1. 1. Friction
• The force acting along the two surfaces in contact which opposes
the motion of one object over the other is called the force of
friction.
5. . Static Friction: The opposing force that comesinto play when one object tends to move over the surface of
another, but the actual motion has not started is called static friction. It prevents the motion and does not
allow two objects to slide upon one another.
6. Limiting Friction:
If you move a heavy box from one place to another the
force required to overcome friction, at the instant the
box starts moving from rest, is a measure of limiting
friction. The static friction at this stage is maximum.
This maximum value of static friction is called the
limiting friction.
Hence, limiting friction is the maximum opposing force
that comes into play, when one object is just at the
verge of moving over the surface of another object.
7. Kinetic Friction:
When we increase the applied force slightly beyond the limiting friction, we see that the box
begins to slide slowly. This does not mean that friction has disappeared. It only means that the
applied force is now greater than the force of limiting friction.
The Force of friction at this stage is called Kinetic friction. Hence, kinetic friction is the opposing
force that comes into play when one object is actually moving over the surface of another object.
There are two types of kinetic motion-sliding friction and rolling friction.
a) Sliding fiction. The opposing sece that comes into play when one object is actually shding over the
surface of another object is called sliding friction (Fig. 124 (4)). For emple, when a log of wood is
moved on a flat surface, it shoes In this case, theopposing once is called siding metion
b) Rolling friction:The opposing force that comes into play when one object is actually rolling over
the surface of another object is called rolling friction For example, when a wheel or a log of
wood rolls over a flat surface, the opposing force is the rolling friction
