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# Stick a round object in a plastic bottle

## Experimental procedure and explanation:

• Fill the plastic bottle with water and put a ping-pong ball on it.
• The ping-pong ball will not fall, even if you slowly turn the plastic bottle upside down.
• At this time, the water pressure in the gap between the plastic bottle and the ping-pong ball is almost atmospheric pressure. At the top of the plastic bottle, the water pressure is lower than the atmospheric pressure because of the height of the water. Because the plastic bottle is sealed, the pressure inside the plastic bottle can be lower than the atmospheric pressure.
(Difference in vertical pressure) = (Density of water) × (Gravitational acceleration) × (Difference in height)
• The water pressure at the bottom is almost atmospheric pressure, but it is slightly lower than atmospheric pressure because of the amount that supports the weight of the sphere. It is the function of surface tension that creates this slight pressure difference.
• • However, keep in mind that surface tension does not support the weight of the water in the plastic bottle. Most of the force that supports the weight of water is the force caused by the pressure difference between the top and bottom of the plastic bottle.
• The easiest way to remove the ball when turning it upside down is when the plastic bottle is almost horizontal. There is a difference in height in the vertical direction because of the gap between the plastic bottle and the ping-pong ball (circumferential gap), and the difference in pressure becomes large, so this must be supported by surface tension. Therefore, it is important that the clearance is as small as possible and that the plastic bottle and the ping-pong ball are as close as possible.
• In the second half, we also try super balls, yo-yo balloons, and polystyrene foam balls. Some of these are difficult to stick to, so hold the bottle upside down with your hand and then let go. If it doesn't stick well, first squeeze the plastic bottle a little harder, then attach a round object and then loosen the squeeze to make it easier to stick. Go ahead and try this with various other objects.
 [Keywords] Depth and pressure, surface tension [Related items] [Reference] "Trick improvement cheat sheet," TPM Age Editorial Division, Ed., JIPM Solution, P74-75. “The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P38-41. “Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, P18-19.
Last Update：4.13.2021