Increasing and Decreasing Friction

A friction force prevents one object from rolling or sliding over another. Although frictional forces, like the traction required to walk without slipping, may be advantageous, they also pose significant resistance to motion. Automobile engines use about 20% of their output to overcome frictional forces in moving parts. The adhesion forces between the contact zones of the surfaces, which are always microscopically uneven, appear to be the primary cause of friction between metals.

Causes of Friction

• The main source of friction is the irregularities on the surfaces in contact, which become interlocked when the surfaces come into contact.

• Smooth surfaces have fewer irregularities, while rough surfaces have more irregularities. As a result, rough surfaces have a high friction coefficient.

• The weight of the body also influences frictional force.

Types of Friction

• Static friction- The frictional force between two surfaces while they are at rest in relation to one another, is known as static friction.

• Dynamic friction- The force of friction that develops between any two surfaces when there is relative motion between them is known as dynamic friction.

How to Calculate the Friction Force Between Surfaces?

We have two types of friction, so we have two formulas to calculate the friction forces.

• Static friction:

${{F}_{s}}$=${{μ}_{s}{F}_{N}}$

${{F}_{s}}$ = Maximum value of static friction force

${{μ}_{s}}$ = Coefficient of static friction

${{F}_{N}}$ = Normal reaction force

• Dynamic friction:

${{F}_{k}}$=${{μ}_{k}{F}_{N}}$

${{F}_{k}}$ = Dynamic/Kinetic friction force

${{μ}_{k}}$ = Coefficient of kinetic friction

${{F}_{N}}$ = Normal reaction force

How is Friction Due to Air Reduced?

Friction is a force that resists object motion and can cause items to slow down. Friction is represented by air resistance. Moving objects are slowed by air resistance. The air resistance of an item is affected by physical features such as its form. Air friction is greatly decreased by streamlining the form of the body moving through air. Example: Jets, supercars and bullet trains have a streamlined design.

Ways for Decreasing Friction

• Making use of lubricants: The application of lubricants like oil, grease, or graphite reduces the roughness of the surface. Lubricants minimise friction by filling the gaps of irregularities between surfaces in contact.

• Polishing: Friction arises due to irregularities on the surface. On polishing, the irregularities decrease, and the surface offers less friction to a body moving over it.

• Proper selection of material: Friction depends upon the nature of the material of surfaces in contact. Using materials with a low friction coefficient can reduce the friction force and improve efficiency. For example, rubber is used for making tires instead of iron.

• Using ball bearings: converting sliding friction into rolling friction: we know that sliding a stone slab is difficult compared to rolling a wheel made of the same stone.

• Streamlining: Air friction can be reduced significantly by streamlining the body's shape. For example, aircraft, missiles, and supercars have a streamlined shapes.

Ways for Reducing Friction

Why should friction be raised if it is the force that lowers productivity and damages equipment parts?

Some ways to increase friction-

• Making the surface rough- cricketers use shoes with spikes to reduce slipping.

• Making grooves on the surface - tires have grooves to offer high friction.

• If we press the surfaces into each other more firmly, we can increase the friction between them.

• Using dry and unlubricated parts.

• Increase the area in contact with fluid: frictional force offered by fluids (liquids and gasses) is called frictional drag. It depends upon the area in contact with fluid; thus, by increasing the area in contact, we can increase friction.

Increasing Friction Examples

The tyres of motor cars and bicycles are rough to increase friction. when a ground or surface becomes slippery after rain or water splashing, it is made rough by spreading sand to increase the friction.

Decreasing Friction Examples

When objects are rolled over the surface, the friction between the rolled object and the surface can be reduced by using ball bearings. Using lubricants such as oil or grease can reduce surface friction. Polishing the surface makes the surface smooth, and friction can be reduced.

Summary

The friction force is a contact force whose magnitude depends directly upon the nature of surfaces in contact. Thus by altering the characteristics of surfaces, we can increase or decrease friction. By making the surface smooth, we can achieve reduced friction, while by making it rough, the magnitude of friction can be increased.

Solved problems

1. Amy is towing a 4 kg toy vehicle previously resting on the floor. Calculate the friction coefficient if the static frictional force is 50 N.

Solution:

Known:

m (Mass) = 4 kg,

${{F}_{s}}$ (static frictional force) = 50 N,

${{F}_{N}}$ (Normal force) = mg

= 4 Kg × 9.8 m/${{s}^{2}}$

${{F}_{N}}$= 39 N

${{μ}_{s}}$ = ${{F}_{s}}$ / ${{F}_{N}}$

${{μ}_{s}}$ = 50/39

${{μ}_{s}}$ = 1.282

2. Force of 60N is exerted on the box kept on the floor with a coefficient of static friction of 0.4. Find the friction force.

Solution:

The force of static friction is given by,

${{F}_{s}}$=${{μ}_{s}{F}_{N}}$

Here, ${{μ}_{s}}$ is the coefficient of static friction, and N is the normal force.

${{μ}_{s}}$= 0.4 and N=60N

Substituting the values in the equation,

${{F}_{s}}$=${{μ}_{s}{F}_{N}}$

${{F}_{s}}$=${{0.4}\times{60}}$

${{F}_{s}}$= 24 N