Lab: Introduction to Motion
(Adapted from: RealTime Physics: Active Learning Laboratories, Module 1 Mechanics,
by David R. Sokoloff, Ronald K. Thornton, and Priscilla W. Laws. New York: John
Wiley and Sons, Inc. 2004.)
Objectives
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To discover how to use a motion detector
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To explore how various motion are represented on a velocity-time graph
To explore how various motion are represented on a distance (position)time graph
To discover the relationship between position-time and velocity-time
graphs
Overview
In this lab you will examine two different ways that the motion of an object that moves
along a line can be represented graphically. You will use a motion detector to plot
distance-time (position-time) and velocity-time graphs of the motion of your own body
and a cart. The study of motion and its mathematical and graphical representation is
known as kinematics.
Investigation 1: Distance-Time Graphs of your Motion
The purpose of this investigation is to learn how to relate graphs of the distance as a
function of time to the motions they represent.
Your will need the following materials:
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graphing calculator or computer-based laboratory system
motion detector
number line on floor in meters (optional)
How does the distance-time graph look when you move slowly? Quickly? What happens
when you move toward the motion detector? Away? After completing this investigation,
you should be able to look at a distance-time graph and describe the motion of an object.
You should also be able to look at the motion of an object and sketch a graph
representing that motion.
How does a motion detector work?
The motion detector sends out a sound wave (air pressure wave) at a very high frequency
that humans cannot hear, 50,000 Hz. Humans can hear up to 20,000 Hz, but most of us
can’t hear frequencies even that high. The motion detector then waits to hear the echo of
the sound wave. Using an estimate for the speed of sound (343 meters per second), it
then estimates how far the object is from the detector by multiplying the waiting time by
343 and dividing by 2. The motion detectors we are using are accurate to 2mm.
Things to note:
 “Distance” is short for “distance from the motion detector.” The motion
detector is the origin from which distances are measured.
 The motion detector detects the closest object directly in front of it
(including you arms if you swing them as you walk.
 The motion detector transfers information to the computer or calculator via
the interface so that as you walk (or jump or run) the graph on the computer
screen displays your distance from the motion detector
 The motion detector will not correctly measure anything closer than some
distance (in our case about 10 cm). When making graphs, don’t go closer
than this distance from the motion detector.
About the calculator interface.
Generally, all you need to do is plug in the motion sensor to the calculator. The interface
program (called EasyData) automatically turns itself on. (In the odd event that nothing
happens when you plug in the motion sensor, click on “Apps” and then choose the “Easy
Data” program.)
The calculator collects data for a specified length of time at a specified frequency when
you press on “Start.” You will occasionally need to adjust the sampling period and the
frequency. To do this, click on Setup, and then choose 2. Time Graph. Then click on
Edit. You can set the sampling length as well as the sampling interval here. Usually a
sampling interval of 0.05 s is about right, unless the object is moving very quickly in
which case it is better to use 0.01 s as the sampling interval.
Activity 1-1: Making and Interpreting Distance-Time Graphs
1. Be sure that the motion detector is plugged into the calculator or computer, and
the interface program running.
2. If you have a number line on the floor and you want to the detector to produce
readings that agree, stand at the 2m mark on the number line and have some
someone move the detector until the reading is 2m.
3. Begin graphing and making distance-time graphs for different walking speeds and
directions, and sketch your graphs on graph paper.
a. Start at the ½ meter mark and make a distance-time graph, walking away
from the detector (origin) medium fast at a constant speed. Sketch the
graph you get and label it “Walking away slowly and steadily.”
b. Start at the ½ meter mark and make a distance-time graph, walking away
from the detector (origin) starting out slowly and steadily speeding
up. Sketch the graph you get and label it “Walking away starting out
slowly and steadily speeding up.”
c. Start at the 2.5 meter mark or so and make a distance-time graph, walking
toward the detector (origin) medium fast at a constant speed. Sketch
the graph you get and label it “Walking toward slowly and steadily.”
d. Start at the 2.5 meter mark or so and make a distance-time graph, walking
toward the detector (origin) starting out slowly and steadily speeding
up. Sketch the graph you get and label it “Walking toward slowly and
steadily speeding up.”
Question 1-1: Describe in a well-written sentence or two the difference between a
graph made by walking away at at constant speed and one made by walking away
speeding up.
Question 1-2: Describe in a well-written sentence or two the difference between a
graph made by walking toward and one made by walking away from the motion
detector.
Prediction 1-1: Predict the position-time graph produced when a person
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starts at the 1-m mark,
walks away from the detector slowly and steadily for 5 s,
stops for 5 s, and
then walks toward the detector twice as fast.
Carefully draw your prediction on graph paper using a dotted line. After you draw it,
compare your graph to the graphs of your group members. Then draw your group’s
prediction with a solid line.
4. Test your prediction. You will need to change the duration of the experiment length
to about 17 s or so in order to test your prediction. By default, the motion detectors
record data for 5 seconds. To change the duration to 20 seconds, click on Setup, and
then choose 2. Time Graph. Then click on Edit. Keep the sample interval to be
0.05 seconds. Press Next. Then change the number of samples to be 375. The press
OK.
Move in the way described in Prediction 1-1, and graph your motion. When you are
satisfied with your graph, draw your group’s final result on your graph paper.
Question 1-3: (To be recorded on your paper) Is your prediction the same as the
final result? If not, describe how you move to make a graph that looks like your
prediction.
Activity 1-2: Matching a Position-Time Graph
By now you should be pretty good at predicting the shape of a position-time graph of
your movements. Can you do things the other way around by reading a postion-time
graph and figuring out how to move to reproduce it? In this activity you will move to
match a position graph shown on the calculator (or computer) screen.
1. Try to match the following graph:
2. To do more distance matching with a calculator so that each of group members has
a turn, click on Setup. Then choose 3. Distance Match. Press Enter. Then click on
Start. The calculator will give you graphs to match. You can try as many times as
you wish. It helps to work as a team. Get the positions right. Each person should take
a turn.
Question 1-4: What was the difference in the way you moved to produce the two
differently sloped parts of the graphs you matched?