Children’s understanding of friction between surfaces develops rapidly because it is something that is continually experienced in a wide range of informal situations. Friction between non-solid materials is less easy to grasp although the general idea that friction between surfaces is reduced when wet is an easy one to begin to develop. Getting children to test the impact of different liquids (water, soap liquid, cooking oil, etc.) on the degree to which they ‘lubricate’ (i.e. reduce frictional force between two surfaces) helps the idea to develop.
Air resistance provides the additional complication of the insubstantial nature of air. Children can experience air resistance by trying to run with open umbrellas held horizontally but in terms of helping them and adults to refine their ideas about the frictional force between air and objects, there is a clear progression in thinking that needs to be considered:
If presented as a frictional force with one of the surfaces as a fluid, the sequence is similar for friction. Additional considerations are:
Air resistance is a force that slow down objects that are moving through the air
This is because the air is made of particles, like any other material and these rub on the surface of a falling object, the resulting friction slowing down the rate of acceleration due to the downward pull of gravitational force
The greater the surface area of the object, the greater air resistance
Obviously the more surface there is, the greater the number of particles that the falling object is likely to collide with resulting in increased friction.
The faster an object is falling through air, the greater the air resistance
The faster that the object is falling increases not only the frequency of impact but the force of impact which results in greater friction forces acting between the object and the air.
When objects falling to the ground are no longer accelerating but travelling at a constant velocity, the forces of air resistance and gravitational force are said to be balanced
The Physics of Falling
Balanced and Unbalanced Forces