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Why do things Float or Sink

Why do some things float and some things sink?

  • Washing up bowl or sink filled with water
  • Empty plastic food container
  • Pebbles and stones from the garden

When you drop things into the water, why do some of them bob right back up to the surface while others sink to the bottom and stay there? Time to do some experiments and find out!

  1. Put your plastic food container into the bowl of water. Does it sink or float?
  2. Guess how many pebbles you think you can put into your plastic container before it sinks.
  3. Now, one by one, put your pebbles into the container. How many pebbles did you put into your container before it sank? Did you guess right?

Why did that happen? Well objects that floate are lighter or less dense than water. The empty plastic container is lighter than the water and so it floats.

If the object is heavier or more dense, it will sink. When the stones are added to the container, it starts getting heavier. Eventually the container with the stones becomes heavier (more dense) than the water and then it sinks.

  • Try putting other objects into the bowl of water - lollypop sticks, feathers, potatoes, and even metal spoons
  • Which ones sink?
  • Which ones float?
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