Types of friction

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We all slow down our vehicles whenever needed by applying brakes. Do you know why a vehicle slows down when brakes are applied?

Not only vehicles any object moving on the surface of another object slows down and stops without any external force acting on it because of "friction". Before going to types of friction, lets know about friction. According to law of physics any object in the world can't be friction-less.

Friction[edit | edit source]

Forces on an object in motion

Forces and motion of object.

Friction is a force that opposes the motion of two contacting surfaces.

Laws of Friction[edit | edit source]

1. Friction depends on the hardness or roughness of the contacting surfaces.Fluid friction depends on the viscosity (the thickness) of the fluid...

2. Friction is directly proportion to the normal force pressing the contacting forces together...

3. Friction does not depend on the area of the contacting surface...

4. In the case of sliding, friction is reduced at very high relative speeds. However, in the case of fluid friction, friction increases with increase in relative speed of movement.

To stop an object in motion, a force must act on it in the opposite direction of motion. The force that opposes the motion of the object is called the frictional force.

Look at the diagram. At first the block is at rest, then the pushing force keeps the block moving. As the block slides over the surface, the frictional force acts on it in the opposite direction. A unit of friction is a Newton, as forces are measured using Newtons.

Friction generally depends on weight of the object and nature of the surface between the moving object and supporting surface.

Types of Friction[edit | edit source]

Different types of motion of the object gives rise to different types of friction. Generally, there are 4 types of friction. They are static friction, sliding friction, rolling friction, and fluid friction. The next sections will explore these forces and when they are applied.

Static Friction[edit | edit source]

Static friction exists between a stationary object and the surface on which it is resting. It prevents an object from moving against the surface.

Example: Static friction prevents an object like a book from falling off the desk, even if the desk is slightly tilted. It helps us pick up an object without it slipping through our fingers.

When we want to move an object first we must overcome the static friction acting between the object and the surface on which the object is resting.

A stationary book on surface

Sliding Friction[edit | edit source]

Sliding friction occurs between objects as they slide against each other.

When sliding friction is acting there must be another force existing to keep the body moving.

Example: When a man is pushing an object on a rough surface the force acting is called "sliding friction".

Rolling Friction[edit | edit source]

Rolling friction is the resistive force that slows down the motion of a rolling ball or wheel. It is also called rolling resistance.

When a force or torque is applied to a stationary wheel, there is a small static rolling friction force holding back the rolling motion. However, resistance from static sliding friction is what really causes the wheel to start rolling. Rolling friction hinders the motion of an object rolling on a surface, that means it slows down the motion of an object rolling on a surface.

Examples: It slows down a ball rolling on a surface and it slows down the motion of tire rolling on the surface.

Like sliding friction here also another force is required to keep the object in motion, in case of pedaling bicycle the bicyclist provides the force which is required for the bicycle to be in motion.

A ball rolling on a surface

Fluid Friction[edit | edit source]

Here on Earth we tend to take air resistance (aka.“drag”) for granted. We just assume that when we throw a ball, launch an aircraft, deorbit a spacecraft, or fire a bullet from a gun, that the act of it traveling through our atmosphere will naturally slow it down. But what is the reason for this? Just how is air able to slow an object down, whether it is in free-fall or in flight? Air friction is experienced by the objects moving through the open air. Air friction acts between the object and the air through which it is moving. It is also called drag. This force depends upon the object's shape, material, speed with which it is moving and the viscosity of the fluid. Viscosity is the measure of the resistance of the air to flow and it differs from one density to another.

Example: It slow downs the motion of airplane flying in the air, here the engine of the airplane helps the plane to overcome the fluid friction and move forward.

A plane in the air

There is another type of friction (a special case)

Limiting Friction[edit | edit source]

Limiting friction is the maximum opposing force that comes into play when one body is just at the verge of moving over the surface of the other body