Why some things float and some sink
We know that a stone dropped into a vessel of water sinks, while something like a piece of wood floats on the water.
But why? The wood is a solid object just as the stone is; the weight of each acts downwards. So what scientific principle makes one sink and the other float?
What we need to find out
- Is there an upward force on an object in water?
- If there is, what fixes its size?
- How is that force related to the weight of the object?
The first clue comes from experience
Hold a floating object like a piece of wood down on the water with both hands, and you feel an upward force from the water against your hands.
Even with an object that sinks, the weight we feel while it is under water is less than its weight in air. A bucket in a well feels light while it is still submerged and heavy the moment it clears the water — that is the everyday version of this.
Giving it a name
This happens because the water exerts an upward force on an object immersed in it. That force is called upthrust.
It is not water alone: any fluid produces an upthrust on an object immersed in it — air included.