1. Motion: Speed & Acceleration
Welcome to Physics

2. Main Principal #1: Physics is the study of moving objects!
We want to be able to predict how objects
will move in all situations

3. But here’s the problem:
Q: How do you know if an object
is actually “moving”?
A: When it’s “changing position” (travelling a certain distance)?
If a person is “standing still”, we
assume he/she is “not moving”
But what about the planet that person is
standing on?
It is moving around a star!!!
Is the person therefore “moving” anyway?
Oh boy; welcome to physics!!!
So how do we tell if something really is “moving”?

4. We use a principal called: “Point of Reference”
“PoR” means: what appears to be moving to you?
It can be tricky to really know what is moving and what isn’t!
Point out the difference of distance
The hands on a clock don’t seem
to move; but they are constantly
moving very slowly!
Are the passengers in a boat moving compared to a person standing on the shore?
Or is the person standing on the shore moving compared to the boat in the water?
Fwew! It can be tricky!

5. Main Principal #2 & 3:
#2: Basically, (if you really think about it) all objects are moving some amount, all the time.
#3: We compare the motion of the object that appears to move to a “stationary” object (one that doesn’t appear to move).
An object is moving “slowly” if it moves a short distance in a long time.
An object is moving “quickly” if it moves a long distance in a short time.
There are several ways we use to describe the motion of an object:

6. “Velocity” and “Speed”
“Speed” means distance travelled in a certain amount of time
“Velocity” is the speed in a certain direction
Think: What are two ways to increase speed (go faster)?
Increase distance traveled in the same amount of time
Decrease the time it took to go the original distance
***Since they are almost the exact same thing,
“Speed” and “Velocity” are used for the same motion
Formula for Speed:
Speed =

7. Graphing Velocity What you want to be able to do is
Graphs can help us
Interpret what has
actually happened!
The shape of the line and steepness of the “slope” can tell us what has happened and how fast it occurred!
That’s right! We are NOT done with graphs!!!
We are only just beginning!!!
What you want to be able to do is
know what the graph is telling
you by looking at the line…
This is NOT the graph for your notes...

8. Interpreting Velocity on a Graph
Draw this graph in your notes…
Interpreting Velocity on a Graph
You can use the slope
of a line to find Velocity
15 m
More Distance
In Less Time
(Faster)
Distance
10 m
5 m
Less Distance
In More Time
(Slower)
5 s
10 s
15 s
Time
Think of the Velocity graph this way: Which one of these balls will fall faster?
The graph with more steepness is the faster velocity. The less steep, the slower.

9. Interpreting Speed on a Graph:
Q: What should you learn from this? A: Steep lines on motion graphs, mean an object is moving quickly! Non-steep lines mean the object is moving slowly!
As a group, describe how fast the object is probably moving as you travel along the line (left to right) (After you try it together, click to see the real motion)
Interpreting Speed on a Graph:
Q: HOW CAN YOU TELL?!?!
It can help to think about the speed of a rolling ball on the slope
15 m
Now it’s moving fast!
Distance
10 m
This ball would roll fast!
This ball would roll slowly…
5 m
Started slowly…
Stopped moving!
This ball won’t roll at all!
5 s
10 s
15 s
Draw this graph in your notes…
Time

10. What should we learn? WATCH YOUR VARIABLES CAREFULLY!
The two lines look different, but still describe the same objects motion!
Graphing Speed:
Let’s dim that last line but keep it here to compare to the new line…
This is basically the line of the data you see in the table…
It all can change if you pick
different variables to graph!
But this graph is not the whole story!!!
Time (min)
Dist.
(meters) 1 2 3 4 5
Speed
100 m/min
100
220
110 m/min
Speed (m/min)
Distance (m)
300
100 m/min
430
107 m/min
500
100 m/min
Now we graph the line of this data…
Time (min)

11. Houston; um…Where are we going?
Acceleration!
Houston; um…Where are we going?

12. Think: How would you describe the motion of floating balloons?
The movement of a balloon
is a great example of an object
Accelerating!
Balloons just kinda move
all over the place. Sometimes they go straight
up, sometimes they fly more sideways. CRAZY!
Think: How would you describe the motion of floating balloons?
Do they move in a “constant” speed or direction? (“constant” means: “staying the same”)

13. What is “Acceleration”?
You probably think it simply means to “speed up.” Well yes, and a bit more:
A: The rate at which velocity changes!
In other words: How fast something speeds up, slows down, or even changes direction.
Speeding up:
Slowing down:
Changing direction:

14. Acceleration = A change in Velocity
You already learned the
Velocity equation:
Here is the equation for Acceleration:
Like this: =
Final Velocity – Start Velocity
Final Time – Start Time
____________________
So how do you do that?
Means “change”

15. Interpreting Acceleration on a Graph
Compare it to the line of
an object with a changing
Velocity = Acceleration!
Notice the
downward curve
15 m/s
It is a nice straight line!
Distance
Velocity
10 m/s
This is the line of an object
with an “constant velocity”
Curved lines mean a change
has occurred = Acceleration!
5 m/s
Straight lines on a graph mean “constant”
It is also possible to have an
object with a negative acceleration
We can call this “Deceleration”
5 s
10 s
15 s
Time
Notice also, the different Y-axis variable!

16. Acceleration Graphing Practice
Time
Dist.
Acc. 1 2 3 4 5
Think: Why is the acceleration staying the same if the car is speeding up more and more?
2 m
2 m/s2
8 m
2 m/s2
18 m
2 m/s2
32 m
2 m/s2
50 m
2 m/s2
Now try and graph both the Speed line AND the Acceleration line.
Compare them! Do this on the left side!