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. Origin or Reference Equal Time Intervals

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

Select the clip you would like to to import into VideoPoint from the directory containing the clips The clip can be previewed here Play the clip using this control bar 2. Importing a Movie Clip

The movie clip will open and be displayed like this Input the number of objects you will study…in this case 1

Main Window Movie Area Table Window Coordinate system window Toolbar

3. Play a Movie Clip This is the movie control bar..use it to play the movie or advance it frame by frame

The object will begin to move Click here and the movie plays

4. Collecting Data Points

The cursor is moved over the object and the mouse button is clicked Be sure to have this icon selected before you attempt to take data This saves a data point with location and time as the coordinates

Continue clicking on the moving object

Continue the point and click routine

You can see the data points on the movie area if you select the trails box Trails Box

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 Click on this icon Insert 1 here and Meter here to set the scale of 1 meter 5. Scaling the Frame

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

We can now graph the data select the graph icon from the toolbar Graph icon Graphing the Data

Select position from the pull down menu Position Leave the Horizontal Axis as time Leave the Vertical Axis as seen

This is a plot of Position (Displacement) vs Time

Now you can have the program fit a curve to the plot Select this icon Select linear since you estimate a linear function constant velocity Analyzing the Data

A best fit curve is drawn through the points and an equation is displayed Equation Curve

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 Notice the time interval for 15 frames per second is seconds Each frame advances by that amount

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 curve which fits the data describes a parabola The formula describing the curve is seen here

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 Click on the image to view it Here is the new origin Notice that it is never in the same place because the camera is panning

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

Now drag this piece to the set point of the beginning frame This shaded area represents the front piece of the current video clip you will remove

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 the shadow are represents what will be cut …first the forward one than the back one

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

Give the new file a name Then select save to save the edited video clip 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