2B40.41b - Buoyancy Paradox - Boat With and Without Atmosphere

Code Number:
Demo Title:
Buoyancy Paradox - Boat With and Without Atmosphere
Area of Study:
Vacuum Pump and Bell Jar System, 9 Inch Evaporating Dish, Metal Boat with 20 Gram Mass on the Inside, and Clear Vacuum Pump Oil.


Place the 9 inch evaporating dish on the vaccum plate and pour in the vacuum pump oil to the 2/3 full level.  Place the boat into the oil.  It may be necessary to add some small masses to the front of the boat so that the level of the liquid is at the red water line on the boat.  Place the bell jar over this and evacuate for at least 24 hours to outgas the pump oil.

Before class you may bring the demo up to atmospheric and remove the bell jar so that the students can see that we are starting the demo with an atmosphere above the boat.  Then place the bell jar over the demo and evacuate.  The point of the demo is that the water line of the boat stays in the same place whether there is atmosphere above the boat or whether there is vacuum as the buoyancy of the liquid is the dominating factor.  You should be able to let the vacuum pump run for quite some time without any observable bubbles if the outgasing was done properly.

  • Paul Hewitt, "Figuring Physics", TPT, Vol. 34, #2, Feb. 1996, p. 108.

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