Science Buddies: "Ask an Expert"

So as apart of my Science grading a few mates and I have volunteered to host a physics fair day. Two of experiments I am doing I can't explain confidently. One of them I have to explain why the the ping pong ball is attach to a piece of string and tape stays in the flowing water current, and the second is to explain why a ping pong ball follows the water current when it is being moved from someone moving the tap. Both experiments can be found just below for more clarification. Thanks!

https://www.youtube.com/watch?v=Gy0SwPSc8Qw

juliusvandenhe00,

Thanks for your questions, sorry for the late reply. But here it goes.

In all these cases, the Bernoulli principle is at play here, which states that for a fluid in motion it's velocity and pressure are inversely proportional to each other. So for an object that is aerodynamically symmetric like a ping pong ball, when the water hits on one side, the pressure on that side is lower than on the opposite side. So as the water is moved (slowly enough) the higher pressure on the opposing side serves to continually push the ball into the region of low pressure where the water is flowing. Similarly for the ball balanced on a column of air. The symmetric nature of the ball attached to the string. When the ball comes in contact with the flow of water, there is an immediate drop in pressure on that side causing the higher pressure to try and push the ball into the steam. Now this is balanced by the physical water hitting the ball on that side which keeps the ball from being pushed all the way into or through the stream and out the other side.

Here's a great video that further explains the relationship between pressure and velocity in Bernoulli's principle: https://www.youtube.com/watch?v=UJ3-Zm1wbIQ

Hope this helps with your projects!