Laboratory Activity 1: Position
Graphs
Group member names:
-Jake Schultz
-Shawn Brown
-
Objectives

Learn how to use a motion detector.

Explore how motions are related to position-time graphs
Equipment

Computer

Ultrasonic motion sensor

USB Link
Activity One: Simple graphs
Position
Prediction 1: Using the line tool on the drawing toolbar of Word, draw a line on
the graph below predicting the shape of a curve for a person walking away at a
slow, steady pace. If you don’t see the drawing toolbar (usually located at the
bottom of the window), check to see that it is enabled with the drawing toolbar
button .
Time
An ultrasonic motion detector is a device that can be used to record an object’s
motion as a function of time. It sends out a series of sound pulses (too high
frequency to hear). These pulses reflect from objects in the vicinity and the
reflected pulses return to the sensor. The computer can record the time it takes
the reflected pulses to return to the computer, and using the speed of sound,
calculate the position of the object.
When using a motion sensor:

Do not get closer than 0.3 meters (one foot) from the sensor because it
cannot record reflected pulses that come back too soon after they are
sent.

The ultrasonic waves spread out in a cone of about 15° as they travel.
They will “see” the closest object. Be sure there is a clear path between
the object whose motion you want to track and the motion sensor.

The motion sensor is very sensitive and will detect slight motions. You
can try to glide smoothly along the floor, but don’t be surprised to see
small bumps in position graphs and even larger bumps later in velocity
and acceleration graphs.

