## Thursday, December 8, 2016

### Suspended Disbelief

I was brainstorming ideas for physics news, and my mom described to me this machine that she’s planning to build. My mom is an early childhood educator, and many of her programs focus on the STEM initiative. In other words, she builds machines and designs experiments that demonstrate cool science phenomena in the hope that she might inspire the next generation of scientists. The machine she had in mind uses a fan and a network of tubes to launch scarves across the room. The mystery of the machine is that while it can shoot scarves impressive distances, ping-pong balls float above the end of the final vertical tube rather than launching. The ball travels through the tubes just as the scarf would, but once it reaches the end of the tube, it becomes suspended in midair. I did some researching and found an explanation: the Coanda Effect.

While suspended, the ball is held in place by a number of forces. There is obviously the downward force due to gravity and the upward force due to the stream of air. We can imagine these forces balancing. However, we must also consider the lateral forces acting of the ball, preventing it from “slipping” away from the stream. When the stream hits the underside of the ball, it is deflected around the ball in a cylindrical column. As the fast-moving air passes by the rings of slow-moving air surrounding the upper-half of the ball, fluid friction causes this slow-moving air to flow with the column. When the air leaves the space around the upper-half of the ball, a small low-pressure zone is created. The Coanda Effect dictates that when this low-pressure zone is created, the stream of air changes course by curving around the low-pressure zone and fixing the ball in place. We can imagine the low-pressure zone as an invisible surface that the air flows against.