Observations and results
Did the longer pendulum have a longer period than the shorter pendulum? Was the longer pendulum's period not quite twice as long as the shorter pendulum's period?
A pendulum's period is related to its length, but the relationship is not linear. A pendulum that is twice as long as another pendulum does not simply have a period that is also twice as long. The exact periods of your longer and shorter pendulums may be slightly less than 1.7 seconds and 1.2 seconds, respectively, because of friction and because their lengths were less than 70 cm and 35 cm (due to strings being used to tie to attachments).
Perhaps the most famous pendulum is Foucault's pendulum, which showed the earth's rotation in the mid-1800s. One of the first known pendulum uses was in about A.D. 100 when a Chinese scientist, Zhang Heng, used it to detect distant earthquakes in a device called a seismograph. Today pendulums have many applications, including measuring local gravity and helping guide ships and aircrafts.
More to explore
"Pendulum Lab" from the University of Colorado at Boulder's PhET project
"Pendulum Exhibits Periodic Motion" from the School for Champions
"Swing Low: Investigate the Motion of a Pendulum" from Science Buddies
This activity brought to you in partnership with Science Buddies