My name is Oliver and my holiday homework is to write about friction.

What is Friction ?

Friction is the forces resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:

• Dry friction resists relative lateral motion of two solid surfaces in contact. Dry friction is subdivided into static friction between non-moving surfaces, and kinetic friction between moving surfaces.

• Fluid friction describes the friction between layers within a viscous fluid that are moving relative to each other.
• Lubricated friction is a case of fluid friction where a fluid separates two solid surfaces.

The normal force is defined as the net force compressing two parallel surfaces together; and its direction is perpendicular to the surfaces. In the simple case of a mass resting on a horizontal surface, the only component of the normal force is the force due to gravity, where N=mg . In this case, the magnitude of the friction force is the product of the mass of the object, the acceleration due to gravity, and the coefficient of friction. However, the coefficient of friction is not a function of mass or volume; it depends only on the material. For instance, a large aluminum block has the same coefficient of friction as a small aluminum block. However, the magnitude of the friction force itself depends on the normal force, and hence the mass of the block.

If an object is on a level surface and the force tending to cause it to slide is horizontal, the normal force N between the object and the surface is just its weight, which is equal to its mass multiplied by the acceleration due to earth's gravity, g. If the object is on a tilted surface such as an inclined plane, the normal force is less, because less of the force of gravity is perpendicular to the face of the plane. Therefore, the normal force, and ultimately the frictional force, is determined using vector analysis, usually via a free body diagram. Depending on the situation, the calculation of the normal force may include forces other than gravity.


Why trains need friction

Trains need friction so that they don't slip off the railway line, and they need friction to grip the rails and start moving. Friction is also needed to slow the train down when the brakes grip the wheels.

The sort of friction between a train and the track is called kinetic friction.


Kinetic (or dynamic) friction occurs when two objects are moving relative to each other and rub together (like a sled on the ground).

Kinetic friction is now understood, in many cases, to be primarily caused by chemical bonding between the surfaces, however, in many other cases roughness effects are dominant, for example in rubber to road friction.

And did you know there is an liquid that you put on the train tracks and the train's wheels carry it along the track with it. This liquid increases the friction between the wheels and the track and it is called KELTRACK . It reduces wheel squeal and energy consumption .




Maglev Trains

Here is a video of a magnetic train





A few countries are using powerful electromagnets to develop high-speed trains, called maglev trains. Maglev is short for magnetic levitation, which means that these trains will float over a guideway using the basic principles of magnets to replace the old steel wheel and track trains. There is no friction between the train and the track, only between the train and the air. 

If you have two magnets they each have a north and south pole. The opposite poles will attract and the same poles will repel. Maglev trains create magnet fields on the track bed and the train of the same polarity. By controlling the generating of the fields you can hold up the train and propel it forward. If there was no friction between the maglev train and the air the train would never stop