1. DO NOW
Come in get a calculator (you cannot use your phone calculator for the quiz).
You will have 10 min to review before the quiz.

2. Average Acceleration
Acceleration is the RATE at which VELOCITY changes.
A truck accelerates from 10 m/s to 30 m/s in 2.0 seconds. What is the acceleration?
Suppose the same truck then slows down to 5 m/s in 4 seconds. What is the acceleration?
“m/s/s” or “m/s2” is the derived unit for acceleration.

3. What do the “signs”( + or -) mean?
Quantity
Positive
Negative
Displacement
You are traveling north, east, right, or in the +x or +y direction.
You are traveling south, west, left, or in the –x or –y direction.
Velocity
The rate you are traveling north, east, right, or in the +x or +y direction.
The rate you are traveling south, west, left, or in the –x or –y direction.
Acceleration
Your velocity(speed) is increasing in a positive direction or your speed is decreasing in a negative direction.
Your velocity(speed) is decreasing in a positive direction or your speed is increasing in a negative direction.

4. Beware – signs can confuse!
Suppose a ball is thrown straight upwards at 40 m/s. It takes 4 seconds to reach its maximum height, then another 4 seconds back down to the point where it was thrown. Assume it is caught with the same speed it was thrown. Calculate the acceleration upwards and downwards.
This negative sign came from using the DELTA
This negative sign came from the DIRECTION of the velocity.
It is no surprise you get a negative answer both ways as gravity acts DOWNWARDS no matter if the ball goes up or down. It is GRAVITY which changes the ball’s velocity.

5. Motion Graphs Lecture 3

6. Motion & Graphs
Motion graphs are an important tool used to show the relationships between position, speed, and time.
It’s an easy way to see how speed or position changes over time
These types of graphs are called kinematic graphs.
There are two types:
Position vs. Time graphs
Speed vs. Time Graphs

7. Position Vs. Time Used to show an object’s position at a given time.
Position: on y-axis
Time: on x-axis

8. You Try It: Graphing Position Vs. Time
Suppose you are helping a friend who is training for a track meet.
She wants to know if she is running at constant speed.
You mark the track in 50-meter increments and measure her time at each position during a practice run.
Create a position-time graph using her data.
Time (s)
Position (m) 10 50 20
100 30
150

9. You Try It: Graphing Position Vs. Time
When you’ve plotted all 4 points, you should get a graph that looks like this…
What would her speed be?
Choose any point, & divide distance (position) over time

10. You Try It: Graphing Position Vs. Time
What would her speed be?
50m/10s = 5 m/s
100m/20s = 5 m/s
Notice that this is a straight line - why??
She is moving at a constant speed - neither slowing down nor accelerating

11. You Try It: Graphing Position Vs. Time #2
Graph the motion of this car.
Your graph should look like this…
What is this car’s velocity?
A constant 10 m/s

12. What does slope have to do with it?
Slope is the ratio of the rise (y-axis) to the run (x-axis) of a line on a graph.
A bigger slope means a steeper line which means a faster speed.

13. Steeper Line = Faster Speed

14. Steeper Line = Faster Speed

15. Negative Slopes What does this graph mean??? And this one?
They show an object that is slowing down - or decelerating.
The first graph is slowly decelerating, while the second graph is quickly decelerating.

16. Basically… You might want to draw this graph in your Motion Math
Little book

17. This is another really good graph to draw in your motion math little book

18. Position Vs. Time - Changing Velocity
Now consider a car that has a changing velocity.
It is not moving at a constant rate, but getting faster by the second.
What would this graph look like?
You try it first…

19. Does your graph look like this?
Be sure you have this one drawn

20. You Try It: GrAPHING Position Vs. Time #3
What would the graph look like for a car that traveled 10 m in the 1st second, 15 m by the 2nd second, 25 by the 3rd second, and 40 m by the 4th second?

21. Predict: What does THIS GRAPH show?

22. This graph is for a car moving with a constant velocity of +5 m/s for 5 seconds, stopping abruptly, and then remaining at rest for 5 seconds.
The straight line means its position is NOT changing.

23. Speed Vs. Time Used to show an object’s speed at a given time.
Speed: on y-axis
Time: on x-axis

24. Speed Vs. Time - Constant Speed
This graph shows the speed versus time for a ball rolling at constant speed on a level floor.
On a speed vs. time graph, constant speed is shown with a straight horizontal line.
If you look at the speed on the y-axis, you see that the ball is moving at 1 m/s for the entire 10 seconds.

25. Speed Vs. Time - Constant Speed
Compare this speed-time graph to the position-time graph for the ball.
Both of the graphs show the exact same motion, even though they look different.
If you calculate the slope of the lower graph, you will find that it is still 1 m/s.

26. You Try It: Graphing Speed Position Vs. Time
Maria walks at a constant speed of 6 m/s for 5 seconds.
Then, she runs at a constant speed of 10 m/s for 5 seconds.
Create a speed-time graph using her data.

27. Speed Vs. Time - Changing Speed
As we know, most objects don’t move at a constant speed.
If a speed vs. time graph slopes up, then the speed is increasing.
If it slopes down, then the speed is decreasing.
If the graph is horizontal, then the object is moving at a constant speed. YOU MAY WANT TO DRAW THESE GRAPHS TOO!

28. Putting it All Together
Which runner won the race?
Albert won the race. He reached 100 meters first.
Which runner stopped for a rest?
Charlie stopped for a rest at 50m.
How long did he stop for?
Charlie stopped for 5 seconds. (13-8)

29. Putting it All Together
4. How long did Bob take to complete the race?
Bob finished the race in 14 seconds
5. Calculate Albert's average speed.
Speed = distance/time
Speed = 100m/12s =
Albert’s Speed = 8.3 m/s