Experiment
13
Air Resistance
When you solve physics problems involving free fall, often you are told to ignore air resistance and
to assume the acceleration is constant and unending. In the real world, because of air resistance,
objects do not fall indefinitely with constant acceleration. When an object is falling with a constant
velocity, we prefer to use the term terminal velocity, or vT.
1
The basic drag equation is, 𝐷 = 2 𝐶𝜌𝐴𝑣 2 .
OBJECTIVES

Observe the effect of air resistance on falling coffee filters.
 Determine how to linearize the data obtained in order to find the coefficient C for a coffee filter.
 Understand how to use Logger Pro to find the cross-sectional area of a coffee-filter.
MATERIALS
computer
Vernier computer interface
Logger Pro
Vernier Motion Detector
5 basket-style coffee filters
PRELIMINARY QUESTIONS
1. (6 points) Using the equation described above in the introduction and a free body diagram,
determine an equation for terminal velocity as a function of mass.
2. (1 point) When the filter reaches terminal velocity, what is the net force acting upon it?
3. (9 points) Sketch graphs of the position vs. time, velocity vs. time, and acceleration vs. time for
one falling coffee filter released from rest.
4. (1 point) If there was no air resistance, how would the rate of fall of a coffee filter compare to the
rate of fall of a baseball?
5. (6 points) Define the following letters in the equation and give units for each:
Letter
C
ρ
A
Definition
Units
Experiment 13
PROCEDURE
1. Plug the Motion Detector into the
DIG/SONIC 1.
Motion
Detector
2. Support the Motion Detector about 2 m
above the floor, pointing down, as shown
in Figure 1. (Or set it on the floor and
ignore the “negatives” you get.)
3. Open the file “13 Air Resistance” from the
Physics with Vernier folder.
4. Click
to begin data collection.
When the Motion Detector begins to click,
release the coffee filter directly below the
Motion Detector so that it falls toward the
floor. Make sure you keep your hands out
of the way.
Interface
Figure 1
5. Get the velocity from the slope of the position vs. time graph. Highlight the linear section and get
the slope from the best-fit line.
6. Record your velocity in the data table.
7. Repeat for two, three, four, five and six coffee filters.
8. Finding cross-sectional area of a coffee filter:
a. Set the stack of coffee filters down on the counter and take a picture from looking straight
down on them. Make sure there is a ruler (or piece of a ruler) in the picture. Transfer it to the
computer somehow.
b. Insert the picture into Logger Pro using INSERT, PICTURE, and then PICTURE WITH
PHOTO ANALYSIS.
c. Click on the
button and use your ruler in the picture to set the distance scale.
d. Click on the
button and place dots all around the perimeter of the coffee filter. Try to get
the last dot on top of the first dot (to the best of your ability). The more dots you put around
the edge, the closer your area will be to the correct area. Don’t go too crazy dotting so that it
takes forever, but don’t do so few that you don’t get a good calculation.
e. Click on the
button to get the cross-sectional area of the filter. When you press this
button, the computer should fill in your area with red and give you the area.
Air Resistance
DATA TABLE (12 points)
Number
of filters
Total mass of
filters (kg)
Terminal Velocity
(m/s)
1
2
3
4
5
6
Cross sectional area of coffe filters:____
Density of air (ρ) = ________________
(assume standard temperature and pressure for this lab)
ANALYSIS
1. (4 points) Make a graph with terminal velocity on the y-axis and total mass on the x-axis.
Perform a curve fit to your data and print the graph with the curve fit showing.
2. (8 points) Determine how to linearize your graph. When you have figured out what you need to
adjust, use the last column of the data table to show your calculations.
2. (4 points) Create a new graph that shows your new linearized version. Add a line of best fit and
print the graph.
3. (1 point) From your linearized graph, write the equation for terminal velocity.
4. (3 points) Using this equation, determine the terminal velocity for 8 coffee filters.
5. (3 points) Using your equipment, measure the terminal velocity for 8 coffee filters.
Terminal velocity = ________________________
How close is this value to your calculated value? If there is a large discrepancy, what
could be causing it?
6. (4 points) From your linearized graph, determine the drag coefficient (C). Show all work.
7. (4 points) Now that you know all pieces of the drag equation, determine the acceleration of 4
filters when the velocity is 1/2 of the terminal velocity.
Experiment 13
AP Physics
Air Resistance Lab
PRELIMINARY QUESTIONS
1. (6 points) Using the equation described above in the introduction and a free body diagram,
determine an equation for terminal velocity as a function of mass.
2. (1 point) When the filter reaches terminal velocity, what is the net force acting upon it?
3. (9 points) Sketch graphs of the position vs. time, velocity vs. time, and acceleration vs. time for
one falling coffee filter released from rest.
4. (1 point) If there was no air resistance, how would the rate of fall of a coffee filter compare to the
rate of fall of a baseball?
5. (6 points) Define the following letters in the equation and give units for each:
Letter
Definition
Units
C
ρ
A
DATA TABLE (12 points)
Number
of filters
Total mass of
filters (kg)
Terminal Velocity
(m/s)
1
2
3
4
5
6
Cross sectional area of coffe filters:____
Density of air (ρ) = ________________
(assume standard temperature and pressure for this lab)
ANALYSIS
1. (4 points) Make a graph with terminal velocity on the y-axis and total mass on the x-axis.
Perform a curve fit to your data and print the graph with the curve fit showing.
Air Resistance
2. (8 points) Determine how to linearize your graph. When you have figured out what you need to
adjust, use the last column of the data table to show your calculations.
2. (4 points) Create a new graph that shows your new linearized version. Add a line of best fit and
print the graph.
3. (1 point) From your linearized graph, write the equation for terminal velocity.
4. (3 points) Using this equation, determine the terminal velocity for 8 coffee filters.
5. (3 points) Using your equipment, measure the terminal velocity for 8 coffee filters.
Terminal velocity = ________________________
How close is this value to your calculated value? If there is a large discrepancy, what
could be causing it?
6. (4 points) From your linearized graph, determine the drag coefficient (C). Show all work.
7. (4 points) Now that you know all pieces of the drag equation, determine the acceleration of 4
filters when the velocity is 1/2 of the terminal velocity.