TYPES OF FORCE
Application of Newton's Laws
CP-04
Page 18
Rev. 0
Figure 9 Frictional Forces
Fluid friction develops between layers of fluid moving at different velocities. This type of frictional
force is used in considering problems involving the flow of fluids through pipes. Such problems are
covered in the Fundamentals Manual on fluid flow. In this section, problems involving rigid bodies
which are in contact along dry surfaces are considered.
The laws of dry friction are best understood by the following experiment. A block of weight W is
placed on a horizontal plane surface (see Figure 9). The forces acting on the block are its weight
W and the normal force N of the surface. Since the weight has no horizontal component, the normal
force of the surface also has no horizontal component; the reaction is therefore normal to the surface
and is represented by N in part (a) of the figure. Suppose now, that a horizontal force P is applied
to the block (see part (b)). If P is small, the block will not move. Some other horizontal force must
therefore exist which balances P. This other force is the static-friction force F, which is actually the
resultant of a great number of forces acting over the entire surface of contact between the block and
the plane. The nature of these forces is not known exactly, but it is generally assumed that these
forces are due to the irregularities of the surfaces in contact and also to molecular action.
If the force P is increased, the friction force F also increases, continuing to oppose P, until its
magnitude reaches a certain maximum value F (see part (c) of Figure 9). If P is further increased,
M
the friction force cannot balance it any more, and the block starts sliding. As soon as the block has
been set in motion, the magnitude of F drops from F to a lower value F . This is because there is
M
K
less interpenetration between the irregularities of the surfaces in contact when these surfaces move
with respect to one another. From then on, the block keeps sliding with increasing velocity (i.e., it
accelerates) while the friction force, denoted by F and called the kinetic-friction force, remains
K
approximately constant.
