What is static friction

Friction is said to be a necessary evil and is all around us. It is a force that acts on a surface that is in contact with another surface, and it impedes the motion of the objects. This means that friction is an opposing force that resists the object’s motion. It is responsible for bringing an object to rest from a state of motion. We slide down a box across a horizontal floor with all our force, but it stops after a point. It happens due to the force of friction. Friction can be broadly divided into two categories – static and kinetic. In this article, we will be focusing on static friction and the laws of static friction.

Static friction

It can be defined as the friction acting upon the objects at rest relative to each other. The force of friction acting upon a box when it is at rest on a floor would be static friction. Once we apply enough force and the box starts moving, the friction at play would be termed kinetic friction.

Static friction is much stronger and more pronounced than kinetic friction. When the two objects are at rest, forces of adhesion between them are more substantial, due to which static friction is more robust in magnitude. 

Friction depends on the roughness of the surface. Even when a surface seems smooth, there would be friction because no surface in the world can be 100% smooth. The magnified version of the surface would reveal several bumps and pits that contribute to the roughness and hence, the friction. The more the roughness of the surface, the higher the friction.

Laws of Static friction

Imagine an object lying on a horizontal wooden plank. It is at rest and would not start moving unless an external force is applied. Now, the plank is made to slowly incline such that the end at which the object is lying is above. The object would not immediately move from its mean position. Nevertheless, there would come an angle of inclination at which the static friction would reach its peak point and then the gravitational pull would overpower the force of friction.

The law of static friction states that the frictional force acting on an object is directly proportional to the normal perpendicular force exerted by the object on the surface. The higher the perpendicular force, the more would be the static friction acting on the object. 

The following example can be understood. There is a 100 kg object on a wooden plank, and on another wooden plank, there is a 10 kg object. Since force is directly proportional to the object’s mass, the force exerted by the 100 kg object on the plank would be much higher than that of the 10 kg object.

Since the normal perpendicular force is higher in the 100 kg case, the frictional force would also be higher. The angle of inclination required to move the 100 kg object from its mean position would be higher because a higher gravitational pull is required to overcome the static friction.

The laws of static friction also include the following points:

1. Static friction is not dependent on the area of the surface in contact. 
2. The coefficient of static friction has a higher value than the coefficient of kinetic friction. The coefficient of friction is a physical quantity that determines how much force of friction is acting upon an object.
3. The direction of the static frictional force is parallel to the surface upon which the object is placed.
4. The static frictional force acts in a direction opposite to the object’s direction of motion.

Impending Motion

Impending motion is defined as the state of motion of an object when it is just about to slip down from an inclined surface. At impending motion, the static friction is at its highest magnitude, known as the maximum static friction. 

The mathematical formula for impending motion is F = 𝜇N, where 𝜇 is the coefficient of friction and N is the normal force.

Conclusion

Static friction comes into play when an object rests on another surface. The strong adhesion forces between the two surfaces make static friction much stronger than kinetic friction. It acts parallel to the surface upon which the object is placed. Also, the higher the object’s mass, the more would be the static friction associated with it. This explains why more work is needed to move a 100 kg box than a 10 kg box. The static friction associated with the 100 kg box is much larger compared to the friction associated with the 10 kg box.