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How to Make a Plastic Cup Submarine That Sinks and Floats

You know if you put a plastic cup on a bowl of water it floats, but if you gradually fill the cup with water it gets lower in the water and eventually the cup sinks. This is because the air is replaced with water and while a plastic cup doesn̵7;t weigh much, once it̵7;s full of water the gravitational pull is greater than the upward force. However, by adjusting the air pressure inside the plastic cup you can make it into a submarine that sinks and floats.

Things You'll Need

Bowl
Small plastic cup
Plastic cling wrap
Sticky tape
Plastic container with tight-fitting lid (about four or five times larger than the cup)

Instructions

- 1
  Fill a bowl with water. Fill a small plastic cup with enough water so when you put the cup in the bowl it only just floats. You may need to experiment to get it just right. This is your submarine.
- 2
  Remove the cup from the bowl of water. Wipe the submarine cup dry using a paper towel.
- 3
  Put a couple of layers of plastic cling wrap over the top of the cup to seal it so the water can̵7;t get out. If it doesn̵7;t seal properly, use a strip of tape and wrap it around the edge of the cup.
- 4
  Fill the plastic container with water almost to the top.
- 5
  Place the plastic cup into the container. Putting the cup in the container might make the water overflow, so do this gently.
- 6
  Put the lid on the container and tighten. The submarine cup will float near the top of the water.
- 7
  Squeeze the plastic container using your hands. The submarine cup starts to sink. Let go of the container and the cup rises to the surface again. The effect of squeezing the container increases the air pressure in the cup. This makes it denser and effectively heavier, so the cup sinks. When you release the container, the air pressure falls, so the cup rises to the surface.

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