Showing that upthrust is real
That upthrust exists can be shown from the reading of a spring balance alone. It takes four steps.
Activity 15.3
- (a) Hang a piece of metal from a spring balance and measure its weight.
- (b) Now apply a downward force to the metal with your hand and read the balance.
- (c) Then apply an upward force to the metal block from below and read the balance.
- (d) Finally immerse the object in water and read the balance.
Figure to be added
The observations
(b) With a downward force, the reading increased.
(c) With an upward force, the reading decreased.
(d) Immersed in water, the reading decreased as well.
Put (c) and (d) side by side
From (b) and (c) we learned this: if a downward force acts on the object the reading rises, and if an upward force acts on it the reading falls.
In (d) the reading fell too. So in (d) an upward force must also have been acting on the object — and the only thing that could apply it is the water.
What this establishes
When an object is immersed in a liquid (a fluid), that liquid produces an upward force on the object.