Weighing Buoyancy

06 September 2009



cups2 plastic glassesStickA stick
Some StringString


Water scalesThe 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?


You should find that the cup you put your finger into, moves downwards.


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.

Upthrust on a fingerReaction force on 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.


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