Phys 131, Fall 2012
TA GUIDE—Lab 2 (Part 1)
Lab 2 (Part 1)—Introduction to Video Capture and Analysis of
Directed Motion & Resistive Forces
Overview:
This is the first week of a two-week lab sequence designed to introduce the students to video
capture and analysis of directed motion. In this first week, they will collect video data for two
separate investigations (hour 1 and hour 2). Next week they will analyze their data and try to
determine how the resistive forces scale with respect to the varied quantity. In the first
investigation (hour 1), they will be asked to analyze the directed motion of either coffee filters falling
through the air or different spheres falling through fluid (one concentration of glycerol). (They are
investigating how resistive forces and terminal velocity scale with the mass of the falling object.) In
the second investigation (hour 2), all students will analyze the directed motion of one sphere falling
through different fluids (different concentrations of glycerol). (They are investigating how resistive
forces and terminal velocity scale with the viscosity of the fluid.) The lab handout will give explicit
instructions on video capture, but no guidance in the performance of the physics skill goals. Your
tasks are: 1) to demonstrate video capture skills when asked, 2) to help with ImageJ when asked, and
3) to act as a guide for the physics skills (see the document on helping student groups learn). By
next week’s lab, they should have the necessary theoretical background to analyze their data
regarding the scaling of resistive force. (It is often a good idea to put the approximate timing on the
board, as well as a short list of the objectives/skill goals—see the next page.)
Materials:





2 per group—webcams for video capture
For coffee filters (hour 1), per group: 5 coffee filters, meter stick w/ stand
For spheres (hour 1), per group:
o same size spheres made of Steel, Brass, and two densities of plastic
o large graduated cylinder with prepared mixture of glycerol and water
o slotted scoop for retrieval of sphere from bottom of graduated cylinder
o whiteboard markers for marking plastic spheres (may be pre-marked)
o meter stick
o paper towels, as needed
For spheres (hour 2):
o Per group—one sphere of either Steel or Brass, paper towels
o For the class—five or more graduated cylinders with different concentrations of
glycerol (marked on cylinders), slotted scoop for each cylinder
1 per person—Video Capture instructions/lab guide
Phys 131, Fall 2012
TA GUIDE—Lab 2 (Part 1), cont.
Introduction:







Groups of three or four students—performing in the Community Lab roles
Mention that resistive forces, like drag resistance and viscous resistance, are proportional
to some power of velocity (v1 (viscous, laminar, non-turbulent flow) or v2 (drag,
turbulent flow))—by taking careful data, we can see which of these is more appropriate.
(Although turbulent and laminar flow are both possible options for either air or fluids,
depending on the Reynolds number—they’ll hear more about this in lecture.)
For fluids, which are HIGHLY relevant to cellular biology, the resistive force is also
proportional to the viscosity of the fluid—by taking careful data, we can see how the
viscosity of a fluid affects fluid drag.
Ask students to define terminal velocity in their own words. Have them think about
what terminal velocity would look like on a v vs. t graph and on an x vs. t graph.
Briefly review the Skill Goals (Physics, Image J, and Video Capture, on the next page)
Remind students that they are ALL expected to master the skills, so they should take
turns and help each other out. Taking notes may not be a bad idea, either.
Inform students that a lab report will be due at the end of the lab next week—but it is a
good idea to write as much as they can of the report this week, so that they don’t forget
what they did!
Summation/Submission:
o Recap the Skill Goals, clarifying any remaining confusion about physics concepts
o Discuss the challenges and considerations with the class, if any have not yet been
addressed. Ask the students what they found most difficult/challenging about either the
video capture or the initial analysis.
ImageJ Skill Goal:

Learn how to determine the distance-to-pixel ratio of an image
Video Capture Skill Goals:



Capture video with the webcam
Determine timing (time between frames) and play rate (number of frames per second)
Consider elements of video construction/planning (What is the best angle? What is the
best time between frames? Is there a known length visible in the video? Are all objects
of interest clearly visible in the video? Is the entire portion of motion in which we are
interested visible in the video? Is the camera (perspective) stationary?)
Phys 131, Fall 2012
TA GUIDE—Lab 2 (Part 1), cont.
Physics Skill Goals:
Analyze 1-D motion from a
video/image sequence
Tabulate the position-at-time for an
object’s motion
Calculate average speed
Understand the concept of terminal
velocity
Understand the scaling of terminal
velocity with mass
Understand the scaling of terminal
velocity with dynamic viscosity
Understand the types of resistive force
in ‘fluids’—Drag vs. Viscous resistance
Analyze motion graphs (y vs. t, v vs. t,
(a vs. t))
Determine uncertainty in video
measurements
Challenges/Considerations:
What is 1-D? How do you separate out a second
dimension? (Does the perpendicular-direction motion
matter? Why?/Why not?)
Where is the object? ( What part do we track?) How do
you match the pixel locations and time slices to the given
scales?
What is the difference between average and instantaneous?
What should the units be?
What does it mean? What does it look like on a y vs. t or
v vs. t graph?
How is vt proportional to m? Is it linear with vt1? Is it
linear with vt2? What are the implications of each of these
options?
How is vt1 proportional to viscosity (μ or η)? Is it linear?
Is it polynomial? Is it exponential? What are the
implications of each of these options?
Which is proportional to vt1? Which proportional to vt2?
What is a ‘fluid’ in physics?
How are the graphs connected? What do
positive/negative values imply? What do
positive/negative slopes imply?
How do you determine uncertainty? Is it the same for all
videos? How can the uncertainty in position be
propagated into the average speed? Is there uncertainty in
time?
Approximate Timing: (~2 hours)
o
o
o
o
o
Introduction:
Data Collection (1st investigation):
Data Collection (2nd investigation):
Class Discussion/Summation:
Data Entry/Data Table:
15 minutes
40 minutes
40 minutes
10 minutes
5 minutes