Friction  (Page 2/5)

Static friction

Static friction comes into play when a body tends to move over another body (surface), but the body has yet not been initiated into motion. When we apply a small force to initiate motion of a block, lying on a horizontal surface as shown in the figure; the temporary joints may not yield to the external force. This means that the force is not sufficient enough to break off the welds, which are formed at contact points. In this case, the external force ( $F_{||}$ ) parallel to the contact surface is equal to the static friction, $f_s$ , i.e.

$$\begin{array}{l}{f}_s=F_{||}=F\end{array}$$

Note that friction force comes into being even before motion is initiated. If we increase the external force in the direction parallel to the contact surface, then friction force increases so that two forces are equal in magnitude and opposite to each other. It is important to understand that friction responds to the net external force parallel to the contact surface to match the magnitude. In this sense, static friction force is a "self adjusting" force against net external force parallel to the contact force (F||) or the component of external forces along contact surface.

Self adjusting force means that if a 2 N force parallel to contact surface fails to initiate motion, then static friction is 2 N; if a 4 N force parallel to contact surface fails to initiate motion, then static friction is 4 N and so on.

We must realize following characteristics of the static friction from the discussion so far :

• So long the body does not start moving, the static friction is equal to the component of net external force in the direction parallel to contact surface.
• So long the body does not start moving, the static friction is a self adjusting force i.e. it varies with net external force in the direction parallel to contact surface.

Also, we must realize that friction force ( $f_s$ ) and component of external force parallel to contact surface ( $f_{||}$ ) are parallel to each other and may not be concurrent in reality. However, we treat them concurrent, while analyzing - for the simple reason that we are considering translational motion. In this situation, the forces can be considered concurrent.