Traction Force

The force used to produce or create motion by the use of dry friction between a body and a tangential surface is known as traction or tractive force. To create friction, commonly, the shear force of the surface is also used.

What is Traction or Tractive Force?

Traction is often expressed as the ratio or fraction of the maximum tractive force to the normal force when the maximum tractive force between a body and a surface is limited by the available friction. When this happens, it is generally called the coefficient of traction, which is similar to the coefficient of friction. By overcoming all the resisting forces like air resistance, rolling resistance, friction, normal loads, etc., this force makes an object move over a particular surface. Hence, traction is a physical process that uses dry friction and transmits a tangential force across an interface between two bodies.

What is the Coefficient of Traction?

The ratio or fraction of the force of friction between two bodies and the force pressing them together is known as COF – coefficient of friction, often denoted by µ; it is a dimensionless scalar quantity. The materials used are a major factor on which the coefficient of friction depends. For instance, steel on ice has a small coefficient of friction as compared to the rubber on pavement, which will have a high coefficient of friction.

The range of coefficients of friction is nearly from zero to greater than one. Between two surfaces of different metals, the coefficient of friction will have a low value, and between two similar metals, the coefficient of friction will have a greater value. For instance, copper has a higher coefficient of friction when moved against copper but less if moved against brass or aluminium.

The force for traction, which is usually divided by the weight on the running gear such as wheels, tracks etc., is known as the coefficient of traction.

Coefficient of traction = Usable traction / Normal force

Factors on Which Coefficient of Traction Depends

There are several factors that affect the traction between two surfaces, and they are:

• The composition of the material of each surface.
• The shape of the surface, i.e. microscopic and macroscopic.
• The normal force that presses the contact surfaces together.
• Contaminants, including lubricants and adhesives at the boundary of the material.
• The relative motion of tractive surfaces also affects the traction, such as a sliding object has less traction as compared to a non-sliding object.
• Relative to some coordinate systems, the direction of traction affects the traction. For example, the traction of a tire frequently varies between restricting, braking and accelerating motion.
• Traction can be amplified by using traction devices for low-friction surfaces, such as off-road or ice, to partially penetrate the surface. Rather than relying solely on dry friction, these devices use the shear strength of the underlying surface.

What is Frictional Force?

When the surface of one object comes in contact with the surface of another, then there exists a force that resists motion between them and that force is known as frictional force. A machine’s work output is reduced by friction; in other words, we can say that because of friction, the ratio of output to input is reduced.

Similarly, one-quarter of the energy of an automobile is used to counteract the limiting friction. Yet, friction is important because it allows the tires of the car to stay on the road, and on the other hand, friction present in the clutches makes it possible to drive. Friction is one of the most substantial phenomena, from matches to machines to molecular structures, in the physical world.

Here are a few factors that affect the force of friction:

• The texture of the surface and the amount of force driving them together is the main cause that affects the frictional force.
• The amount of frictional force also gets affected by the angle and position of the object.
• The frictional force will be equivalent to the weight of an object if that object is placed flat against any other object.
• The frictional force will be improved if an object is pushed against the surface and becomes more than the weight of the object.

Frictional Force Explained

What is the Coefficient of Friction?

The ratio or fraction of the resisting frictional force to the normal force pressing the two surfaces in contact and in motion is known as the coefficient of friction. It is generally denoted by mu (μ).

Mathematically we can express, μ = F/N,

where F = Frictional force and

N = Normal force

The coefficient of friction is dimensionless because both F and N are measured in units of force such as newtons or pounds. For static friction and kinetic friction, the coefficient of friction has different values.

The force that is applied to an object is resisted by the frictional force in static friction, and until the force of static friction is overcome, the object remains at rest. On the other hand, the frictional force that resists the object in motion is kinetic friction. The coefficient of kinetic friction is approximately 0.5 for the case of a brick sliding on a clean wooden table, which indicates that to overcome friction in keeping the bricks moving along at a constant speed, a force equal to half the weight of the bricks is required, and the coefficient of static friction is approximately 0.6.

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