1. Chapter 12 Preview Section 1 Measuring Motion
Matter in Motion
Preview
Section 1 Measuring Motion
Section 2 What Is a Force?
Section 3 Friction: A Force That Opposes Motion
Concept Map

2. Chapter 12
Section 1 Measuring Motion
Bellringer
Describe the location of a favorite place, using a reference point and reference directions. For example, “The library is at the corner of Madison Street and Monroe Street. It is two blocks west of the school and four blocks north of the bus stop.”
Write your response in your Science Journal.

3. Chapter 12 What You Will Learn
Section 1 Measuring Motion
What You Will Learn
Properties used to describe the motion of an object include a reference point, direction, speed, velocity, and acceleration.
Average speed can be calculated by dividing total distance by total time.
A change in velocity is due to a change in speed, direction, or both.
Speed and acceleration can be represented on graphs.

4. Motion and Reference Points
Chapter 12
Section 1 Measuring Motion
Motion and Reference Points
An object is in motion if it changes position over time in relation to a reference point.
A reference point is any object that appears to stay in place.
Features on Earth’s surface are often used as standard reference points.

5. Motion and Reference Points, continued
Chapter 12
Section 1 Measuring Motion
Motion and Reference Points, continued
Reference directions—such as north, south, east, west, up, or down—can be used to describe the direction of an object’s motion.
A grid can be used to describe motion in a two-dimensional system.

6. Chapter 12
Section 1 Measuring Motion
Average Speed
The speed of an object is the rate at which the object moves.
The speed of an object is rarely constant. So, it is useful to find the average speed of an object.

7. Average Speed, continued
Chapter 12
Section 1 Measuring Motion
Average Speed, continued
Average speed is the total distance traveled divided by the total time taken.
A graph of position versus time can show an object’s speed. On a graph of position versus time, the slope of the line is equal to the speed of the object.

8. Velocity: Direction Matters
Chapter 12
Section 1 Measuring Motion
Velocity: Direction Matters
Velocity is the speed of an object in a particular direction.
Velocity and speed do not have the same meaning.
Velocity must include a direction.

9. Velocity: Direction Matters, continued
Chapter 12
Section 1 Measuring Motion
Velocity: Direction Matters, continued
The velocity of an object is constant only if the speed and direction of the object do not change. So, constant velocity is always motion along a straight line.
The velocity of an object changes if the object’s speed, direction, or both change.

10. Chapter 12
Matter in Motion
Velocity

11. Chapter 12
Section 1 Measuring Motion
Acceleration
Acceleration is the rate at which velocity changes over time.
An object can accelerate by changing speed, direction, or both.
The units for acceleration are the units for velocity divided by a unit for time, usually meters per second per second, or (m/s)/s, which is also written as m/s2.

12. Chapter 12
Matter in Motion
Acceleration

13. Acceleration, continued
Chapter 12
Section 1 Measuring Motion
Acceleration, continued
Centripetal acceleration occurs when an object moves at a constant speed in circular motion.
A graph of speed versus time can show positive acceleration, no acceleration, and negative acceleration.

14. Chapter 12
Matter in Motion

15. Chapter 12
Section 2 What Is a Force?
Bellringer
A force is a push or a pull on an object. Imagine that you are trying to push a box up a ramp. Describe as many forces (pushes and pulls) as you can in that situation. Be sure to include the direction of each force (push or pull) in your description.
Write your answers in your Science Journal.

16. Chapter 12 What You Will Learn
Section 2 What Is a Force?
What You Will Learn
A force is a push or a pull that acts on an object. Forces have magnitude and direction.
Net force is the combined effect of two or more forces acting on an object.
Balanced forces do not cause changes in motion.
Unbalanced forces cause changes in an object’s velocity.

17. Forces Acting on Objects
Chapter 12
Section 2 What Is a Force?
Forces Acting on Objects
A force is a push or a pull exerted on an object in order to change the motion of the object.
All forces have two properties: direction and magnitude, or size.
Forces can cause acceleration.

18. Forces Acting on Objects, continued
Chapter 12
Section 2 What Is a Force?
Forces Acting on Objects, continued
Forces always act on objects. For any push to occur, something has to receive the push.
A newton (N) is the SI unit used to describe the magnitude, or size, of a force.
A force can act on an object without causing the object to move.

19. Chapter 12
Matter in Motion
Force

20. Combined Effect of Forces
Chapter 12
Section 2 What Is a Force?
Combined Effect of Forces
The net force is the combination of all the forces acting on an object.
When forces act in the same direction, you add the forces to determine the net force.
When two forces act in opposite directions, you subtract the smaller force from the larger force to determine the net force.

