GateWay CC
PHY101 Physics Lab
Measuring the distance
and velocity
The motion detector is used in the lab activity in order to generate graphical representation of
position and velocity of an object (student). Motion detector emits a very high-pitched, high
frequency "signal," in the direction of an object. The “signal” then detects an object. If an object
is flat, then the signal will bounce off of an object and travel back to the detector. The detector
will store the information on the signal traveled. The information will then be used by the
computer to generate displacement (distance) and velocity graphs.
In the computer, two sets of information’s will be stored:

Time how long it takes from when the signal first leaves the detector to when it returns
after bouncing, and

The speed at which the signal travels
Using the formula for the distance calculation:
d
v
t
Position in m
4
2
computer can calculate the distance between the detector and the object from which the signal
bounced. This information will then be displayed on the computer screen in a digital or graphical
form. By keeping track of how the distance from the detector changes with time, the computer
can show the change of the distance as a function of time. The graph may look like one in figure
1.
2
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Time in s
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Figure 1. Distance in meters as a function of time in seconds
The graph on Figure 1 consists on three sections:

In section one an object is moving away from the detector

In section two an object stays still (not moving – distance is not increasing), and

In section three, an object is moving toward the detector.
From a graph similar to one in Figure 1, student can estimate the instantaneous velocity at any
moment. In this lab, we will be using the detector and the computer to which it is connected
through the interface box to determine the instantaneous distance from the detector, and
velocity of a moving object.
GWC – motion.doc
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GateWay CC
Activity 1: Using the motion detector and its software
HARDWARE CONNECTIONS
1.
Connect the Science Workshop interface to the computer, turn on the interface, and turn
on the computer.
2.
Connect the Detector stereo phone plug to Digital Channel 1 on the interface. Yellow
connector on 1.
3.
4.
Open the Data Studio or Science Workshop software.
5.
Open a graph distance as a function of time. (Click and drag). Select position versus time
graph.
6.
Open a digital display. (Click and drag).
Select the proper probe - sensor on the software. (Click and drag the motion sensor detector).
To run this experiment click on “RUN” button at the upper left corner of your computer screen.
When you do this you will notice that the motion detector emits a clicking sound. Move yourself
or some object toward or away from the motion detector while it is clicking and watch what
happens with your graph. In the RUN mode computer will not store the data collected. When you
became more familiar with the lab setup, and want to store data, click on REC button.
Each person in your group should try being the "moving object" and make a distance versus
time graph, while looking at the distance graph on the screen. Adjust the computer screen so
that you can watch it while you are moving. Try constant distance (standing still), constant
velocity (moving forward or backward with constant speed), and constant acceleration (moving
forward or backward with uniformly increasing (or decreasing) speed).
To hand in for activity 1: Nothing.
Activity 2: Matching a position versus time graph
Position in m
2
1
In this activity you need to match as close as possible the graph shown in Figure 2. Each student
should start from about 0.4 m in front of the detector. First, look at the graph in Figure 1 you
will be trying to match. Discuss with your lab partners when you will be walking toward the
detector and when you will be walking away from the detector and when you will be sitting still
– not walking. In addition, figure out when your speed will be increasing, when it will be
decreasing, and when it will be constant. Now click "REC" and try walking in such a way that the
distance versus time graph of your motion matches the distance versus time graph in Figure 1.
Each person should repeat this step until a good match is obtained. Make a printout of each
person's best run, and write that person's name on the printout.
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Time in s
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Figure 2. Distance in meters as a function of time in seconds
To hand in for activity 2: One graph for each student: Graph of attempted match for each
member of the group.
GWC – motion.doc
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Activity 3: Estimating average velocity from a distance versus time graph
Position in m
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1
In this activity students will generate one distance versus graph shown in Figure 3. The graph
consists of two separate trials. One line shows the students walking away and one walking
toward the detector. Follow the same general procedure as in activity 2 above. Have one
student in your group make a distance versus time graph while moving at constant velocity, and
while watching the distance versus time graph. The distance versus time graph should be a
straight line sloping up or down shown in Figure 2. It may take a few tries to get a nice graph.
Make a printout of the graph, and estimate your velocity by finding the slope of the distance
versus time graph. In other words, find how much distance you covered and divide it by how
much time you took to cover it. Show your calculation on the graph, and write your estimated
average velocity on the graph. Was your average velocity positive (away from the detector) or
negative (toward the detector)?
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Time in s
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Figure 3. Distance in meters as a function of time in seconds
Identify any two points on the increasing graph, and two points on the decreasing graph.
x1
y1
x2
Y2
Decreasing Graph
Increasing Graph
When the two points on the straight line are given the slope of each line can be calculated:
slope 
y2  y1
x2  x1
Use this formula to calculate the slope of each plot.
Slope of the increasing graph: __________________
Slope of the decreasing graph: __________________
To hand in for activity 3: One distance versus time graph with calculation of estimated
average velocity,
Activity 4: Drawing a graph from a description of the motion
Draw a distance versus time graph and a velocity versus time graph for an object which:

First moves with a constant slow speed away from the origin for 2 s,

Then moves with a constant faster speed away from the origin for the next 2 s,

Then stands still for the next 2 seconds,

Finally, moves with a constant slow speed toward the origin for the last 2 s.
GWC – motion.doc
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Time in s
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Figure 4. Distance in meters as a function of time in seconds
To hand in for activity 4:

Hand drawn position versus time graph (only one per group),
Activity 5: The relationship between velocity vs. time and position (distance) vs. time
graphs
Position in m
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Velocity in m/s
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In following series of paired position vs. time graphs and velocity vs. time graphs, one graph in
each pair has been completed. Use a solid or dotted line to complete the graphs that is not
completed.
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