Friction – Static Friction

Introduction

Friction is the resistance offered against the movement of one object relative to another. In other words, friction is that force that resists the motion when the surface of an object comes in contact with another object. Frictional forces develop whenever there is a tendency for one surface to slide or move against another surface in contact. They oppose this tendency to move or slide, hence friction is a retarding force.

There are two types of friction, static and kinetic friction. Static friction is the force that operates between two surfaces that are not moving concerning each other. Kinetic friction operates between moving objects. There are other types of friction like sliding friction, rolling friction, and fluid friction. 

What does Static friction mean?

Static friction acts between two surfaces that are at rest and not moving relative to each other. We come across static friction in almost all walks of our daily lives. When you try to pull heavy objects, they might not move initially. This is static friction. When you push it harder with a greater force, you overcome the static friction, and the object moves. 

When we increase the force, we are trying to move the object. The frictional force tends to increase in equal magnitude to the applied force. It acts in the opposite direction to the applied force. This frictional force blocks the movement of the object until point A. After this point, when the applied force is greater than the friction, the object begins to move. Only a small force is required to move the object beyond that point. The friction between starting point  and point A, namely Fs, is static friction. When the object moves or slides beyond A, the static friction reduces to kinetic friction.

The maximum value of static friction that can be generated between the surfaces in contact, to limit the object’s motion is called limiting friction. In other words, if the applied force exceeds the limiting value of static friction, it tends to move. 

Frictional force examples

1. A car parked on a hill stays despite the gravitational forces because static friction between the vehicle and the ground holds the car at rest.  

1. A nail holds on to a wall due to the frictional force acting between the nail and the wall. The nail will not move until the applied force is within the limiting force. 

1. Spectacles stay intact on our nose and face due to the static friction between the nose and spectacles. Else, it would have slipped down the face. 

Coefficient of static friction

The force of static friction is the product of normal force and coefficient of static friction. It can be written as,

Fs = μsN 

where, 

Fs -the maximum force of static friction or the limiting friction, 

μs  -Coefficient of static friction,

N – Normal force,

• When the normal force N is lesser than the static frictional force Fs, N < Fs, the object is at rest, and we call it a static equilibrium condition.

• When the normal force N is equal to the static frictional force Fs, N = Fs, the object is in limiting equilibrium. 

• When the normal force N is greater than the static frictional force Fs, N > Fs, the object is in motion, and so Fs becomes Fk, the kinetic friction. 

Coefficient of static friction, μs = Limiting frictionNormal force

The coefficient of static friction is a scalar quantity. It does not have any unit or dimensions. The coefficient of static friction usually ranges between 0 and 1. If it is close to zero, there is no friction between the two objects. On the other hand, if it is close to one, the friction between the objects is equal to the normal force. Sometimes, the coefficient is more significant when the frictional force is stronger than the normal force. 

Laws of static friction

1. The direction of static friction is always opposite to the direction in which an object or body moves.
2. The coefficient of static friction is always higher than the coefficient of kinetic friction. 
3. The force of static friction adjusts on its own. It increases with an increase in the applied force. The force of static friction is greater than or equal to the applied force until the motion begins.
4. When an object is moved, the force of static friction between the surfaces in contact is the highest. This maximum force of friction is called limiting friction. 
5. The magnitude of limiting friction between the surfaces is directly proportional to the magnitude of the applied normal force. i.e., Fs ∝ N. 
6. The direction of the limiting friction is always opposite to the object’s direction of motion. 
7. The limiting friction is always tangential to the surfaces in contact. 
8. The limiting friction depends on the type of object, its texture, and the nature of surfaces in contact. It is high for rough surfaces and low for smoother ones. 
9. The limiting friction is independent of the area and shape of the surfaces. 
10. The limiting friction is directly proportional to the mass of the object. 

Differences between static and kinetic friction

Conclusion

It is almost impossible to imagine how the world would be without the force of friction. We can never hold objects, or we can never drive uphill without the frictional force. Static friction is the force that acts to keep the object at rest. Kinetic force is a retarding force that acts to slow down the moving object. The value of static friction increases with the applied force to a limit. Thus, static friction acts when two bodies in contact are at rest concerning each other.