# Static friction

In this article, you will understand how static friction appears in everyday life, in addition to knowing formulas and other explanations regarding friction, which is a physical quantity that arises in the contact between two surfaces. Also see how the topic appears in Brazilian entrance exams.

## What is static friction?

Static friction can also be called just friction force , which is only suitable for situations in which there is no movement between surfaces. It would be like the example of a very heavy wardrobe, a baby trying to push a very large object, or similar situations.

Take a good look at the examples mentioned in the paragraph above, which concepts appear in common? If you thought about the difference in size and weight between bodies, you’re right. It is a fact that the force of static friction is closely related to the weight of that object, the larger it is, it will probably suffer more friction.

At the same time, the sliding surfaces also influence the magnitude of friction. It’s like carrying a cardboard box across a smooth, flat, uneven floor; and then try to slide the same box over a cemented floor, full of reliefs, pebbles, irregularities and roughness.

Each of these aspects appears in the formula for the force of static friction. In the case of weight (P), it will be represented by the normal vector (N), that vector that is always perpendicular to the object, with a direction opposite to that of the weight force.

The question of the qualities and characteristics of the sliding surface is resolved with the coefficient of static friction (µ E ). It is a mathematical value that expresses the behavior of two surfaces when sliding against each other. Therefore, each pair of materials will have a different µ E coefficient .

From this, the formula that expresses the static friction force (F AE ) is:

AE = µ E .N

## Particularities of static friction

### Static friction on an inclined plane

On horizontally inclined surfaces, the value of a body’s normal will be equal to its weight force. So, if an object weighs m = 10 kg, under the action of gravity at g = 10m/s 2 , then its weight will be P = mg = 10.10 = 100 N. This means that in the horizontal position, its normal will be P = N = 100 N.

Now, what happens to the normal force when the object is on an inclined plane? Observe the image.

Note that the object is stationary on the inclined plane, but the weight force and the normal force do not balance each other. While the weight points downwards with a value of 100 N, the normal (in green) has an unknown value and is perpendicular to the surface the body is on.

### The Main Point

The point is that, for the object to remain stationary in that position, all its forces must balance. This is when the friction force becomes so important. In specific situations, static friction prevents the object from sliding on the plane, balancing the components of weight and normal force.

The plane is tilted in such a way that the object would tend to slide to the left. Friction always opposes movement, so it will appear parallel to the surface, in the opposite direction of sliding, as shown in the image below.

Note that it was necessary to draw a Cartesian plane that has the X axis parallel to the surface, while the Y axis is perpendicular. From this, the weight vector was decomposed into its components P Y and P x , so that P Y + P x = P.

From then on, as the object is static, all these forces must add up to 0. In other words, P X pulls to the left with the same intensity as the static friction force pulls to the right. At the same time, the action of gravity on P Y is equal to the normal between the surface and the object.

### How to overcome friction?

Sometimes, it is necessary to eliminate friction in everyday life, to make some procedures easier and less tiring. Because, in general, the greater the friction force, the greater the force used to try to overcome it.

The easiest point of action would be to reduce the coefficient of static friction between the two surfaces. To do this, it is necessary to make the two sliding blades as polished, flat and smooth as possible.

Another widely used method is oils and lubricants. They create a greasier film, thus reducing the influence of the roughness and hardness of the surfaces and, therefore, facilitating sliding.