Rocks in Water
Many children enjoy dropping rocks into water and watching the splashes. After a few attempts at dropping rocks, they may notice that a few factors have a large impact on the size of the splash. The rock’s size, shape and the height it is dropped from.
The children may want to experiment with how changing these features changes the size of the splash. We call the features of our experiment that we will measure variables. There are different types of variables depending on whether we will change them throughout the experiment.
Independent variables are variables that change throughout the experiment; this could be the height the rock was dropped from or the mass (similar to weight but technically different) of the rock. When performing an experiment, it is good practice to only have one independent variable; otherwise, it becomes difficult to work out what changed the outcome.
Dependent variables are the variables that we measure in our experiment. Here, this is the size of the splash. We could either do this by taking a video of the splash and comparing the highest point or the area of the splash. We could also just use the children’s judgement for the size of the splash.
Control variables are variables that remain the same and don’t change. If we are changing the mass of the rock (this is our independent variable), then the height that we drop the rock from should be kept the same and be a control variable. We should also try to keep the shape of the rock similar; this is quite hard in practice. One alternative that could make this experiment fairer is to replace the rock with some playdough; this way, we could have more control over the shape of the object being dropped.
In this experiment, it is better to drop the rock and not throw it, as the amount of energy given to the rock by a throw can differ, making the test unfair. When the rock is dropped, it accelerates due to the force of gravity acting on it. You can read more about this in our article about acceleration due to gravity.
Some children performing this experiment were able to recognise that there was a correlation between the height dropped and the splash size, and the size of the rock and the splash size when dropping some rocks multiple times. The size of the splash created depends on the amount of energy transferred to the water by the rock. If there is more energy, the splash will be larger (there are other considerations that are not discussed here). The energy of the rock is determined by the mass of the rock (similar to weight but technically different) and the height from which the rock is dropped. Before the rock is dropped, it has potential energy equal to: \begin{equation} \text{energy } = \text{ mass } \times \text{ height } \times \text{ acceleration due to gravity}. \end{equation}
When it the dropped from a greater height, the rock has more potential energy, and when the rock has greater mass, it has more potential energy. As it falls, the rock accelerates and the potential energy becomes kinetic energy. When the rock hits the surface of the water, the energy is transferred to the water, which then influences the size of the splash. For more of a discussion about energy, see our article on conservation of energy.
