Friction

Friction

Friction is opposing force that is set up at the surface of contact when one body slides or rolls or tends to do so on the surface of another body.

Classical view

According to the old view, a surface is seen smooth to the naked eye has many irregularities when seen through a microscope as shown in the figure. If two surfaces come in contact, there is interlocking between irregularities and force is required to break it. This force measures the force of friction.

Modern View

When two surfaces come in contact, the actual microscopic area of contact is much less and the molecules at these points come so close to each other that a strong intermolecular force acts on each other. So a force is required to break it. When the normal reaction increases, the area of contact also increases. That is why the force of friction also increases.

Static Friction and Kinetic Friction

When a block is resting on a horizontal surface, the block presses the surface due to its weight mg. The surface pushes up on the body with a force perpendicular to the surface called normal reaction. If we apply a small horizontal force F on the block, it does not move. If we go on increasing the force, the block remains at rest till to certain magnitude of the force and then, starts to slide. The friction of friction between two surfaces when the body is in static condition is called static friction. The maximum value of static friction is called limiting friction.

Initially, it requires more force to break the interlock between the irregularities of two surfaces. But once the block is in motion, it takes the time to interlock between the irregularities of two surfaces. The body gain inertia of motion. So the force of friction is reduced. The force of friction when block is sliding is called kinetic or dynamic friction. For a limiting speed, the kinetic friction remains constant throughout the motion. Kinetic friction is always less than limiting friction.

A graph between the applied force and force of friction is as shown in the figure. AD represents the maximum static friction called limiting friction. CB represents the kinetic friction. Kinetic friction may be of two types i.e. sliding and rolling friction.

Sliding Friction

The opposing force that comes into play when one body is actually sliding over the surface of another body is called sliding friction. For example, when a flat block is moved the flat surface of a table the opposing force is sliding friction.

Rolling friction

The opposing force that comes into play when one body id actually rolling over the surface of the other body is called rolling friction. For example, when a wheel or a circular disc or a ring or a sphere or a cylinder rolls over a surface, the force that opposes it is the rolling friction.

Laws of Friction

Following are the laws of the friction:

1. The frictional force between two surfaces opposes their relative motion.
2. The frictional force depends upon the roughness of the surface.
3. The frictional force acts parallel to the surfaces in contact.
4. The frictional force is independent of the area of contact between the two surfaces.
5. The kinetic friction is independent of the relative velocities of the surfaces.
6. The magnitude of the frictional force is directly proportional to the normal reaction.

Coefficient of Friction

Let a body be at rest on a horizontal table as shown in the figure. When a force F_a is applied to it horizontally, the body just begins to slide along the surface. From the laws of friction, the force of friction F is proportional to the normal reaction R.

\begin{align*} F &\propto R \\ \text {or,} \: F &= \mu R \\ \end{align*}

where µ is a constant of proportionality called coefficient of friction between two surfaces in contact.

$$ \therefore \mu = \frac FR \dots (i) $$

The coefficient of friction is the ratio of friction force to the normal reaction. It is a unitless quantity and its value depends on the material of the objects. If the object is in the motion, it is then we replace F by F_k and µ by µ_k. So equation (i) becomes

$$ \mu _k = \frac {F_k}{R} $$

Where F_k is the force of kinetic friction and µ_k is the coefficient of kinetic friction.

Static Friction is a Self-Adjusting Force

When a block is at a rest and no force is applied, the force of friction is zero. When the magnitude of the applied force is increased till it does not move, the magnitude of the static friction also increases and the direction of static friction and applied force are opposite. That means, the magnitude and direction of static friction and applied force are opposite. That means, the magnitude and direction of static friction adjust itself according to the magnitude and direction of the applied force.