21. Chapter 12
Matter in Motion

22. Balanced Forces: No Change in Motion
Chapter 12
Section 2 What Is a Force?
Balanced Forces: No Change in Motion
When the forces on an object produce a net force of 0 N, the forces are balanced.
Balanced forces do not cause a change in the motion of an object.

23. Balanced Forces: No Change in Motion, continued
Chapter 12
Section 2 What Is a Force?
Balanced Forces: No Change in Motion, continued
Objects that are not moving and have balanced forces acting on them are called static objects.
Tension and compression are two forces that often act on static objects.

24. Balanced Forces: No Change in Motion, continued
Chapter 12
Section 2 What Is a Force?
Balanced Forces: No Change in Motion, continued
Tension is a force that is exerted when matter is pulled or stretched.
Compression is a force that is exerted when matter is pushed or squeezed.

25. Unbalanced Forces: Velocity Changes
Chapter 12
Section 2 What Is a Force?
Unbalanced Forces: Velocity Changes
When the net force on an object is not 0 N, the forces on the object are unbalanced.
Unbalanced forces cause a change in the velocity of an object.

26. Unbalanced Forces: Velocity Changes, continued
Chapter 12
Section 2 What Is a Force?
Unbalanced Forces: Velocity Changes, continued
Unbalanced forces are needed to cause static objects to start moving.
Objects do not always move in the direction of the unbalanced force.

27. Section 3 Friction: A Force That Opposes Motion
Chapter 12
Bellringer
Imagine that you are trying to push a heavy desk across the floor. There are two opposing forces on the desk: the force you apply, and the force of friction between the desk and the floor. What will happen if the force of friction is the same magnitude as the force that you apply? What will happen if the force that you apply is greater than the force of friction?
Write your answers in your Science Journal.

28. Chapter 12 What You Will Learn
Section 3 Friction: A Force That Opposes Motion
Chapter 12
What You Will Learn
The magnitude of the force of friction can vary.
Kinetic friction is a force that, when unbalanced, can change the velocity of a moving object.
Static friction balances an applied force and can prevent motion.
Friction can be both harmful and helpful.

29. Chapter 12 The Source of Friction
Section 3 Friction: A Force That Opposes Motion
Chapter 12
The Source of Friction
When the hills and valleys of one surface stick to the hills and valleys of another surface, friction is created.
Friction is a force that opposes motion between two surfaces that are touching.

30. The Source of Friction, continued
Section 3 Friction: A Force That Opposes Motion
Chapter 12
The Source of Friction, continued
As the force pushing surfaces together increases, the friction between the surfaces increases.
Friction is usually greater between materials that have rough surfaces than it is between materials that have smooth surfaces.

31. Chapter 12
Matter in Motion

32. Chapter 12 Types of Friction
Section 3 Friction: A Force That Opposes Motion
Chapter 12
Types of Friction
There are two types of friction: kinetic friction and static friction.
Kinetic friction is friction between moving surfaces.
The force of sliding kinetic friction is greater than the force of rolling kinetic friction.

33. Types of Friction, continued
Section 3 Friction: A Force That Opposes Motion
Chapter 12
Types of Friction, continued
Static friction can balance an applied force and can prevent changes in motion.
Some static objects do not move because the force of static friction balances the other forces acting on the object.

34. Chapter 12
Matter in Motion

35. Friction: Harmful and Helpful
Section 3 Friction: A Force That Opposes Motion
Chapter 12
Friction: Harmful and Helpful
Because friction can be both harmful and helpful, sometimes it should be decreased and sometimes it should be increased.
Lubricants are substances that are put on surfaces to decrease friction between the surfaces. Some examples of lubricants include motor oil, wax, and grease.

36. Friction: Harmful and Helpful, continued
Section 3 Friction: A Force That Opposes Motion
Chapter 12
Friction: Harmful and Helpful, continued
Harmful friction can be decreased by using lubricants or by smoothing the surfaces that rub against each other.
Helpful friction can be increased by making the surfaces that rub against each other rougher or by increasing the force pushing surfaces together.

37. Chapter 12
Matter in Motion
Concept Map
Use the terms below to complete the concept map on the next slide.
speed
time
velocity
motion
acceleration

38. Chapter 12
Matter in Motion
Concept Map

39. Chapter 12
Matter in Motion
Concept Map