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Static Friction

It is a force that keeps an object at rest. This friction is experienced when we try to move a stationary object on a surface, without actually triggering any relative motion between the body and the surface. The force that we apply is balanced by the static frictional force by the surface or floor on which the object is placed.

One interesting thing about static friction is that it is a self-adjusting force. That is, as we increase our force of the push, the force of static friction increases too and the two forces are still balanced. But as we keep on increasing the pushing force a point will come when it overcomes the static friction and the object starts moving.

Consider pushing a sofa across the floor. The floor will exert a force of static friction on the sofa.

Let us plot this situation on a graph.

The force you apply is a horizontal force represented on the X-axis. And the frictional force applied by the floor is represented on the Y-axis.

Now as you increase the horizontal force applied, the frictional force also increases accordingly to keep the sofa at rest. This continues till a point where the sofa just starts moving. And that’s where the static friction ends and the sliding friction begins. The sliding friction is more or less constant if you notice.

 The part of the graph represented by a sloping line is where static friction acts and the part represented by a straight line parallel to the X-axis is where sliding friction acts.

From the above, we can also conclude that the force of static friction is greater than the force of sliding friction and we already know that the force of sliding friction is greater than the force of rolling friction.

Force of Static Friction > Force of Sliding Friction > Force of Rolling Friction

Summary