Friction in Solid

The developed theory of solid surface friction is based on the assumptions that the adhesion theory of friction is correct and that the asperities at which contact occurs deform according to the stress is proportional to strain. A roughness model is proposed in which asperities (either spherical or cylindrical) 

(a) are distributed uniformly over the surface, 

(b) have the same radius of curvature, and 

(c) all protrude from the surface by the same amount. 

The analysis leads to the equation F= CL , which relates the frictional force F to the normal load L and has been discovered empirically by several researchers. 

The parameter C is shown to be dependent on the physical properties of the sliding materials, the degree of surface roughness, and the apparent area of contact, and thus on the geometrical arrangement of the sliding surfaces, whereas the parameter is only dependent on the physical properties of the sliding materials.

What is Friction?

Friction is that kind of force which prevents one solid object from slipping or rolling over another one. Frictional forces, such as the traction required to walk without slipping, can be useful, but they can provide a significant amount of resistance to motion. Approximately 20% of an automobile’s engine power is wasted in overcoming frictional forces in moving components. Adhesion, that is the forces of attraction, in between the contact zones of the surfaces, that are always microscopically uneven, appear to be the main cause of friction between metals. Friction is caused by shearing these “welded” joints and by the irregularity of the harder surface ploughing through the softer surface.

Example – 

Dragging a chair around

A hefty chair does not move when a tiny amount of force is applied to it. This clearly indicates that a force is being applied from the opposite direction that is bigger in magnitude than the external force used to move it. This is referred to as the force of friction. A person must exert a force larger than the size of the frictional force in order to move the chair.

Formula

The formula for the force of friction is given by

f = μN

Where, f = friction force, μ = coefficient of friction, N = Normal Force

Solid Friction

Solid friction refers to the friction that exists between two unlubricated surfaces. The two surfaces may be at rest, or one surface may be moving while the other is at rest. The rules of solid friction are as follows. 

1. Friction works in the opposite direction that the surface has a tendency to move. 

2. As long as the surface is at rest, the force of friction is equal to the force applied to it. 

3. The highest frictional force occurs when the surface is at the point of motion, and this maximum frictional force is referred to as the limiting friction force. 

4. The limiting frictional force is proportional to the normal response between two surfaces in a constant ratio.

5. The limiting frictional force is independent of the form and area of the in-contact surfaces. 

6. When the two surfaces are moving, the ratio of limiting friction to normal response is slightly lower. 

7. Frictional force is independent of sliding velocity. The above solid friction rules are sometimes known as static and dynamic friction laws or the law of friction.

Let’s understand this with an example 

When three bricks are dragged along a table, the friction is three times larger than when only one brick is dragged. As a result, the friction F to load L ratio remains constant. 

This constant ratio is known as the coefficient of friction, and it is typically represented by the Greek letter mu (μ). In mathematics,μ = F/L.

Frictional Force

Frictional force is a kind of the force that is created when two surfaces come into touch with each other and slide against each other.

Several things influence frictional force:

• The surface texture and quantity of force impelling them together have the greatest influence on these forces.

• The quantity of frictional force is always affected by the angle and location of the object on which it is applying by the surfaces.

• When an object is placed flat against another object, the frictional force is equal to the object’s weight.

• When someone pressed an object against the surface, the frictional force increases and it also exceeds the weight of the object.

Conclusion

The developed theory of solid surface friction is based on the assumptions that the adhesion theory of friction is correct and that the asperities at which contact occurs deform according to the stress is proportional to strain. Approximately 20% of an automobile’s engine power is wasted in overcoming frictional forces in moving components. Adhesion, that is the forces of attraction, in between the contact zones of the surfaces, that are always microscopically uneven, appear to be the main cause of friction between metals. Solid friction refers to the friction that exists between two unlubricated surfaces. As long as the surface is at rest, the force of friction is equal to the force applied to it.