Coffee Filter Lab
As objects fall, they increase their speed due to the downward pull of gravity. Air resistance
counteracts gravity's pull by resisting the downward motion of the object. The amount of air
resistance depends upon a variety of factors, most noticeably, the object's speed. As objects
move faster, they encounter more air resistance. When the
amount of upward air resistance force is equal to the downward
gravity force, the object encounters a balance of forces and is said
to have reached a terminal velocity. The terminal velocity value
is the final, constant velocity value achieved by the falling object.
A group of physics students are investigating the terminal
velocity values obtained by falling coffee filters. They videotape
the falling filters and use video analysis software to analyze the
motion. The video is imported into the software program and the
filter's position in each consecutive frame is clicked on (see
Figure 1). The software uses the position coordinates to generate
a plot of the vertical velocity as a function of time. Figure 2
shows the velocity versus time
graph that resulted from the
analysis of the motion of a
single filter.
The lab group then
investigated the effect of mass
on the motion of the falling
filters. They stacked varying
numbers of pleated coffee
filters tightly together and
analyzed the motion of the
stacks of filters. They
determined the terminal
velocity of the stacks of filters. The students also measured the mass of the filters to determine
their weight and used the value to determine the amount of air resistance encountered by the
filters. The results of several trials are shown in Table 1. The terminal speed (i.e., velocity) as a
function of mass is shown in Figure 3.