2. Chapter: Forces and Changes in Motion
Table of Contents
Chapter: Forces and Changes
in Motion
Section 1: Motion
Section 2: Forces and Motion
Section 3: The Laws of Motion

3. Motion 1
What is motion?
One way you can describe the motion of an object is by how it changes position.
There are two ways to describe how something changes position. One way is to describe the entire path the object travels. The other way is to give only the starting and stopping points.

4. Motion 1
What is motion?
Displacement is the distance and direction between starting and ending positions.

5. Relative Motion 1 Something that is in motion changes its position.
The position of an object is described relative to another object, which is assumed to be not moving. This object is called the reference point.

6. Motion 1
Speed
When you are moving, your position is changing. How quickly your position changes depends on your speed.
Speed is the distance traveled divided by the time needed to travel that distance.
Speed increases if it takes less time to travel the same distance.

7. Motion 1
Constant Speed
If you are riding in a car with the cruise control on, the car’s speed doesn’t change. This means that the car’s speed is constant.
For an object traveling at constant speed, the object’s speed at any instant of time doesn’t change.

8. Motion 1
Average Speed
The speed of the car at one instant of time is the car’s instantaneous speed.
Average speed is represented by the symbol and can be calculated from this equation:

9. Velocity 1 Velocity is the displacement divided by time.
Motion 1
Velocity
Velocity is the displacement divided by time.
If you were to travel 1 km east in 0.5 h, you would calculate your velocity as

10. Motion 1
Acceleration
Acceleration is the change in velocity divided by the amount of time required for the change to occur.
If an object changes its speed, its direction, or both, it is accelerating.

11. Speeding Up and Slowing Down
Motion 1
Speeding Up and Slowing Down
An object is accelerating when its velocity changes.
Velocity can change if the speed of an object changes, whether the speed increases or decreases, or if it changes direction.
If an object slows down, its speed changes. Therefore, if an object is speeding up or slowing down, it is accelerating.

12. Motion 1
Turning
When an object turns or changes direction, its velocity changes. This means that any object that changes direction is accelerating.

13. Section Check 1
Question 1
Which is the distance and direction between starting and ending positions?
A. acceleration
B. displacement
C. speed
D. velocity
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14. Section Check 1
Answer
The answer is B. Displacement can be zero if you travel in a path that leads you back to where you started.
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15. Question 2 Answer 1 Explain what is meant by relative motion.
Section Check 1
Question 2
Explain what is meant by relative motion.
Answer
The position of an object is described relative to another object. Taking some other object as a reference point, we can say whether or not a given object is in motion.
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16. Section Check 1
Question 3
What is calculated by figuring the displacement over time?
A. acceleration
B. average speed
C. speed
D. velocity
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17. Answer 1 The correct answer is D. The formula for velocity is v = d/t.
Section Check 1
Answer
The correct answer is D. The formula for velocity is v = d/t.
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18. Forces and Motion 2
Force
A force is a push or a pull that one object exerts on another object.
Objects like floors, chairs, and Earth also exert forces on other objects, including people.

19. Force Has Direction and Size
Forces and Motion 2
Force Has Direction and Size
Just like velocity and acceleration, a force has both size and direction.
The direction of a force is the direction of the push or pull.
Pushing or pulling harder increases the size of the force you exert. The size of a force is measured in newtons (N).

20. Forces and Motion 2
How Forces Combine
When more than one force acts on an object, the forces combine.
The combination of all the forces acting on an object is the net force.

21. Combining Forces in the Same Direction
Forces and Motion 2
Combining Forces in the Same Direction
When two forces act in the same direction on an object, like a box, the net force is equal to the sum of the two forces.

22. Combining Forces in Opposite Directions
Forces and Motion 2
Combining Forces in Opposite Directions
If two forces of equal strength act on the box in opposite directions, the forces will cancel, resulting in a net force of zero.

23. Combining Forces in Opposite Directions
Forces and Motion 2
Combining Forces in Opposite Directions
When two unequal forces act in opposite directions on the box, the net force is the difference of the two forces.

24. Balanced and Unbalanced Forces
Forces and Motion 2
Balanced and Unbalanced Forces
When the net force on an object is zero, the motion of the object doesn’t change.
The forces acting on an object are balanced forces if the net force is zero.
The forces acting on an object are unbalanced forces if the net force is not zero.

25. Contact and Non-contact Forces
Forces and Motion 2
Contact and Non-contact Forces
A force is exerted when one object pushes or pulls on another.
A force that is exerted only when two objects are touching is a contact force.
Non-contact forces are forces that can be exerted by one object on another even when the objects aren’t touching.

26. Forces and Motion 2
Gravity
Gravity is a non-contact force that every object exerts on every other object due to their masses.
The gravitational force between two objects is an attractive force that tends to pull the two objects closer together.

27. Forces and Motion 2
Gravity
The size of the gravitational force between two objects depends on the masses of the objects and the distance between them.
The gravitational force between two objects increases if the mass of one or both of the objects increases.
If two objects move closer together, the gravitational force between them increases.

28. Forces and Motion 2
Mass and Weight
Because weight is a force, it is measured in newtons.
Weight is not the same as mass. Mass is the amount of matter an object contains, and is measured in kilograms.

