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Take a look at the reeds in this video…

That’s right, they are floating along on the canal.

Have you ever stopped to wonder why some things float and some things sink? Well, most explanations of why things float revolve around the Archimedes Principle, but he stated that principle over 2,200 years ago and we have a much better model to work with these days.

The Archimedes Principle is stated as:

Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.

But I could just as well say, “Any object resting on a table is supported by a force equal to the weight of the object”.  But what is useful about that statement? And, what is misleading?

Why Some Things Float

Well, you know that if the table wasn’t there, the object would fall towards the Earth.  So, in fact both statements are specific cases of Newton’s third law:- For Every Action There Is An Equal And Opposite Reaction. What’s misleading is, it doesn’t tell you where that force, or more correctly, reaction, comes from.


Here’s an experiment you can do to see what’s really going on:

You Need:

Place the bucket of water on the parcel scale and note the reading (or zero it). Attach the brick to the luggage scale and note the reading. Lower the brick into the bucket of water and observe:

The second observation tells you that the “force” does not come from the water. An amount of weight equal to the buoyant force is transferred to the parcel scale. Indeed, when an object is supported by a table, the “force” does not come from the table. On the table, the weight of  the object is transferred  to whatever the table is resting on by a process called “Strain”.  Ultimately, the weight is transferred to the Earth.  

WHY THINGS FLOAT OR SINK

In our experiment, the water was displaced upwards because it is contained in a bucket. The water in a canal is contained by the sides and ends of the canal stretch, so any displacement of water must also upwards. We can say the same thing about lakes. Now here’s where you need a bit of imagination. All the bodies of water in the world, including the oceans, are contained by gravity. The water has already gone as far down and as far out as it can go, it can only go upward.  Therefore, water is always displaced upwards.

Newton’s third law, “for every action there is an equal and opposite reaction”, means that without mechanical advantage (gears, levers, pulleys etc) a body can only lift its own weight. Therefore a body placed into water will sink into it until either:

THE RELEVANCE OF DENSITY

Water has a density of 1 Kg/L (1 kilogram per litre). Any object that has an average density greater than 1 Kg/L will not displace its own weight of water and will sink. Any object that has an average density less than 1 Kg/L will displace its own weight of water and will float.