Physics Lab #1: Basic Linear Motion
Overview:
Linear motion is exactly what the name implies- motion in a straight line. It is important to note that linear does
not imply one direction, only that any direction change must be along the same axis. In other words, “forward” and
“reverse” are allowed, but not “forward then left” or “reverse then right.” Basic linear motion is very simple to
describe, and thus predictions about an object in basic linear motion are also easy to make (i.e.- a car traveling North
at 50mph drives for 2hr... how far does it drive?). We will use the simple concepts here to introduce a very
important part of physics- the motion graph, and a very important part of this class- the Vernier data collecting
system.
The Motion Sensor
Some cameras equipped with ‘auto focus’ use the same technology found in the motion sensor- ultrasound pulses.
Sound travels through air at approximately 340m/s, plus or minus a few depending upon the temperature, barometric
pressure, and humidity. For motionless objects (or objects traveling significantly below this speed), sound pulses
can be used to determine the distance between two objects by measuring the time between the pulse generation at
one location and the returning echo from another location:
0.6s
total time = 1.2s
0.6s
So how far apart are the two objects? 0.6s x 340m/s = 204m.
The motion sensor operates by sending out a short burst of sound and recording the time it takes for the reflection to
return. Knowing the speed of sound, it’s pretty easy to find the distance between the device and another object.
The motion sensor has an effective range of 0.45m – 6.0m (1.5 – 19 feet). All data readings will need to be made
between these distances.
Using a motion sensor, we can determine the distance of an object from the motion sensor at any given time. The
motion sensor is capable of sending and reading many bursts of sound each second. Therefore, if the location of an
object changes over time (i.e.- is moving), the motion sensor will give a record of the object’s distance at any point
in a given time interval.
Procedure:
Connect the motion sensor to the LabQuest via the provided patch cable.
The sensor should auto-detect, and display “DIG 1: Position” on the main screen. If not, set this up maually:
Tap SENSORS, SENSOR SETUP, DIG 1, MOTION DETECTOR
At the home screen, you should note that the data collection window at right displays a collection length of 5s. If
not, set this up manually by tapping this window and selecting the following:
MODE: Time Based, RATE: 20 samp/sec, LENGTH: 5s, TRIGGERING: Disabled. Return to home
screen.
Tap the graph image at the top menu bar, Select GRAPH, SHOW GRAPH, GRAPH 1. Return to home screen.
Part 1
1) Stand approximately 5m (15’) from the sensor. Sensor should be stationary (set on lab table edge or held firmly).
2) Aim sensor face at your stomach/chest. Walk very slowly toward your sensor at an even, constant speed.
3) Partner should press the “Run” button on the LabQuest immediately after you begin walking.
4) Stop when you close to within 0.5m (1.5’) from the sensor or when the sensor completes the readings.
5) The graph that appears is a distance vs. time (d vs. t) graph, one of the staples of the study of motion. If your
graph does not resemble a straight line, redo the trial.
6) Make a sketch of the graph. Label both axes as shown on the calculator. Trace along the graph.
Part 2
Repeat the trial in Part 1 with the following change:
Walk at a faster pace toward your sensor. Remember to maintain a constant speed and to stop when you
are within 0.5m from the device. Remain motionless until the sensor finishes data collection.
Sketch and label the graph as above.
Part 3
Repeat the trial in Part 1 with the following change:
Walk at a gradually increasing speed toward your sensor. Start at a very slow speed and
finish fast! This one may take a little practice. The resulting graph should be
fairly smooth, though not necessarily straight.
Sketch and label the graph as above.
Part 4
Repeat the trial in Part 3 with the following change:
Walk at a gradually decreasing speed. Start fast!
Sketch and label the graph as above.
Questions:
Graphing in the physical sciences is much more than the “visual representation of data” you were taught in grade
school. Graphing data and analyzing the slope and intercepts can obtain huge amounts of important quantitative
(numerical) and qualitative (descriptive) information. The following questions deal with the slope of the graph.
1) Compare the graph in Part 1 with that of Part 2.
a) What is indicated by a steep vs. a shallow slope? To answer, think about how these trials were different.
b) These two graphs are approximately straight lines. What does this tell you about the nature of the
motion graphed? Again, think about your motion during these trials.
c) How would the graphs be different if you were to walk away from the sensor instead of toward it?
For your answer, make a quick sketch representing constant speed motion away from the sensor.
2) Compare the graph in Part 3 with that of Part 4.
a) What information is communicated by a gradually increasing slope?
b) What information is communicated by a gradually decreasing slope?
c) What is the common term for this kind of motion (think- what do you call this kind of motion when
you’re in a car)?
3) What is the primary difference between the motion that generated a d vs. t graph with a positive slope and the
motion that generated one with a negative slope? So what does the sign of the slope communicate?
4) Consider the following graph produced by a motion sensor trained on a moving object:
d(m)
t(s)
What information can be deduced from the graph regarding the motion of the object?