An apparatus that at once shoots a billiard ball horizontally and drops another one vertically from an equal height. Even though the two have different initial velocities, they both accelerate in the same direction and at the same rate due to Earth's gravity--this is confirmed by seeing and hearing both balls land simultaneously.While we did our best to make sure the setup is level and the apparatus is precise, the video shows the balls actually land about 0.02 s apart from each other (the slow-motion part was done in 60 fps, and there seems to be a difference of about 1 frame). We consider this difference to be negligibly small. Normally the apparatus is positioned about 2 meters above the floor, and the difference in landing time is just as imperceptible.
You will notice the "drop-ball" bounces towards the center of the picture. We think this is mostly because the concrete floor has small pockmarks and other local irregularities, which on average combine to form a level surface but individually can cause funny bounces. The ball may also have a very small horizontal velocity due to the way in which it is dropped.
For more details on our setup see http://sciencedemonstrations.fas.harvard.edu/icb/icb.do?keyword=k16940pageid=icb.page80669pageContentId=icb.pagecontent194040&state=maximize&view=view.do&viewParam_name=indepth.html#a_icb_pagecontent194040
Thanks to Daniel, Doug, and Nils for their help.
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