What properties of a ball influence how it rolls down an inclined plane? This question can be used to introduce a playground investigation. A group of students is likely to bring up mass, size, material and possibly friction. Depending on their background they may or may not need a prompt to list more specific material properties, including density, elasticity and surface properties.
A next step is to come up with experiments to test the different possible effects, with access to a number of different balls, e.g. as shown in figure to the right. Students may need hints that rolling pairs of balls next to each other gives a more direct comparison than measuring the time required for individual balls.
Can you find pairs of balls that roll with nearly the same acceleration down the slide? Can you find balls that roll with a different acceleration?
You may also compare bottles or jars filled with sand, rice of water.