We will consider k as a coefficient to be found by fitting this
simple model to the experimental data. (Deriving an expression
for k from first principles may be a rather daunting task,
well beyond our present scope. See Ref. 1 for more details.)
Defining a characteristic time s ¼ ð3=2Þðm=kÞ and a characteristic
distance v ¼ ð3=2Þðm=kÞ2g sin h, Eq. (4) can be
written in the following dimensionless form