Discussion With Students
Students will likely be surprised at these results because
they go beyond what is generally found in standard
physics textbooks. However, there are summaries
in the literature that support the conclusion that static
and kinetic friction forces are more complicated than
a simple expression such as Eq. (1) suggests.2-5 Before
getting into any sort of explanation of why the surface
temperatures may affect friction forces, it is interesting
to allow students to put forth their own hypotheses.
Many times students will develop a model that stems
from thermodynamics and chemistry classes, where
they think greater temperatures at the surface mean
molecules are vibrating faster and at greater amplitudes,
and this volatility may create larger “hills and
valleys” that interlock at the surface boundary. Many
physics textbooks will present a friction model in these
shearing terms. This is actually a useful way of picturing
what happens at a microscopic scale, and it is important
for students to try and think of what may be
happening at such small scales.