The upward push
Upthrust is the force a fluid exerts upwards on an object immersed in it.
Upthrust
Not water alone but any fluid produces an upthrust on an object immersed in it. It acts vertically upwards.
Where does it come from?
The answer is already in the earlier part of this chapter: liquid pressure increases with depth. The lower face of an object immersed in water is deeper than its upper face, so the pressure on the lower face is greater.
The difference between two forces
A downward force on the upper face — from the smaller pressure.
An upward force on the lower face — from the greater pressure.
The upward force is therefore the larger, and the difference between the two is a resultant acting upwards — which is the upthrust.
Apparent weight
An object's weight does not fall in water — what changes is its apparent weight. Neither its mass nor its weight alters; the balance shows the difference between the weight and the upthrust.
Over the next two pages we demonstrate this with a spring balance, and then measure how large it is.