Forces and Motion

Materials: floating and sinking

Physics Narrative for 11-14 Supporting Physics Teaching

Density and floating

Why do some objects float and others sink? We've just noticed that the buoyancy force exerted is equal to the gravity force acting on the water displaced in the bottle-pushing experiment. Now maybe we could replace the material inside the bottle with something other than water. If the replacement makes the bottle heavier than the water, the bottle will make a larger hole to compensate, if lighter, than a smaller hole. If the hole the bottle makes is not large enough to compensate for the added pull of gravity, then the bottle will sink.

But we could forget the bottle, and move on to materials. Now the volume of the material will set the size of the hole made, and so the buoyancy force, whilst the mass of the material will set the pull of gravity.

A challenge

Here is a challenge for you. Consider the list of objects below and decide which ones will sink and which will float when placed in water.

  • 3 kilogram block of iron
  • 10,000 kilogram steel hull car ferry
  • 0.02 kilogram piece of iron
  • 10,000 kilogram wooden hull car ferry
  • 500 kilogram block of wood
  • 5 gram brass screw
  • 10 kilogram piece of wood
  • 5 gram plastic clothes peg
  • 0.5 kilogram football
  • 3 kilogram block of ice

As a general rule, for solid objects, everything made of wood floats and everything made of a metal sinks.

The particles in wood are not so closely packed. There is more space in between the particles and fibres, which gives wood its low density. Metals have their particles closely packed such that there is little space inside. This makes metals denser than either wood or water. The volume of water displaced (the size of the hole made) is small compared to the pull of gravity for high density materials.

We think it's helpful to imagine weighing a set of blocks of materials, all the same size. The blocks that weigh less than the same volume of water will float. The stuff from which they are made is not so densely packed. The blocks that are heavier than the same volume of water will sink. They have a densely packed structure.

A football floats because it contains plenty of air. Air is less dense than water. A football is a combination of a plastic outer shell and air interior. Overall the football is not very dense. For the same reason, steel boats float because their interiors contain so much air that their average density is less than that of water. So it is not strictly true to say that all metal objects sink. If they are shaped so as to contain air they may float: it all depends on the average density.

For solid objects (not shaped to trap air or another low-density material), the rule for floating is that an object will float in water if its density is less than that of water.

This is a reliable rule of thumb – but it's not an explanation.

So floating and sinking are all about the close or loose packing of particles – a property which can be measured by finding the density of a material.

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