1. Section 1: Measuring Motion
Chapter 11: Motion
Section 1: Measuring Motion

2. Observing Motion How do you determine if something is moving?
Is a person sitting in a driving car moving?
What about a character in a video game that is apparently running across the TV?
If the earth is orbiting the sun, are you moving now?

3. Observing Motion
The motion of an object must be observed in relation to objects that stay in place, or reference points
Frame of Reference: used to describe the motion of an object relative to reference points
When an object changes position with respect to a frame of reference, the object is in motion
Mythbusters

4. Observing Motion Consider the person sitting in the car…
Describe their motion from each frame of reference:
Inside the car
Moving very fast
A car driving past
The person is slowly moving backwards

5. Observing Motion Distance: measures the path taken
Displacement: the change of an object’s position
always includes direction
Bill Nye Frame Of Reference and Law of Inertia

6. Speed and Velocity
Speed: the distance traveled divided by the time interval in which the motion occurred
Velocity: the speed of an object in a particular direction
Speed tells us how fast an object moves, and velocity tells us how fast the object moved (speed) AND the direction the object moved

7. Velocity Velocity is described relative to a reference point
North, west, east, south, etc.
It can also be described as motion from a fixed line
Direction is described as positive or negative along the line of motion
Up and right are usually positive velocity
Down and left are usually negative velocity

8. Velocity Could something have a negative velocity?
Yes, that means it is going backwards
Is it possible for something to have a negative speed?
No, there is no specific direction
Is it possible for something to have a constant speed but a changing velocity?
Yes, all the object has to do is turn
Remember, velocity includes direction, speed does not

9. Calculating Speed
Average speed is calculated as distance divided by time
Constant Speed: equal distances in equal amounts of time
Instantaneous Speed: the speed at any given time

10. Units The SI unit for speed is meters per second (m/s)
Remember! We use the metric system in science
Instead of using miles per hour (mph), we use meters per second (m/s) or kilometers per hour (km/h)
The smaller the unit, the more precise the measurement
You will need to make conversions in your math problems!

11. Math skills: Velocity List the given and the unknown values.
Metal stakes are sometimes placed in glaciers to help measure a glacier’s movement. For several days in 1936, Alaska’s Black Rapids glacier surged as swiftly as 89 meters per day down the valley. Find the glacier’s velocity in m/s. Remember to include direction.
List the given and the unknown values.
Given: time - t = 1 day
distance - d = 89 m down the valley
Unknown: velocity - v = ? (m/s and direction)

12. Math skills: Velocity
2a. Perform any necessary conversions. To find the velocity in meters per second, the value for time must be in seconds. t = 1 day = 24 h x 60 min x 60 s 1 hr 1 min t = 1 day = 24 x 60 x 60 t = 86,400 s = 8.64 x 104 s

13. Math skills: velocity 3. Insert the known values and solve.
2b. Write the equation for speed
3. Insert the known values and solve.

14. Section 2: acceleration
Chapter 11: Motion

15. Acceleration
Acceleration: the rate at which velocity changes over time
Acceleration can be a change in speed
An increase or decrease in speed is an acceleration
Acceleration can be a change in direction
A motorcyclist who rides around the inside of a barrel is constantly accelerating
Acceleration can be BOTH a change in speed AND direction
NFL Science of Football: kinematics

16. Acceleration
For straight line motion, a positive acceleration means the object’s velocity is increasing
The object is speeding up
A negative acceleration means that the object’s velocity is decreasing
The object is slowing down
Centripetal Acceleration: acceleration that occurs in circular motion

17. Acceleration A constant (unchanging) velocity means no acceleration
A constant acceleration means a constantly changing velocity

18. Acceleration
To calculate acceleration subtract the initial velocity (vi) from the final velocity (vf) and divide by time
a = vf – vi or a = v
t t
The SI unit for acceleration is m/s2 (m/s/s)

19. Math skills: acceleration
A flowerpot falls off a second-story windowsill. The flowerpot starts from rest and hits the sidewalk 1.5 s later with a velocity of 14.7 m/s. Find the average acceleration of the flowerpot.
1. List the given and the unknown values.
Given: time - t = 1.5 s
initial velocity - vi = 0 m/s
final velocity - vf = 14.7 m/s down
Unknown: acceleration - a = ? (m/s2 and direction)

20. Math skills: Acceleration
2. Write the equation for acceleration. 3. Insert the known values into the equation, and solve.

21. Graphing motion
Speed is shown with a distance vs. time graph of its motion
x-axis: time (ind. variable)
y-axis: distance (dep. variable)
The slope of the line represents the speed
The speed can be found by calculating the slope
A straight line means a constant speed
How to read and interpret graphs

23. Graphing Motion Acceleration is shown with a speed vs. time graph
x-axis: time (ind. variable)
y-axis: speed (dep. variable)
The slope of the line represents the acceleration
A positive slope means an object is speeding up
A negative slope means an object is slowing down

25. What’s Going on? at rest (0 velocity) Acceleration Constant Velocity
Negative velocity
(backwards)
Constant Velocity