1. Chapter 2 Moving Objects
Motion
Speed
Instantaneous Speed
Constant Speed
Average Speed
Velocity
Acceleration
Force
Net Force
Balanced Forces
Inertia
Friction
Gravity
Weight
Mass
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General Science Chapter 2

2. General Science Chapter 2
Section Motion
Motion
Speed
Instantaneous Speed
Constant Speed
Average Speed
Velocity
Acceleration
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General Science Chapter 2

3. General Science Chapter 2
Motion
You don’t have to see something move to know that motion has taken place.
You only have to know that its position has changed. Best described as a change in position.
You need a reference point to know if position has changed.
Depends on the observer’s frame of reference.
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General Science Chapter 2

4. General Science Chapter 2
Speed
How fast something moves
The rate of change in position
The distance traveled in a given amount of time
The rate of motion
Rate means change over time
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5. General Science Chapter 2
Instantaneous speed
The rate of motion at a given instant
Speedometer
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6. General Science Chapter 2
Constant speed
Speed that does not vary
Cruise control
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General Science Chapter 2

7. General Science Chapter 2
Average speed
Total distance traveled divided by total time of travel
Used when the object in motions makes changes in speed, including starts and stops.
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General Science Chapter 2

8. General Science Chapter 2
Calculating speed
If you know distance and time, you can calculate speed.
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9. General Science Chapter 2
Example
You skate a distance of 100 m in s. What is your average speed?
Known:
d = 100 m
t = 67 s
Unknown
v = ?
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10. General Science Chapter 2
Example continued
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11. General Science Chapter 2
Graphing speed
When distance is graphed on the y-axis and time is graphed on the x-axis, we call this a distance-time graph.
The slope of a distance-time graph is the speed.
What kind of graph would you use to graph distance vs time?
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General Science Chapter 2

12. Section 1: Discussion Questions
What is motion?
What two quantities do you need to know to calculate average speed?
What does a flat line on a segment of a distance-time graph tell you about the average speed for that time period?
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General Science Chapter 2

13. General Science Chapter 2
Velocity vs. speed
Velocity describes both speed and direction.
Speed describes only rate of change in position.
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14. General Science Chapter 2
REVIEW #1
What is the difference between instantaneous and average speed?
What is the difference between speed and velocity?
What is the equation used to solve for average speed?
What SI units are used for speed?
What is motion?
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General Science Chapter 2

15. General Science Chapter 2
Acceleration
The rate of change of velocity.
Acceleration occurs if either the speed or direction changes, or if both change.
“Slowing down” is a change in speed, so when something slows down, it is accelerating.
When speed decreases, acceleration is opposite in direction to velocity.
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General Science Chapter 2

16. General Science Chapter 2
Acceleration
The size of acceleration depends on both the change in velocity and the time interval over which the change takes place
If the change in velocity is large, the acceleration will be large.
If the time interval is short, then acceleration will be large.
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General Science Chapter 2

17. Calculating acceleration
The triangle is the Greek letter delta, and stands for “change in”
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18. General Science Chapter 2
Units on acceleration
Meters per second per second or
Meters per second squared
Written as m/s2
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19. General Science Chapter 2
Example
A car increases its velocity from 10 m/s to 32 m/s in 3 seconds. What is the acceleration of the car?
Known:
vi = 10 m/s
vf = 32 m/s
t = 3 s
Unknown:
a=?
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20. General Science Chapter 2
Example continued
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21. Discussion Questions, Part 2
What is acceleration?
What are three ways an object can accelerate?
What equation is used to determine acceleration?
What is the SI unit of acceleration?
Is the SI unit of acceleration a derived unit?
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General Science Chapter 2

22. General Science Chapter 2
Section Forces
Force
Balanced Forces
Net Force
Inertia
Friction
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23. General Science Chapter 2
Force
A push or a pull one object exerts on another
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24. General Science Chapter 2
Balanced forces
Forces that are equal in size and opposite in direction are called balanced forces.
Tug-of-war
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25. General Science Chapter 2
Net Force
When forces are unbalanced, there is a net force.
A net force always causes a change in velocity.
Could be a change in speed, a change in direction, or both.
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26. General Science Chapter 2
Inertia
The tendency of an object to resist any change in its motion.
If it is moving, it tends to keep moving at the same velocity unless a force acts on it.
If it is not moving, it tends to remain at rest unless a force acts on it.
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27. General Science Chapter 2
Mass and inertia
The more mass an object has, the greater its inertia is.
Objects with greater inertia take more force to change their velocity.
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28. General Science Chapter 2
Newton’s first law
An object moving at a constant velocity keeps moving at that velocity unless a net force acts on it.
An object at rest stays at rest unless a net force acts on it.
An object at rest/in motion will stay at rest/in motion unless a net force acts on it.
Also called the law of inertia.
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29. General Science Chapter 2
Seatbelts
When a car crashes, a force stops the car and any passengers in the car.
A passenger who isn’t wearing her seatbelt isn’t acted on by the force of the seatbelt, so she keeps moving at the same velocity.
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30. General Science Chapter 2
Friction
The force that opposes motion between two surfaces that are touching each other.
Enables us to walk, hold things, etc.
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31. General Science Chapter 2
Friction
The amount of friction depends on two things
The kinds of surfaces
The force pressing the surfaces together
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32. General Science Chapter 2
Discuss
What is a force?
What are balanced forces?
What is inertia?
What is friction?
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33. General Science Chapter 2
Section Gravity
Gravity
Weight
Mass
Scales
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34. General Science Chapter 2
Gravity
Every object in the universe exerts a force on every other object
That force is gravity
Often, the force is too small to notice
Example – the force of gravity between two people
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35. General Science Chapter 2
Gravitational force
The amount of gravitational force between two objects depends on two things
Their masses and the distance between them.
The mass of a person is small, so the gravitational force between two people is weak.
The mass of the Earth is large, so the gravitational force between the Earth and a person is strong.
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36. General Science Chapter 2
Gravitational force
Gets weaker as the distance increases
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37. General Science Chapter 2
Weight
The measure of the force of gravity on an object.
We usually use the term weight when we talk about the gravitational force between the Earth and an object near its surface.
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38. General Science Chapter 2
Weight
Related to mass
Objects with greater mass have greater weight.
Weight is determined by the gravitational pull on the object.
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39. General Science Chapter 2
Weight and mass
Weight is measured in Newtons (N).
1 kg of mass at the Earth’s surface has a weight of 9.8 N
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40. Weight on Earth vs. Weight on Moon
Since the Earth has more mass than the moon, it exerts a greater gravitational force than the moon.
So, things weigh less on the moon than they do on Earth.
Do they have less mass on the moon?
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41. General Science Chapter 2
Scales
Use the principal of balanced forces to measure how much something weighs.
Scales use springs to balance the force of an object’s weight.
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42. General Science Chapter 2
Discuss
What is the force of gravity?
What is weight?
Weight is measured in what SI units?
What instrument is used to measure weight?
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43. General Science Chapter 2
Chapter Review
Do you know the difference between:
Speed and velocity
Average speed and instantaneous speed
Motion and speed
Balanced forces and net forces
Mass and Weight
A balance and a scale
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General Science Chapter 2