Purpose: To study the relationship between air drag forces and the velocity of a falling body.
Equipment: Computer with Logger Pro software, lab pro, motion detector, nine coffee filters, meter stick
FD = k |v|n, where the power n is to be determined by the experiment.
Procedure:
NOTE: You will be given a packet of nine nested coffee filters. It is important that the shape of this
packet stays the same throughout the experiment so do not take the filters apart or otherwise
alter the shape of the packet. Why is it important for the shape to stay the same?BECAUSE IF THE SHAPE CHANGES SO WILL THE DRAG.
1. Login to your computer with username and password. Start the Logger Pro software, open the
Mechanics folder and the graphlab file. Don’t forget to label the axes of the graph and create an
appropriate title for the graph. Set the data collection rate to 30 Hz.
2. Place the motion detector on the floor facing upward and hold the packet of nine filters at a minimum
height of 1.5 m directly above the motion detector. (Be aware other of nearby objects which can
cause reflections.) Start the computer collecting data, and then release the packet. What should the
position vs time graph look like?
the graph should look like a line with a negative slope
Verify that the data are consistent. If not, repeat the trial. Examine the graph and using the mouse,
select (click and drag) a small range of data points near the end of the motion where the packet
moved with constant speed. Exclude any early or late points where the motion is not uniform.
3. Use the curve fitting option from the analysis menu to fit a linear curve (y = mx + b) to the selected
data. Record the slope (m) of the curve from this fit. What should this slope represent? TERMINAL VELOCITY
.
Repeat this measurement at least four more times, and calculate the average velocity. Record all
data in an excel data table.
4. Carefully remove one filter from the packet and repeat the procedure in parts 2 and 3 for the
remaining packet of eight filters. Keep removing filters one at a time and repeating the above steps
until you finish with a single coffee filter. Print a copy of one of your best x vs t graphs that show the
motion and the linear curve fit to the data for everyone in your group (Do not include the data table;
graph only please).
5. In Graphical Analysis, create a two column data table with packet weight (number of filters) in one
column and average terminal speed (|v|) in the other. Make a plot of packet weight (y-axis) vs.
terminal speed not velocity (x-axis). Choose appropriate labels and scales for the axes of your
graph. Be sure to remove the “connecting lines” from the plot. Perform a power law fit of the data
and record the power, n, given by the computer. Obtain a printout of your graph for each member of
your group. (Check the % error between your experimentally determined n and the theoretical
value before you make a printout – you may need to repeat trials if the error is too large.)
6. Since the drag force is equal to the packet weight, we have found the dependence of drag force on
speed. Write equation 1 above with the value of n obtained from your experiment. Put a box around
this equation. Look in the section on drag forces in your text and write down the equation given there
for the drag force on an object moving through a fluid. How does your value of n compare with the
value given in the text? our value of n is close to 2, matching n=2 therefore it is parabolic and our calculations were accurate.
Errors: cross sectional area, messing up shape of filters, air coming in and pushing the filter away. human interference with the motion detector.
conclusion:we were able to learn about the relationship between air drag forces and velocity of a falling object, when speed increases drag increases, when acceleration decreases drag increases.
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