Chapter 2 Review of Forces and Moments - Brown University

Another test configuration consists of two disks that are pressed into contact and then rotated with
different speeds. The friction force can be deduced by measuring the torque required to keep the disks
moving. An example (from
http://www.ms.ornl.gov/htmlhome/mituc/te53.htm ) is shown below
If you want to see a real friction experiment visit Professor Tullis’ lab at
Brown (you don’t actually have to go there in person; he has a web site
with very detailed descriptions of his lab) – he measures friction
between rocks, to develop earthquake prediction models.
A friction experiment must answer two questions:
(i)
(ii)
What is the critical tangential force that will cause the surfaces to start to slide? The force
required to initiate sliding is known as the static friction force.
If the two surfaces do start to slip, what tangential force is required to keep them sliding? The
force required to maintain steady sliding is referred to as the kinetic friction force.
We might guess that the critical force required to cause sliding could depend on
(i) The area of contact between the two surfaces
(ii) The magnitude of the normal force acting at the contact
(iii) Surface roughness
(iv) The nature of the crud on the two surfaces
(v) What the surfaces are made from
We might also guess that once the surfaces start to slide, the tangential force needed to maintain sliding
will depend on the sliding velocity, in addition to the variables listed.
In fact, experiments show that
(i) The critical force required to initiate sliding between surfaces is independent of the area of contact.
This is very weird. In fact, when Coulomb first presented this conclusion to the Academy Francaise, he
was thrown out of the room, because the academy thought that the strength of the contact should increase
in proportion to the contact area. We’ll discuss why it doesn’t below.