Physics Lab Activity
Kinematics Graphs
Printer-friendly versions of this page:
Purpose:
When you finish this activity, you should be able to:
- Sketch a position vs. time graph for a described
one-dimensional motion.
- Describe a one-dimensional motion given a position vs. time
graph.
- Sketch a velocity vs. time graph for a described
one-dimensional motion.
- Describe a one-dimensional motion give a velocity vs. time
graph.
Equipment:
Description:
A graph of position vs. time and/or velocity vs. time is one of
the most effective ways to describe motion. In this lab, you will use
a "motion detector" linked to a computer or calculator to
automatically draw kinematics graphs in real time for
your one-dimensional motions. This makes it easy and
natural for you to connect kinematics graphs and the motions they
represent.
This activity is divided into two parts. In the first part, you
will be asked to predict what the graph of a motion would look like,
and then actually execute that motion and compare the resulting
graph. In the second part, your lab partners will draw a graph and
challenge you to perform the motion that will produce the graph.
It will help to understand what's going on. "Motion detectors"
don't actually detect motion. They work by sending out a
high-frequency sound pulse which strikes an object (you, in this lab)
and reflects back to the detector. By precisely measuring the time
for the pulse to return, the software can determine the distance to
the reflecting object.
For example, suppose that it takes a signal 0.017 seconds to travel
to you and reflect back to the detector. Knowing that the speed of
sound is about 340 m/s, the distance traveled by the pulse is easy to
calculate:
total distance traveled = (average velocity)(time) = (340
m/s)(0.017 s)
total distance traveled = 5.8 m
The distance from the detector to you is half of the total
distance = 5.8 m/2 = 2.9 m
Knowing the time and distance to the object it is pretty
straightforward to draw the graph, and the software can construct the
graph in "real time", so you can watch the graph being drawn as you
move.
The software calculates your velocity from the distance and time
data that it collects. For example, if you were detected 2.9 meters
from the detector at 1.55 seconds, and 3.0 meters from the detector
at 1.60 seconds, your velocity would be calculated as:
Hints:
Right now you are probably saying "Wow! This lab is computerized!
What could possibly go wrong?" Well...
The reflection of the sound waves is not entirely reliable. Be
sure that you are performing your motion in a clear area - no nearby
walls, all furniture and extra people out of the way. If you can't
get a good graph, try re-aiming the detector or moving nearby objects
out of the way. It may help to hold a large piece of cardboard in
front of you to reflect the waves better.
In any case, your graph won't be perfectly "clean" and smooth. You
will need to ignore the "glitches" in the graph, if possible (unless
you really are jumping several meters and back in a hundredth of a
second...). Keep in mind that since velocity is calculated from
position, any slight problems in the position vs. time graph will be
magnified in the velocity vs. time graph. Since accelerations are
calculated from velocities in this software, acceleration vs. time
graphs are pretty much impossible, but you can try them if you have
time. If your software allows "smoothing" of your graphs, take
advantage of it.
Keep in mind that the motion detector will not work well for
objects closer than about 40 cm or farther than 4-6 meters.
Procedure:
- Position the motion detector so that it is abdomen/chest high,
with a clear area at least 4 meters long (and 2-3 meters wide) in
front of it.
- For easy reference as you move, put small strips of masking
tape on the floor at 1 meter, 2 meters, 3 meters, and 4 meters
from the detector. Don't forget to remove this tape when you are
finished!
- Part 1:
- Double click the experiment file "position_vs_time.ds".
This will start the DataStudioTM software and load
the experiment file The screen should look like the picture
below.
- How it works:
- When you press the Start Button, the computer
will delay for 3 seconds, and then the motion detector will
run for 10 seconds and automatically shut off. A graph of
position vs. time for your motion will be displayed.
- To graph a second motion, just press Start again. To
make the display easier to read, you can pull down the Data
Menu and click on the previous run to uncheck it and remove
it from the graph.
- For each motion described in the position
vs. time worksheet, first draw what you predict the graph
will look like for the described motion on the axes on the left
{"Predicted"). This prediction might not be very accurate at
first, but you should get better with practice. Then, one
person in your group will actually perform the motion and you
can sketch the graph of the motion on the axes on the right
("Measured"). Be sure to take your turn performing the
motions.
- Part 2:
- Open the experiment file "velocity_vs_time.ds".
It operates just like the position vs. time experiment file,
except that it produces a graph of velocity vs. time.
- For each motion described in the velocity
vs. time worksheet, first draw what you predict the graph
will look like for the described motion on the axes on the left
{"Predicted"). This prediction might not be very accurate at
first, but you should get better with practice. Then, one
person in your group will actually perform the motion and you
can sketch the graph of the motion on the axes on the right
("Measured"). Be sure to take your turn performing the
motions.
- Part 3: One of your lab partners will sketch a graph on the
axes on the left side of the page on the "challenge" worksheet,
and your task is to reproduce the graph using the motion detector,
as nearly as possible, with an actual motion. Be sure to use the
experiment file "position_vs_time.ds" for position vs. time
graphs, and the experiment file "velocity_vs_time.ds" for velocity
vs. time graphs.
last update November 22, 2005 by JL
Stanbrough