43 kg
CA Standards Practice 4

77. What balanced forces are acting on a book sliding across a table?
SCI 2.d A B C D
What balanced forces are acting on a book sliding across a table?
A gravity
B gravity and friction
C sliding friction, normal force, and gravity
D gravity and normal force
CA Standards Practice 5

78. Concepts in Motion 1

79. Image Bank

80. Interactive Table

81. Interactive Table

82. End of Resources