Science in the shop

My son gave me a day warning that he wanted some help for a presentation he was giving in science class, so last night we spent a couple hours in the shop building and filming. Edson is in 8th grade and his class is working through the fundamental laws of physics. He was assigned Bernoulli's principle. He knew the venturi effect relied on Bernoulli's principle so he wanted to shoot some video of the venturi burner I built for the forge I built him. Edson has been fascinated with blacksmithing, so I built a forge, but back to the science.

Edson also wanted to replicate an exhibit we saw at the Exploratorium. The exhibit consists of air blowing down through a hole. When a disk is placed over the hole it is held up by the air blowing down. Very counter intuitive but an amazing demonstration of Bernoulli's Principle. We used part of the rig  from Lauren's science fair project to measure the terminal air velocity of various objects (old vacuum motor on a dimmer switch). A design borrowed from the Mythbusters (thanks Adam).

How does it work? The increase in air speed reduces the pressure at the area around the hole but covered by the disk. In this area the thin layer of air is accelerated. The reduced pressure of the accelerated air creates a pressure differential. The force of the higher air pressure in the room pushes up on the dis and is greater than the combined force of gravity and the air blowing the disk down.