# Upthrust in fluids

Question: Define upthrust and give examples

Upthrust – this is all to do with objects immersed in fluids, either partially (some of the object sticks out of the fluid) or totally (all the object is below the fluid).

A fluid can actually be a liquid or a gas but to make life easy we will stick to a liquid – say water. We all know what that is!

Upthrust is defined as the upward force on the object provided by the liquid because the object has "displaced" some of the fluid. The volume below the water level that is now occupied by the object used to be filled with water and if the object has a different density than water there will be an upward force on the object. If this force is less than the weight of the object the object will sink.

If you lift a heavy stone by a rope and then dangle it in water you will find it appears to weigh less, this is because of the upthrust pf the water on the rock. It is much easier to lift things in a swimming pool – the upthrust of the water helps support them.

If something floats the upthrust is equal to the weight of the object.

Archimedes explained this by his principle:
When a body is wholly or partially immersed in a liquid there is an upthrust which is equal to the weight of liquid displaced.

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