Friction in Solids and Liquids, Physics tutorial

Concept of Friction:

We are familiar that if a solid is in contact by the other, the roughness of their surfaces prevents them from sliding freely over one other. Such action is brought around by the consequence of friction. Frictional forces act tangential to the surface of separation among the two bodies in contact. It doesn't appear unless there is relative motion or a force tending to generate motion, then it for all time appears to oppose the motion or the force tending to generate motion.

You have to overcome a few frictional forces prior to an object can move over other. The frictional force carries on acting as long as one body moves over the other. If you withdraw the force producing the motion, friction slows down the moving body till it ultimately comes to the rest.

- Friction is stated as a force that acts at the surface of separation among two objects in contacts and tends to oppose the motion of one over the other.

Merits and Demerits of Friction:

Advantages of Friction:

1) With no friction, it would be not possible for us to walk or to stop walking after getting started. Friction protects us from slipping while walking.

2) Friction lets the automobile tires to make a firm grip by the road therefore aiding their movement. The threading pattern of vehicle tires is designed to provide the tire a good grip on the road.

3) Friction facilitates the locomotive engine to pull the train on the nail.

4) Friction is employed in fan belts employed over pulleys or wheels in machinery. The belting which drives the wheels in machines based on friction to make the wheels turn devoid of slipping.

5) Friction lets a screw or a nail to stay in place after being screwed into position. Such nails therefore hold pieces of wood altogether due to the friction between the nails and the wood.

6) Friction is employed in the grindstone to sharpen chisels and knives. The rough surface of grindstone wears away the metal surface to sharpen them.

7) Friction lets the brakes to stop the train and car.

Disadvantages of Friction:

1) Friction leads to wear and tear on the moving portions of machines.

2) Friction decreases the effectiveness of machines as it causes loss of energy. Much useless work is however done by machine in overcoming friction that opposes their motion.

3) It mainly causes heating of engines. Energy which should be employed to overcome friction is transformed into heat. This outcome in the bearing of machines getting hot whenever they are running. At times, the energy is transformed to sound.

Methods and Reasons for Reducing Friction:

There are three main methods of decreasing friction. You might have been using such methods in decreasing friction in the past and you may as well know other methods. These are:

1) The utilization of lubricants such as oil, grease, air and graphite. You can utilize lubricants to remain two metal surfaces from coming into direct contact. You can as well do this by keeping a layer of air or other gases among them.

2) The utilization of ball or roller bearing. If a wheel is turning on an axle, the use of ball or roller bearings decreases the friction. These are similar to wheels in wheels and are precisely shaped and have particularly hardened surfaces. In roller bearings, the surfaces don't slide however roll over the others and rolling friction is much less than the sliding friction.

3) The streamlining of the body shapes of moving objects. You can decrease friction by shaping a body to the streamlines of the fluid via which it is moving. This decreases the frictional force or retarding force. You will obtain that modern car, ships and aero- planes are made in such way as to lessen the friction among them and the fluid or the medium via which they move.

Types of Friction:

There are mainly two kinds of frictions that we are going to illustrate.

These are:

1) Static or Limiting Friction:

It is the maximum force which should be overcome before a body can just begin to move over the other.

2) Kinetic or Dynamic Friction:

This is the force which should be overcome so that a body can move by uniform speed over the other body.

Laws of Friction:

The laws of Friction are as follows:

1) Friction resists the relative motion among the two surfaces in contact. It acts in a direction opposite to that of the motion.

2) The force of friction rises to the similar extent as the force that tends to begin the motion.

3) Frictional force based on the two surfaces in contact - the nature of the substance and the conditions of the surfaces like a rough, smooth, polished, dry, wet and others.

4) This is independent of the area of the surfaces in contact.

5) It differs directly by the normal force pressing the surfaces altogether. That is, it is proportional to the normal reaction R.

Coefficient of friction:

It is the measure of the amount of resistance which a surface applies on or substances moving over it, equivalent to the ratio among the maximal frictional force which the surface applies and the force pushing the object toward the surface. The coefficient of friction is not for all time similar for objects which are motionless and objects which are in motion; motionless objects frequently experience more friction than moving ones, needing more force to put them in motion than to keep them in motion.

  • The static coefficient of friction is the coefficient of friction which applies to objects and is motionless.
  • The kinetic or sliding coefficient of friction is the coefficient of friction which applies to objects which are in motion.

Concept of Viscosity:

Frictional forces as well exist among different layers of liquid surfaces. Such frictional force can be considered as viscosity. The viscosity of a liquid is low when the frictional force is low and vice-versa. For illustration: if you throw an object into water and a similar object into palm oil, the object moving via water will experience less resistance than if it moves via palm oil as the viscosity of water is Lower than that of palm oil. This makes the object to move faster in water.

The attraction between molecules in the neighboring layers of a liquid is responsible for the viscosity of the liquid.

Viscosity can be stated as the property of a fluid that tends to prevent motion of one portion over the other.

This is the internal friction between layers of a liquid or gas in motion. The viscosity of a liquid quickly reduces as its temperature increases.

Effects of Viscosity:

The viscosity of oil is much significant in choosing appropriate lubricants for different parts of the machines. We utilize more viscous oils for heavier machines than in light ones. Oils, grease and air are employed as lubricants due to their viscosity.

We employ engine oils in lubricating engines to keep metal surfaces from rubbing against one other.

Water is not employed as a lubricant as it consists of low viscosity. The viscosity of oils and greases reduces with temperature. Thus, their lubricating consequences are lowered at high temperature.

The viscosity of the air makes the bob of a swinging pendulum come to rest faster with a cord joining to the string than devoid of a cord.

The broader the area of the cord the greater the viscous force opposing the pendulum's motion and the pendulum bob comes to rest faster.

Laws of Viscosity:

1) It mainly based on the area of the surface in contact (dissimilar to the solid friction).

2) It opposes motion (such as solid friction).

3) It is based on the relative motion among the layers (dissimilar solid friction).

4) It mainly based on the nature of the liquid.

5) This is independent of normal reaction (dissimilar to solid friction).

Similarities and Differences between Viscosity and Friction:

The similarities and differences which exist between viscosity and friction are as follows:


a) Both the forces oppose relative motion between surfaces.

b) Both are based on the nature of the materials in contact.


a) Friction doesn't based on the area of surfaces in contact; viscosity based on regions of surfaces in contact.

b) Friction is dependent on the normal reaction, viscosity is not.

c) Friction takes place in solids, viscosity occurs in liquids and gases (fluids).

c) Friction doesn't based on the relative velocities between the two layers; viscosity based on the relative velocity among layers.

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