29. Forces and Motion 2
Friction
Friction is a contact force that resists the sliding motion of two surfaces that are touching.
Friction causes a sliding object to slow down and stop.
Friction also can prevent surfaces from sliding past each other.

30. Forces and Motion 2
Static Friction
Static friction is the force between two surfaces in contact that keeps them from sliding when a force is applied.

31. Forces and Motion 2
Sliding Friction
Sliding friction is the force that opposes the motion of two
sliding surfaces in contact.
Sliding friction exists between all sliding surfaces that are touching.

32. Forces and Motion 2
What causes friction?
When two surfaces are in contact, the surfaces stick to each other where the dips and bumps on one surface touch the dips and bumps on the other surface.
Friction is caused by the sticking of the two surfaces at these bumps and dips.

33. Forces and Motion 2
The Buoyant Force
The buoyant force is a force exerted by a fluid on an object that is in the fluid.
The buoyant force is always upward.
If you are floating in water, the buoyant force is large enough to balance your weight.

34. Forces and Motion 2
Air Resistance
Air resistance is a contact force that opposes the motion of objects moving in air.
Just like friction, air resistance acts in the direction opposite to an object’s motion.
Air resistance is less for a narrow, pointed object than for a wide, flat object.

35. Question 1 Answer 2 A force is a _______ or a _______.
Section Check 2
Question 1
A force is a _______ or a _______.
Answer
Force is a push or a pull.

36. Section Check 2
Question 2
A force exerted between two objects that are not touching is known as _______.
Answer
The answer is a long-range force. Gravity is a long-range force; although we are not touching the Sun, our planet is being acted on by the Sun to keep it in orbit.
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37. Section Check 2
Question 3
You try to push a desk across the floor one direction but a friend is trying to push it the opposite direction. As a result, the desk doesn’t move. This is an example of what kind of forces?
A. balanced forces
B. continuing forces
C. extended forces
D. unbalanced forces
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38. Section Check 2
Answer
The answer is A. When forces are balanced, the net force is zero.
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39. The Laws of Motion 3
The First Law of Motion
According to the first law of motion, if the forces acting on an object are balanced, then an object at rest remains at rest and an object in motion keeps moving in a straight line with constant speed.
When the forces on an object are balanced, the motion of the object doesn’t change.

40. The Laws of Motion 3
The First Law of Motion
The forces on an object are unbalanced when the net force on the object is not zero.
According to the first law of motion, for an object to change speed or direction, the net force acting on the object must not be zero.

41. The Laws of Motion 3
Changing Direction
A moving object changes direction when the net force acting on the object is not in the same direction as the
object’s motion.
Then the direction of motion curves toward the direction of the unbalanced net force.

42. The Second Law of Motion
The Laws of Motion 3
The Second Law of Motion
Newton’s second law of motion describes how the net force on an object, the mass of the object, and the acceleration of the object are related.
According to the second law of motion, the acceleration of an object depends on the net force acting on the object and the object’s mass.

43. The Laws of Motion 3
Acceleration and Mass
Acceleration is larger when the speed of an object changes more quickly.
If the net force is the same, the acceleration of an object decreases as the mass of an object increases.

44. Acceleration and Direction of the Net Force
The Laws of Motion 3
Acceleration and Direction of the Net Force
According to the second law of motion, when a force acts on an object, its acceleration is in the same direction as the force.
When an object slows down, the direction of its acceleration is opposite to its direction of motion.

45. Acceleration and Net Force
The Laws of Motion 3
Acceleration and Net Force
When you lift your backpack, its speed depends not only on its mass, but also on the force you exert.
According to the second law of motion, the acceleration of an object increases when the net force on the object increases.

46. The Laws of Motion 3
The Third Law of Motion
object exerts an equal force in the opposite direction on the first object.
The third law of motion, states that when one object exerts a force on a second object, the second

47. The Laws of Motion 3
Force Pairs
The forces two objects exert on each other are called force pairs.
The forces in a force pair act in opposite directions and are always equal in size.

48. The Laws of Motion 3
Action and Reaction
One force of the force pair is called the action force, and the other force is the reaction force.
For every action force, there is a reaction force that is equal in size, but in the opposite direction.

49. The Laws of Motion 3
Action and Reaction
The swimmer’s hands exert a backward action force on the water.
The water then exerts a forward reaction force on the
swimmer. This is the force that propels the swimmer forward.

50. Section Check 3
Question 1
Newton’s second law of motion states that _________ of an object is in the same direction as the net force on the object.
A. acceleration
B. momentum
C. speed
D. velocity
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51. Section Check 3
Answer
The answer is A. Acceleration can be calculated by dividing the net force in newtons by the mass in kilograms.
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52. Section Check 3
Question 2
The forces two objects exert on each other are called _______.
A. action pairs
B. force pairs
C. gravity
D. reaction pairs
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53. Section Check 3
Answer
The answer is B. The forces in a force pair act in opposite directions and are always equal in size.
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54. Section Check 3
Question 3
When an object slows down, the direction of its acceleration is _______ its direction of motion.
A. the same as
B. opposite
C. behind
D. in front of
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55. Section Check 3
Answer
The answer is B. When you stop a ball from rolling, the force of your foot is opposite the direction of motion of the ball.
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