# Friction

Friction is a force that resists motion, making an object in motion or at rest stay that way. It's kind of like when you try to slide a book across a table and it stops moving. That's friction at work. We rely on friction to walk and drive cars. It's caused by atoms and molecules interacting, even though objects may seem smooth on the surface. Sometimes friction can be a pain, so we use lubricants to reduce it. For example, machines need oil-based lubricants to keep parts from wearing down. Friction is pretty important in our daily lives!

When two surfaces touch, even if they look smooth, there are actually tiny bumps and dips that cause friction. It's impossible to make something perfectly smooth. The law of conservation of energy says that energy can't be destroyed, only transformed. So, when friction happens, it creates heat energy that spreads through the objects and medium around them.

## Static frictional force

When objects aren't moving, the frictional force between them is called the static frictional force. Basically, this is the force that keeps from sliding around. This force is measured in Newtons, which is a unit of force. The frictional force acts in the opposite direction to the applied force. For example, if there's a block with a mass of m and a force F acting on it, but it's not moving, that's because of the static frictional force.

An object has four forces acting on it: the gravitational force (mg), the normal force (N), the static frictional force (fs), and the applied force (F). When these forces are balanced, the object remains in equilibrium. If the applied force is greater than the frictional force, the object will start moving. The frictional force is proportional to the normal force, which is the force that holds the object up against gravity. This means that lighter objects have less friction. To remove the proportionality, we use a constant called the coefficient of static friction (μs). The maximum static frictional force is μs⋅N, where N is the normal force and μs is the coefficient of static friction. If the applied force is less than or equal to μs⋅N, the object will remain at rest. If the applied force is greater than μs⋅N, the object will start moving. We can express this as:

F ≤ μsN, where N = mg (the normal force is equal to the object's weight).

## Kinetic frictional force

When an object is in motion due to an unbalanced external force, the frictional force is known as kinetic friction. There are three types of kinetic friction: sliding friction, rolling friction, and fluid friction. Sliding friction occurs when a non-circular object can only undergo translational motion, rolling friction occurs when an object can freely rotate around an axis, and fluid friction occurs when an object is moving through a medium such as water or air.

The coefficient of kinetic friction (μk) is the proportionality constant for kinetic friction, just like the coefficient of static friction is for static friction. The values of μk and μs depend on the nature of the surfaces in contact, with μk generally being less than μs. Typical values range from 0.03 to 1.0, and the value of the coefficient of friction can never be negative.

When an external force is applied to an object, the frictional force initially increases until it reaches the maximum static frictional force. After that, the object starts moving and kinetic friction comes into play. The magnitude of kinetic friction decreases initially and then remains constant throughout, unlike static friction which increases linearly.

The geometric relation between static and kinetic friction can be represented graphically as a function of the applied force. Consider a block of mass m on a surface with an external force F applied parallel to the surface. As the applied force increases, static friction increases until it reaches its maximum value, after which kinetic friction takes over and remains constant. This transition from static to kinetic friction can be seen as a sudden drop in the frictional force graph.

As discussed earlier, the force applied is a linear function of static friction, and it increases to a certain value, after which kinetic friction comes into action. The magnitude of kinetic friction decreases until a certain value is attained. The value of friction then remains almost constant with the increasing value of external force.

## Friction on an inclined plane

Thus far, we have focused on objects on a horizontal surface. Now, let us consider an object at rest on an inclined plane, which forms an angle θ with the horizontal.

When analyzing the forces acting on an object, we must consider the gravitational force, friction, and the normal force. When the object is in equilibrium, these forces should cancel each other out. We can choose our Cartesian axes as per convenience. For instance, we can choose the axes along the inclined plane.

The horizontal component of the gravitational force is mg sinθ, which balances the static friction acting in the opposite direction. The vertical component of gravity is mg cosθ, which is equal to the normal force acting on the object. Writing down the balanced forces algebraically, we get:

mg sinθ = fs

mg cosθ = N

When the incline angle is increased until the object is on the verge of slipping, the force of static friction reaches its maximum value μsN. This angle is called the critical angle θc. Substituting this, we get:

mg sinθc = μs mg cosθc

The normal force is:

N = mg cosθc

Taking the ratio of the two equations, we get:

tanθc = μs

Here, θc is the critical angle, and the value of the coefficient of friction can be determined by measuring the angle of inclination of the plane. When the incline exceeds the critical angle, the object starts accelerating downwards, and kinetic friction comes into action.

In the case of a hockey puck resting on the surface of a frozen pond, the frictional force acting on the puck can be determined by finding the maximum static frictional force. If the coefficient of friction is given, we can use the equation:

fs ≤ μsN

Substituting the given values, we get the value of the maximum static frictional force.

Some key takeaways from the concept of friction are:

• There are two types of friction: static friction and kinetic friction. They exist independently and do not come into action simultaneously.
• The coefficient of friction depends only on the nature of the surface.
• On an inclined plane, the coefficient can be determined solely by the angle of inclination.
• Typical values of the coefficient of friction do not exceed 1 and can never be negative.
• Frictional forces are universal, and it is practically impossible to have a frictionless surface.

## Friction

What is friction?

When two or more objects are in contact or surrounded by a medium, there is a resistive force that tends to oppose any motion. This is known as friction.

What type of energy is produced by friction?

Heat energy.

What causes friction?

Friction is caused by the interaction between molecules of different objects at a microscopic level.

How can we reduce friction?

Lubricants of various types are used to reduce friction.

What are the three types of kinetic friction?

The three types of kinetic friction are sliding friction, rolling friction, and fluid friction.

Start creating interactive learning content in minutes with Shiken. 96% of learners report 2x faster learning.
Free 14 day trial
Cancel anytime
20k+ learners globally