Some objects like bulky sweaters are good sound absorbers and may not
be “seen” very well by a motion sensor. You may want to hold a book in
front of you if you have loose clothing on.
Set up the equipment: Obtain a USB Link and a motion detector as pictured.
Start up the computer if it is not already running. Plug the USB link into the back
of the computer as shown. Note that there is only one direction in which the
cable will plug in. Then plug the cable of the motion detector into the USB Link.
Note that there is again only one direction in which it will
plug in.
Within a few seconds the computer should announce,
“Sensor detected” and ask you what to do. Select “Open
DataStudio” and wait until it has finished loading and
detected the sensor. Click on the “Start” button along the
bar at the top and move something away and towards the
motion detector to verify that it is working. You should hear it making a clicking
sound and see the line being graphed move up and down. Click on “Stop” (the
same button as “Start”) and clear the data by going to “Experiment” on the top
menu, selecting “Delete Last Data Run” and answering “OK” to the prompt.
Open the experiment file called Distance.ds to display distance (position) vs.
time axes. You can find it by going to the networked disk “PLAB” (usually disk
G:), in that opening the folder “Plab”, then finding the folder for Physics 201, and
then looking in the folder for Lab1.
Experiment: Set the detector on the edge of the table and have one member in
your group stand about 0.5 meter (½ yard) away from the detector and walk
away backwards at a slow, steady pace, while watching the graph on the screen.
It may help to get a smoother graph to hold a book in front of you away from your
body at the level of the detector. Let everyone have a chance. When you have a
nice graph, copy the graph and paste it in the section below. If you have a graph
that messes up, you can delete it by selecting “Delete last data run” from the
“Experiment” menu. If you get so many runs it is hard to see, you can remove
them all with the “Delete all data runs” from the Experiment menu. To copy the
graph to MS Word, first click on the graph in DataStudio to make sure it is
selected, then select “Copy” from the “Edit” menu in DataStudio. Then click in
the space below and then do a “Paste Special” in the “Edit” menu of Word.
Select the bitmap image option and click OK.
-paste graph here-
Question 1: How did the result compare to your prediction? If it was different,
do you now understand why it is the way it is? Note: you should not go back and
change predictions. There is absolutely nothing wrong with making a prediction
and then finding out it wasn’t quite correct. In fact, that can show that you are
learning.
It was the opposite of our prediction. We thought that the closer position would
be a greater starting position and that the further away you got the more it would
drop position as you moved away.
Prediction 2: How will the graph be different if you walk away from the detector
at a faster pace? How will the graph be different if you start further from the
detector and walk toward it?
The slope will be greater the faster you change position.
Experiment: Without erasing your last data run, start about ½ meter from the
detector and walk away at twice the rate. Then take another run, starting away
from the detector and walking toward it. Label each run as to which it is by
double clicking on the labels “Data Run 1” in the menu list on the upper left-hand
window and giving them more descriptive names. Then copy and paste your
graph in the space below the same way as above.
-paste graph here-
Question 2: What does the magnitude of the slope (how fast the over-all line is
increasing or decreasing) tell you about the motion? What does the direction of
the slope (increasing or decreasing) tell you about the motion? What would a
graph look like for someone standing still?
The increasing and decreasing slope of the line indicates the speed of the object
moving towards or away from the device. If the slope is increasing then the
object is moving away from the detection device, if it is decreasing the object is
moving toward the device. It would be a horizontal line with a slope of zero.
Activity Two: More complicated motion
Prediction 3: Sketch on the graph below a prediction for the following motion:
walking away from the detector slowly, stopping for a while, and then walking
faster away from the detector.
Position
Time
Question 3: Try walking the way described. How does it compare to your
predictions?
Our prediction was very accurate. The graphs look relatively similar.
Activity Three: matching graphs
(Note: if your group had some technical difficulties and are running out of class time, skip to the
summary questions and come back and finish as much of this activity as you can.)
Open the experiment file called PositionMatch.ds. A position graph like that
shown below will appear on the screen.
(from RealTime Physics (Electronic Version) by Prisilla Laws, et al., John Wiley and Sons, NY, 1999)
Prediction 4: Describe in words how you would move in front of the motion
detector in order to create the graph.
You would start out standing still for about 6 sec. Then move away quickly for 2
seconds. Standing still for 4 seconds, then moving away again for 4 seconds at a
slower rate of speed then the first time you moved away. You finish by standing
still for an additional 4 seconds.
Experiment: Walk in front of the motion detector and try to create the graph.
You may try a number of times. It helps to work in a team. Get the times right.
Get the positions right. Each person should take a turn.
Experiment: Try walking to produce a graph like each of these bellow. First
discuss with your team how you would need to move to produce each graph, and
then work as a team to do each.
(from RealTime Physics (Electronic Version) by Prisilla Laws, et al., John Wiley and Sons, NY, 1999)
Question 5: How do you need to walk to match graph A?
Start out moving at a relatively slow pace away from the detector and gradually
increase speed as you move away from the detector (graph a).
How do you need to walk to match graph B?
Start out at a fast pacing moving away from the detector and gradually slow
your pace down as you continue to move away from the detector.
How do you need to walk to match graph C?
You start out pretty far away from the detector and move towards it at a slow
pace with an increasing pace as you near the detector.
Summary
The following questions will help you get the main ideas out of this lab. You
should find these straightforward questions, but take the time to talk it over with
your team and write complete answers to these questions. You may find your
answers here to be the most useful part of this lab down the road.
Summary 1: In a position graph, what does the horizontal distance from the axis
of the graph mean? What doe vertical distance from the axis mean?
Horizontal measures time, and vertical measures distance.
Summary 2: In a position graph, what does the slope of the line mean? What
does a line sloping down mean? What about a line sloping up? A horizontal line?
What does the steepness (how fast it is increasing/decreasing mean)?
Slope means velocity. A downward sloping line indicates that an object is moving
toward the detector. An upward sloping line indicates that an object is moving
away from the detector. A horizontal line indicates that the object is sitting still at
a constant distance from the detection device. The steepness of the line
indicates the velocity of an object.
Summary 3: How do you know your above statements are true? What
experimental observations and/or logical reasoning can you give to justify what
you said in summary questions 1&2?
In the experiments we did in moving away and towards the detector gave us the
results that we indicated in the above questions. Also we varied the speed in
which we were moving.
This activity was adopted from materials in the following curricula: Workshop Physics for Life
Sciences, Beth Ann Thacker, RealTime Physics, David Sokolof, Ronald Thorton and Priscilla
Laws, and University of Northern Iowa General Physics Activities, Larry Escalada and
Malcomber.