Transcript Analysis of Motion

Analysis of Motion
An Introductory Tutorial using VideoPoint in
the WINDOWS platform
By
David G. Iadevaia, Ph.D.
June 2000
TYC Physics Workshops for the 21st Century
A Project of Joliet College, Lee College and the National Science
Foundation
Preface
The purpose of this tutorial is help students start using the Videopoint program to collect and
analyze data.
It was designed for a student who has never used the Videopoint program.
By following the step by step format the student will be able to:
import a video clip and collect a number of data points from the clip.
produce a graph of the data.
fit a curve to the data (model).
describe the motion from an equation generated by the curve fit.
Specific examples were used to analyze an object with a constant velocity and a constant
acceleration.
The tutorial also contains information on how to edit a video clip using QuickTime Pro.
A separate folder is included with several original video clips which can be used for analysis.
A Very Brief Review of
Motion According to
Galileo
Aristotle’s ideas about motion did not
satisfy Galileo’s experimental results.
In order to understand and to predict the
motion of an object, Galileo abandoned
Aristotle’s point of view.
Galileo invented the concept of inertia.
That is, an object will continue to do
whatever it is currently doing.
If the object is moving it will continue to
do so.
If the object is at rest it will remain at rest.
Galileo’s experiments helped him
understand the concept of inertia.
His experiments helped him formulate his
ideas about two important concepts dealing
with moving objects.
The concepts are velocity and acceleration.
In order to understand these concepts we
must understand what it means when an
object moves.
We will say a change of location of an
object defines motion. See the object
below
New Position
You notice that the object’s position
constantly changed as time went by.
This is proof that motion has occurred.
That which the moving object has is called
velocity.
Watch again.
New Position
If you could mark distance intervals and
measure the time it takes for the object to
move through the intervals you would then
measure its velocity.
Equal Time Intervals
If the object moves through a distance in the same time
interval for each distance interval then the object has a
constant velocity.
If we could mark the location of the object in distance
and time from a reference then we could easily measure
how it moved. Click the mouse to mark the location of
the object after each time interval. The object below has
a constant velocity.
Origin or
Reference
Equal Time Intervals
If the object moves through larger distances in the same time for
each distance then the object has an acceleration.
If we could mark the location of the object in distance and time
from a reference then we could easily measure how it moved.
Click the mouse to mark the location of the object after each time
interval.
The object below has an acceleration.
Equal Time Intervals
Origin or
Reference
Click on the image below and watch an
actual moving object.
If there was a method of marking the actual
video you just viewed it would be possible to
take measurements and calculate the velocity
of the rolling ball using a computer.
There is such a method and it utilizes a
powerful video analysis program called
Videopoint.
We will now use the short video clip and
analyze it using Videopoint.
The following sequence will take you
through the steps needed to analyze a
video clip using Videopoint.
1. Starting the Program
2. Importing a Movie Clip
3. Playing a Movie
4. Collecting data points
5. Scaling the Frame
6. Analyzing the Data
1. Starting the program
The Videopoint program is already loaded in the computer.
Click the Videopoint icon .
Here is the first screen you see…click on it to start
Select OPEN MOVIE
The clip can be previewed here
2. Importing a Movie Clip
Play the clip
using this
control bar
Select the clip you would like to to import into VideoPoint from the directory containing the clips
Input the number
of objects you will
study…in this case
1
The movie clip will open and be displayed like this
Main Window
Movie Area
Coordinate
system
window
Toolbar
Table Window
3. Play a Movie Clip
This is the movie control bar..use it to
play the movie or advance it frame by
frame
Click here and the movie plays
The object will begin to move
4. Collecting Data Points
Be sure to have this icon selected before you attempt to take data
The cursor is moved over the object and the mouse button is clicked
This saves a data point with location and time as the coordinates
Continue clicking on the moving object
Continue the point and click routine
Continue the point and click routine
Continue the point and click routine
Trails Box
You can see the data points on the movie area if you select the trails box
5. Scaling the Frame
Click on this
icon
Insert 1 here and Meter here to set the scale of 1 meter
It is useful to scale the frames…that is to select an object of known length
so the the program “knows” how big a distance interval is
There is meter stick in the frame click on the beginning of the meter stick then…….
