Weighing the three objects
The answer to why objects A, B and C behave differently comes from comparing the weight with the upthrust.
Activity 15.5
You need three objects made of different materials: A must float partly immersed in water; B must float fully immersed; C must be an object that sinks.
One like B can be made by putting a suitable amount of sand into a small bottle that can be closed tightly so no water gets in.
Using a spring balance, measure the weights of A, B and C.
Now measure the apparent weight of A while it floats, of B while it floats fully immersed, and of C while it rests submerged.
Figure to be added
The readings one student obtained
| Object | Weight of the object (N) | Apparent weight in water (N) | Observation |
|---|---|---|---|
| A | 1.1 | 0 | floats partly immersed |
| B | 1.8 | 0 | floats fully immersed |
| C | 2.4 | 0.5 | sinks |
The upthrust worked out
| Object | How it sat in the water | Weight of the object (N) | Upthrust (N) |
|---|---|---|---|
| A | floated partly immersed | 1.1 | 1.1 |
| B | floated fully immersed | 1.8 | 1.8 |
| C | sank | 2.4 | 1.9 |
The observation
A and B — for both floating objects the weight is equal to the upthrust.
C — for the object that sinks the weight (2.4 N) is greater than the upthrust (1.9 N).
That the apparent weight of both floating objects is zero says the same thing — the spring balance feels no weight at all, because the object's weight is entirely balanced by the upthrust.