Together with another phys-L-er, Richard Hodson, the air resistance on a
balloon was measured using an ultrasonic motion detector. We will try
to turn this into a student laboratory activity. Here is quick and dirty
report from our first trial.
A mouth-inflated air ballooon (10 of them for $2 at Toys R Us) was loaded
by suspending a small Pb sphere from it. The mass of the sphere and the
balloon was 0.1 kg. The balloon was egg-shaped; the maximum horizontal
"diameter" was 20 cm while the vertical "diameter" was 22 cm. It was
released below the motion detector and traveled about 2.5 meters. Then
the floor was hit at v=4 m/s. The last acceleration was 6.5 m/s^2.
Fnet=m*g-R --> m*a=m*g-R --> R=m*(g-a)
The data lead to a convlusion that R=0.08*v, where R is in N and v in m/s.
Mac Motion was used to collect data at the "rate" of 20 per second, the
"averaging" was 7. We would use higher averaging if more distance were
available to record the fall.
Ludwik Kowalski