HPP A7
You're really hip
Exploration
GE 1.
1. Tape a DOT on someone's side, right above the hip. Observe the dot as the
person walks in a straight line. Does the dot move in one direction only?
2. Obtain either a pencil or a small puppet. Let a pencil (or puppet) be a model
of a person walking. The eraser will be the head. Move the pencil
horizontally in two ways: a) as if it is a person moving on a conveyer belt
(such as you find at the airport) and b) as if it is a person walking. What is
different about the pencil motion between a) and b)?
A good description of walking ( and running and jumping) requires more than our simple onedimensional motion model developed so far. We will develop a more realistic picture in this
activity.
Understanding human motion is greatly facilitated by using video. Let's learn how to obtain
quantitative data from a video.
Using VideoPoint
We will be analyzing video clips of motion throughout this course. This is a great technique for
obtaining quantitative information about the motion of objects. Essentially we will look at a
video clip one frame at a time, and make measurements of an object’s position. From knowledge
of how many frames were photographed per second we can get information about time.
Activity Guide
 2010 The Humanized Physics Project
Supported in part by NSF-CCLI Program under grants DUE #00-88712 and DUE #00-88780
HPP A7
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The data taking is greatly facilitated by using a computer application called VideoPoint. Before
using VideoPoint, download the "Man Walking" video clip from the course web site.
1. Start VideoPoint by doing the following:
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click on the Start button
select Programs
select the VideoPoint folder
select VideoPoint
You will see a title window, which you can close.
2. First you must select the video clip to analyze. You do this by
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selecting Open Movie
browsing to find the QuickTime video clip showing a man walking, which should be in the
Phy107 folder
selecting the video clip file.
3. You will be asked how many objects are to be located. Select 1 (which should be the default).
We are just going to follow the hip as the man walks, for right now.
4. Now you will collect position data on each frame of the video.
Position the cursor over the hip.
Click on it. Notice that the video clip advances automatically to the next frame.
Instead of clicking on the hip on every frame, skip two frames by using the Right Arrow cursor
key, and then click on the hip. Continue this process until you run out of frames: click, skip
frame, skip frame; click, skip frame, skip frame.
When you finish you should notice a table with the hip positions measured in pixels. VideoPoint
has already calculated the time of each frame on the assumption that there are 30 frames per
second. This may not be correct for the video you are looking at. The first or second frame
should have the actual frame rate printed on it. If it is not 30 fps, then
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select Movie menu
select Select Frame Rate
select Other
type in the correct frame rate
5. Now you must convert the pixel scale into a more useful meter scale. VideoPoint can do this
for you with a little bit of help.
First locate an object in the video of known length.
Select the Movie Scale icon (a ruler).
Activity Guide
 2010 The Humanized Physics Project
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Type in the real length of the known object.
Click on OK.
Position the cursor over the left side of the object and click.
Position the cursor over the right side of the object and click.
VideoPoint should now know how many pixels correspond to a meter and can calculate the
actual position of the hip. You should see the actual x and y coordinates appear in the data table.
6. You should save your VideoPoint file by selecting the File menu and selecting Save As. An
appropriate name for the file might be "walkingManGroup1.vpt", where you put your own group
number in.
GE 2.
1. Create an x-t graph of the hip motion. You can ask the instructor how to do
this. Does the graph suggest constant velocity motion in the x (horizontal)
direction?
2. If the hip only moves horizontally while walking, what would a y-t graph of
the hip look like?
3. Create a y-t graph of the hip motion. Does the graph suggest that the hip
moves vertically while walking?
Invention
The hip undergoes a vertical wobble during normal walking. The figure below shows this
feature. The circles indicate the position of the hip as the man walks.
Activity Guide
 2010 The Humanized Physics Project
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We can describe the hip motion mathematically by specifying separately the horizontal
coordinate versus time and the vertical coordinate versus time. This is a significant new idea:
complex two (or three) dimensional motions can be understood by considering horizontal and
vertical aspects of the motion separately.
Let's develop a mathematical model of hip motion. We will start with the horizontal aspect: the
relationship between x and t. To facilitate our mathematical model development we will transfer
the position data over to a spreadsheet program.
Copying data to Excel
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Select the data table in VideoPoint.
Select the time column, press CTRL, and select the x column.
Select the Edit menu.
Select Copy Data. Your data is now on the clipboard.
Start Excel
Click in a cell where you want to begin pasting data.
Select the Paste icon, or go to the Edit menu and select Paste.
Creating an Excel Spreadsheet
Now we will set the spreadsheet up to estimate the instantaneous velocity and acceleration from
your position data. Your spreadsheet at this point should look similar to the figure below.
Activity Guide
 2010 The Humanized Physics Project
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As you did in Activity 2.6, you should first eliminate every other position measurement. Do this
by clearing the number, not by deleting the row. The spreadsheet will be similar to the figure
below.
Activity Guide
 2010 The Humanized Physics Project
HPP A7
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Now you must type in an Excel formula for calculating the average velocity over a small time
interval that surrounds each time of interest. Do this in column C. The figure shows an example.
You should copy the average velocity calculation formula into the appropriate cells in column C.
Activity Guide
 2010 The Humanized Physics Project
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Finally, type in a formula for calculating average acceleration over a small interval surrounding
the time of interest into column D. The figure below shows an example.
Copy the acceleration formula into the appropriate cells of column D.
Application: Creating a Mathematical Model of horizontal hip motion
Now you have data for position versus time, and estimates of the instantaneous velocity versus
time, and the instantaneous acceleration versus time. Create a graph of each in Excel and paste
below. Make sure the time scale is the same for each graph to aid in comparing.
Graphs:
GE 3.
1. What is the general shape of your position-time graph? Does it suggest
constant velocity or changing velocity? What about your x-t graph supports
your view?
2. Do the velocity-time and acceleration-time graphs support your answer to
question 1?
Activity Guide
 2010 The Humanized Physics Project
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3. What kind of equation would best describe the position-time graph? Write
out the general form of the equation here using t as the independent variable
and x as the dependent variable, so that you have x = … , where the right hand
side contains t.
4. Use your x-t graph to estimate the parameters in the equation you wrote out
in question 3. Write out your x-t equation here.
5. A mathematical model of data has four aspects: numbers, words, graph, and
symbolic. Have you considered all four aspects for your hip position-time
data?
6. Microsoft Excel has some powerful statistical tools to aid in developing
mathematical models of data. One of these is the Trendline tool. Here are the
steps for using this tool:
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Select the graph
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Click on one data point. This selects the entire data series.
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Select the Chart menu.
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Select Add Trendline
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Select the model you wish to try
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Select Options
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Check Display equation on chart
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Check Display R-squared value
Do these steps now and type in the "best-fit" equation that Excel produces
here.
7. Could the same kind of equation be used to describe the vertical (y-t)
motion? You do not have to do a complete analysis of this right now. Just
look at the y-t graph.
Part of your homework assignment will be to collect position-time data for the knee during
walking and create a spreadsheet that estimates velocity-time and acceleration-time. If time
permits, your should go ahead and collect your postion data now. Paste the position-time data
into Excel. Save your spreadsheet in your group File Exchange area, so that everyone can access
it.
Activity Guide
 2010 The Humanized Physics Project