Click on the other end….the program now knows how long a meter is in the frame
6. Analyzing the Data
Analyzing the Velocity Data
Graphing the Data
Graph icon
We can now graph the data select the graph icon from the toolbar
Leave the Vertical Axis as seen
Leave the Horizontal Axis as time
Position
Select position from the pull down menu
This is a plot of Position (Displacement) vs Time
Analyzing the Data
Select this icon
Select linear since you estimate a linear function constant velocity
Now you can have the program fit a curve to the plot
Equation
Curve
A best fit curve is drawn through the points and an equation is displayed
Notice the time interval for 15 frames per second is 0.067 seconds
Each frame advances by that amount
Here is the data table for the data collected notice that time went ahead but nothing was recorded
it wasn’t until the 43 frame that displacement data was collected
Analyzing Acceleration Data
View the following clip by clicking in the black box above
Import the video clip as before
Collect the data as you did with the velocity video clip
By displaying the trails you can easily see that the distance the object travels increases
for a fixed time interval……this is proof that the object is accelerating
Select the type of curve…in this case
a polynomial
It is easily seen that this is not a straight line but a curved line
The formula
describing the
curve is seen
here
The curve which fits the data describes a parabola
Several More Advanced
Techniques
Changing the Origin
Editing a Video Clip
Changing the Origin
The origin of the movie is a fixed point in
each frame.
That fixed point is the reference for the
coordinate system.
The origin by default is found at the bottom
left of the frame.
It remains there for each frame of the clip.
Here is the (0,0) point, the origin
for each frame in the clip.
Each (X,Y) pair is determined
from this point
For controlled videos the origin will be the
same from frame to frame.
But suppose the origin moves because the
camera was moved as in panning to keep the
object in the field of view.
You will have to move the origin in each
frame to an object which is seen in each
frame.
Selecting a New Origin
Here is the
new origin
Notice that it
is never in the
same place
because
the camera is
panning
Click on the image to view it
You must step through each frame one at a time
in order to mark the same point in each frame as
the origin.
To do that you must open the clip as before and
just like you marked the moving object’s position
you now mark the origin position in each frame.
This ensures that the origin is always located by
the program and the program always has a point
of reference.
Editing Movie Clips
Here is an example of a clip which will
need to be edited
Click on the image above to see the clip
Start the QuickTime Player
Select FILE from the pull down option and select OPEN MOVIE
Using this program you can open an AVI file
The unedited video clip is now opened
By dragging this control locate the frame in the clip to set
as the beginning of the edited clip
This shaded area represents the front piece of the current video clip you will remove
Now drag this piece to the set point of the beginning frame
From the pull down EDIT select the CUT option
The slide control now moves to the beginning position…..
Now move the control to the end or last frame you want in your video clip
Now drag the controls to the back end of the frame marker
…first the forward one than the back one
the shadow are represents what will be cut
Select the EDIT from the pull down and then select CUT to cut the remainder of the clip
Notice the last frame is in the window and the control marker is all the way to the end
of the control bar indicating the edit is now done
To save the video as a .MOV file select EXPORT from the FILE pull down
Then select
save to save the
edited video clip
Give the new file a name
Save the file as a MOV file for direct input
to the Videopoint program
Final edited video clip
Click on the image to view it
Making A Video Clip
Making a video clip is straight forward.
You can use any video camera to do this.
However, the following points should be considered in
producing a clip suitable for analysis using the
VideoPoint program:
Use a tripod
Level the camera
For the most accurate
results the video camera
should be perpendicular to
the moving object.
Click on image to view
The object should move across the video camera’s
field of view (FOV) without the camera having to
pan as the object moves.
Click on image to view
Keep the FOV constant during the taping….
DO NOT zoom in on the moving object.
Click on image to view
A reference object of know length should be included
in the FOV. This object could be a ruler or a know
dimension in the FOV such as the width between
fence posts or the size of the moving object.
Click on image to view
Remember in a laboratory setting you can control
many variables which would affect the accuracy of
the final video clip you will use for analysis.
The technique used in producing your own
controlled video clip in the laboratory will be the
basis on which you plan your recording of an
event in the field.
1. Maintain perpendicular alignment with the moving
object.
2. Select a vantage point to video the event such that
you do not have to pan the camera.
3. Include an object of know length in the FOV with the
moving object.
4. Keep the FOV constant during the taping.
All of the video clips included on this CD-ROM have
been made using the previous 4 criteria.
Examine the included video clips and you will start to
understand how to produce accurate video clips
yourself.
Eventually the technique you develop will allow you to
collect accurate data from your video clips.
Last Slide