Buoyancy

A fun experiment with water is to see which objects float. Children could have many theories for why different objects float. It can be interesting for them to notice that it is not just the size of the object that affects whether an object floats. For instance, a feather will normally float, but a marble won’t even though the marble might be smaller in size than the feather.

Ultimately, an object will float if it is less dense than water. For an object to float, the forces of the weight of the object and the upthrust of the water must be balanced. They become balanced through the dispersion of water. If the forces can be in balance by dispersing an amount of water which has a volume smaller than the object, the object will float. If an object can float, we can say it is buoyant. To see more discussion about forces, you can see our article on air resistance and gravity.

Experimenting with string could be a fun activity to try. A dry string floats as it is less dense than water. If we then submerge the string in water (so it soaks up water), it now sinks as it has become more dense than water. In the images below, we can see a dry piece of string floating on the surface of the water and a piece of string full of water that has sunk to the bottom.

The density of an object is determined by the mass and volume of the object. As given in the equation below: \begin{equation} \text{density } = \frac{\text{ mass }}{\text{volume}} \end{equation}

In the case of the string, soaking it in water maintains the volume but increases the mass, so the density increases. The density becomes larger than the density of water, and so the string sinks.

We could also decrease the density of an object by changing its volume. A good example of this is popcorn. When a popcorn kernel pops, its volume increases. Its mass decreases a little too, but this change is less significant here. As the volume has increased, the popped popcorn has a lower density than the kernel. As the kernel is denser than water and the popped popcorn is less dense than water, the kernel sinks and the popped popcorn floats. In the image below, we can see a popped piece of popcorn floating on the surface of the water while the kernel sinks to the bottom.