Sticking a Plastic Sheet to the Ceiling

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Watch the video on YouTube.

What type of experiment is this?

Experimental procedure and explanation:

• Affix a suction cup to a plastic sheet (18 cm × 25 cm) and stick the plastic sheet to the ceiling. Rub it with a towel so that there are no gaps.
• When the suction cup is pulled downwards, the plastic sheet does not come off easily.
• Next, when we use a smaller plastic sheet (12 cm × 18 cm) and hang a 10 kg weight from it, we find that it can withstand a weight of 10 kg. After this experiment, when we attached a spring scale and pulled the suction cup downwards to find out how much force it could withstand, it supported up to approximately 18 kg. This is approximately the same as the force needed to pull up a plastic sheet on top of a table (the size of the force differs slightly because of the surface conditions, etc.).
• The operating principle here is the same as that in “Sticking a Plastic Sheet to a Table,” which can be explained as follows. At the parts where the plastic sheet is in contact with the ceiling, which is only a small part of the total area, the pressure is less than atmospheric pressure, and therefore the force can be supported because of the difference between the contact pressure and the atmospheric pressure on the bottom surface of the plastic sheet. This force, which was a maximum of 18 kg in this experiment, is much smaller than the force due to the atmospheric pressure acting on the bottom surface (approximately 216 kg for a 12-cm × 18-cm plastic sheet), and therefore the plastic sheet can support only a small part of the force due to atmospheric pressure.
• Although this is not shown in the video, when the plastic sheet is pulled down very quickly, a larger force acts on it. This is because of the added force caused by the viscous friction when air flows into the gap at the top, which depends on speed, as well as the force needed to accelerate the air near the top and bottom surfaces of the plastic sheet, which depends on acceleration.

[Note]	When performing an experiment that applies a force over 2 or 3 kg, be sure to conduct a safety check with a teacher. The explanation “a vacuum is formed between the plastic sheet and the ceiling” is incorrect. Mistakes are often found in introductory science textbooks, which explain the operating principle here as that “it is pushed by atmospheric pressure.” When considering the forces acting on an object under atmospheric pressure, be sure to consider the forces on both the front side and the reverse side, rather than on one side only. This experiment is one such example.
[Keywords]	Atmospheric pressure
[Related items]	Sticking a Plastic Sheet to a Table, The Secret of Suction Cups
[Reference]	“Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, P16－17. “The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, P34－37.