What is static friction?

Friction , also called friction , is the resisting force that opposes relative sliding motion between two objects whose surfaces are in contact.

This force is not the same when objects are at rest and when they are in motion. That is why two types of friction are distinguished, dynamic friction (also called kinetic friction) and static friction .

Concept of friction and static friction

Dynamic friction is the force of resistance to motion that occurs when objects are already moving. For its part, static friction is the resistance force that opposes the beginning of the movement and that occurs, therefore, when the objects are at rest.

For example, if we leave an object on a ramp, it will not slide down the ramp on its own if a small initial push is not applied. This initial force has to overcome static friction for the object to start moving.

Once sliding, dynamic friction comes into play, which is generally less than static friction, which is why it is necessary to apply less force to keep the object moving than to start it .

What is friction due to?

In friction, both static and dynamic, numerous factors intervene, but without a doubt the most important are the imperfections or roughness in the contact surfaces and the perpendicular force that holds the two surfaces together. This force is known as the normal force and the most common example is the weight of the object caused by gravity.

The action of the normal force and the surface roughness create a resistance to sliding between both surfaces, which is what we call friction . The force required to overcome this resistance is known as the friction force or shear force.

No surface is completely smooth. If observed under a microscope, imperfections that create friction are always found, although the differences between materials are obviously enormous. For example, a sheet of polished aluminum is obviously much smoother than the surface of an asphalt road.

In addition to surface roughness, other important factors that can influence friction are electrostatic attraction and the type of intermolecular chemical bonds that exist between the two surfaces.

How is static friction calculated?

To put an object in motion on a surface we have to apply a force that is proportional to the normal force that keeps the object attached to the surface. The constant of proportionality between both forces is called the coefficient of friction and is generally designated by the Greek letter ? (mu) . The friction force would have the following formula:


  • f is the fiction or friction force at the threshold of motion
  • ? is the coefficient of friction
  • N is the normal force

If there are no other types of interactions, the normal force of an object on a flat horizontal surface is equal to its weight. That is, its mass times the acceleration due to gravity:

The maximum force that can be applied without causing the object to move is known as the motion threshold . Below this threshold, the force applied to the object coincides with the friction force, both cancel each other out and motion is prevented. Both are known to equalize because otherwise the object would move in some direction.

If we measure the maximum force that we can apply to the object without producing movement, we will know the force of static friction at the threshold of movement. Since we already know the friction force and the normal force, we can isolate the coefficient of static friction from the previous formula.


The coefficient of friction can be calculated in this way for any pair of solid materials, for example, wood – wood, wood – metal or metal – metal. The coefficient of static friction is generally designated e to distinguish it from the coefficient of dynamic friction, d .

The ? d is usually smaller and is similarly calculated by measuring the maximum force that can be applied to keep the object moving at constant speed (no acceleration).

Static Friction vs Kinetic Friction

As explained, static friction is that which occurs below the threshold of movement. Once this threshold is exceeded, the object begins to slide on the surface and, in general, the friction between the two becomes less and is called dynamic friction.

The fact that dynamic friction is less than static friction implies that the force needed to keep an object in motion is less than the force needed to start the object moving. We can observe this effect around us every day and it is usually a great advantage.

For example, the friction between the asphalt and the car’s wheels is intentionally high to give the driver greater control over the vehicle. This creates a lot of static friction and you need to use a lot of force to get the car moving. That is why you have to use the low gears of the engine.

Once in motion, to maintain the speed of the car we need less force since dynamic friction comes into play, less than static friction. Now we can use the high gears of the engine that apply less force and consume less fuel .



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