Upthrust on an object in a liquid

When an object is put into water (or any other liquid) there are forces on the object due to the liquid. The deeper down in the liquid you go and the bigger the density of the liquid the greater these forces are.

You can see from the drawing that the pairs of forces on the two sides of the box are equal and so cancel out but the forces on the bottom of the box are greater than those on the top. This means that there is a difference between the forces on the top and those on bottom of the box.

The resulting force on the bottom of the box pushes it upwards and this is called the UPTHRUST on the box.

The upthrust on an object stays the same no matter how deep it goes (as long as it is completely submerged). This is because the upthrust depends only on the DIFFERENCE between the forces on the top and bottom of the object.

If the upthrust on the object is bigger than its weight the object rises, some of it comes out of the water until the upthrust is equal to its weight and the object floats.

If the upthrust on the object is smaller than its weight the object sinks to the bottom of the liquid.

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.