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.