Why Do Some Objects Float While Others Sink?

Why do some things float and others sink? Imagine a bathtub filled with water. If you drop in a small rock, it sinks, but if you put a big boat in, it floats! It’s all about how much water is pushed out of the way when something goes into the bath. A boat pushes out more water than its own weight, so it floats, while a rock pushes out less, and it sinks.

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Examples

A toy boat floats because it pushes out a lot of water, more than its own weight.
A metal ball sinks in water but floats in mercury because mercury is denser.
A balloon filled with helium floats up because the air it displaces weighs more than the balloon and the gas inside.

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Categories: Physics · buoyancy· density· floatation· science for kids· physics basics