1. Chapter 2
Motion and Speed

2. Sec. 1 Describing Motion
Motion occurs when an object changes position.
You don’t need to see it move to know that motion has happened.
You just need a reference point

3. Distance and Displacement
Distance—how far an object moved
The SI unit for distance is the meter,m
Displacement—the distance and direction of an object’s change in position from the starting point.

4. Distance Vs Displacement Animation

5. Distance can be longer than Displacement
Displacement cannot be longer than Distance
Distance and Displacement can be the same.

7. Speed Rate—any change over time
Speed—the distance an object travels per unit of time.
Speed = distance s = d
time t
Ex. You ran 2 km in 10 min, what is your speed?
s = 2 km / 10 min
s = 0.2 km/min

8. Average & Instantaneous Speed
Average speed—total distance traveled divided by total travel time.
Used when speed is changing
Instantaneous speed—the speed at a given point in time.
Ex. Speedometer

9. Velocity Speed describes only how fast something is moving.
Velocity—includes the speed of an object AND the direction of its motion
Ex: 40 mi/hr North or m/s up

10. Change in Velocity Velocity can change in 2 ways Change in speed
Change in direction
Or Both
Ex: a race car has a constant speed of 100km/hr around an oval track.
Speed is constant, but velocity changes

12. Graphing Motion
On a distance vs. time graph, time goes on the x-axis and distance goes on the y-axis.
The slope (steepness) of the line represents the speed of the object.
 Distance
Speed
Time↓

13. Positive Speed
Positive Speed: Moving away from the origin or reference point
Graph has a positive (upward) slope

14. Constant, Positive Speed
Straight Line
Speed isn’t
changing

15. Fast vs. Slow Positive Speed
Slow: has a shallow, gradual slope
Fast: has a steep slope

16. Changing, Positive Speed
Graph has a curve
The slope is changing, so speed is changing
Speeding Up
Slowing Down

17. Constant, Negative speed
Negative Speed: Moving toward the origin or reference point
Graph has a negative (downward) slope
Constant, Negative speed

18. Fast vs. Slow Negative Speed
Fast: steep, downward slope
Slow: shallow, gradual downward slope

19. Changing, Negative Speed

20. Not Moving (0 speed)
A horizontal line (zero slope) indicates no motion (or no change from the origin)

21. Which one is the fastest? Which isn’t moving?

24. Rearranging the speed equation
The original speed equation is s=d/t
This can be rearranged to solve for d or t.
s = d t
d = s x t (how far?)
t = d (how long?) s

26. #1
How long will it take a bike rider to travel 450 meters at a constant speed of 2 m/s?

27. #2
If a car traveled 2500 m in 20 minutes, what is the average speed of the car?

28. #3
If a bug travels for 50 seconds at 4 meters per second, how far does it travel?

30. Acceleration Acceleration: Change in velocity over time
When something speeds up, it has a positive acceleration
When something slows down, it has a negative acceleration
Units for acceleration have one distance unit and 2 time units
Example Units: m/s/s or m/s2, mi/hr/min

31. Changing Direction
A change in velocity can be either a change in speed or a change in direction.
Any time an object changes direction, its velocity changes and it is accelerating.

32. Calculating Acceleration
Acceleration Equation:
acceleration = change in velocity
time
Change in velocity =
final velocity (vf) – initial velocity(vi)
We can write the acceleration
equation as a = (vf – vi) t
(vf -vi)
a t

34. Graphing Acceleration
On a Velocity vs Time Graph, the slope of the line is the acceleration
Positive acceleration has an upward slope
Positive Acceleration (speeding up)
+ acceleration

35. Negative Acceleration (Slowing Down)
Negative acceleration has a downward slope
- acceleration

36. Zero Acceleration
When an object has a constant velocity, it has 0 acceleration, so the graph is a horizontal line.
velocity
0 acceleration
time

37. An Object Changing Acceleration

38. Acceleration Example #1
An airplane starts at rest and speeds up to 80m/s at the end of the runway in 20 s. What is its acceleration?
a = (vf – vi) = (80 m/s – 0 m/s)
t 20 s
= 4 m/s2
The airplane is speeding up, so the acceleration is positive.

39. Acceleration Example #2
A skateboarder is moving at 3 m/s and comes to a stop in 2 s. What is the acceleration?
a = (vf – vi) = (0 m/s – 3 m/s) = -1.5 m/s2
t s
The skateboarder slowed down, so the acceleration is negative.
Note: words like at rest and stop mean velocity = 0.