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Pascal's Law

Pressure applied to a confined liquid increases the pressure of that liquid uniformly.

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How Does a Submarine Manage To Sink And Rise On Command?

The Secret of the Submarines

Credit: Jim Ellwanger
Source: http://www.flickr.com/photos/65497908@N00/5826616161/in/photolist-9SSVZk-JH2j9-9ZryZn-6sCzzf-6sz2F5-6sDpv9-4AYmmz-agg5iE-4rcRkd-agg4Bj-agdiuT-agg3Xb-actmsG-ab1L4N-abgt1B-actmvE-4T61bT-2DimC-abjj27-6AjdFf-4usfm1-ejgzqc-abjjc3-abZ5SQ-egw99r-5Cog9N-5mQK7j-5fBa7T-5fFvJo-4LUTvT-8F26Lc-8Pi7cr-5qiem8-6WYd1Z-c9rcjq-PCKKm-58dt4R-f1kr75-f16cex-4wkUs-A2p86-9ksSQQ-4zb6Mk-8kM2u-7mXZkP-9JUerV-bAcXyf-7MUShF-7MUP6R-7MYMHY-7MUNRH
License: CC BY-NC 3.0


So how does a submarine sink and rise in the ocean on command? Pascal’s Law can explain this. Try this experiment to understand it better.  You’ll need matchsticks, a plastic bottle, and a balloon.

  1. Fill the bottle completely with water.
  2. Cut off the heads of the matches and drop them into the water.
  3. Cover the mouth of the bottle with the mouth of the balloon.

The match heads should be floating right? Try pressing into the middle of the balloon. They will sink to the bottom! When you lift your finger the match heads should float back to the top. Why is this happening? Pascal’s Law! Pascal’s Law states that pressure applied anywhere to a confined body of fluid will transmit equally to all directions in the container. Basically the matches move down due to pressure that is transmitted through the water. When you press the balloon, the increased pressure pushes a bit of water will go to each match head, making it dense enough that it will sink. When you remove your finger, air pressure in the match heads make them rise again.

How does this relate to submarines? Submarines are similar. The average density of the submarine makes it act the way it does. If it is greater than that of the water, the submarine will float and if it is less then the submarine will sink. The crew can change the density of the submarine by pumping in and pumping out water in ballast tanks.

Creative Applications

  1. Instead of matchsticks, what if you used rocks? Why wouldn’t that work?
  2. Would the time for the matches to sink be different? Try this experiment with different sizes of bottles!

  3. How else is Pascal’s Law used in other machines? Do some research!




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