What you Need
What to do
The idea is to build some scales, with a glass of water on each side. So first you want a way of attaching the cups to some string.
One way of doing this is by cutting two holes in each cup, slightly below the rim.
Then tie the cups to the stick, and attach a piece of string to the centre of the stick.
Now fill the two glasses almost full.
Add water so that the stick balances when hung from the centre piece of string.
Put your finger in one cup, without touching the sides. What happens?
What may happen
You should find that the cup you put your finger into, moves downwards.
Why does it happen?
One way of thinking about this, is that if you put anything in water - a boat, a rubber duck or just your finger, it will feel an upthrust force pushing up on it. This is the force which makes boats float, and is equal to the weight of water that is displaced by the object.
Issac Newton worked out that if you apply a force to something, you will feel an equal and opposite force (every action has an equal and opposite reaction). So if the water is applying an upwards, upthrust force to your finger, your finger must be applying a downward force to the water.
Another way of thinking of it is that when you put your finger in the water, it will increase the level of water in the cup. This means that there is more water pressure at the bottom of the cup, acting on the same area, so there is a greater force.
We received this question
The force on the bottom of the cup is the pressure times the area of the bottom of the cup.
Here was my reasoning, before seeing the results: I will push down, or actually hold my finger up less, exactly the same amount as the buoyancy the water provides. Thus there will be the equivalent weight pushing down. Here is how I would test this - set up the same experiment, but use an inanimate object *placed on the floor* (or other surface). It would have to be an inverted U of some kind, or held in a clamp for example. I suspect that as it will *not* compensate for the up thrust from the water, there will be no increased down thrust (weight). The 'water is higher => more pressure => more weight' idea does not seem possible to me, or a taller container would have an effect, which obviously it would not (assuming an equal weight of the two containers). Jerome, Sun, 3rd Jan 2010
Galileo came up against these forces when under house arrest on the instructions of the Vatican.
You feel the upthrust because your finger is less dense than water. If your finger was made of iron, you would feel no upthrust. The extra weight shown on the scale is the weight of the water displaced by the part of the finger that is under water.q76pqv Al, Sun, 25th Apr 2010
If your finger was made of iron, there would still be an upthrust. The way I visualise displacement is to think of the volume of water that is being displaced being 'lifted' at the top of the container. To lift the displaced fluid takes a force, and this is the bouyancy that is felt. That is why the bouyancy of a non-floating object is related to the volume, not the mass or weight. Ian, Tue, 15th Nov 2011