Lesson 3 of 8
Floating and sinking
Goal: Explain floating and sinking using upthrust.
Water pushes up on anything in it. An object floats if this upthrust can balance its weight.
- Upthrust comes from pressure. The bottom of an object is deeper than its top. So the water pushes up on the bottom harder than it pushes down on the top.
- It floats when the upthrust equals its weight. The forces are balanced.
- It sinks when its weight is bigger than the biggest upthrust the water can give.
- Density decides which. A material that is denser than water, such as steel, sinks. One that is less dense, such as cork, floats.
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Key words in this lesson
Pressure The force on each unit of area
Upthrust The upward push of a liquid or a gas on an object in it
Density How much mass is packed into each cubic centimetre