Rolling friction

The opposition to relative motion when a body is rolling over a surface is called rolling friction. It is less in magnitude when compared with kinetic friction.

A body like a ring or a sphere rolling without slipping over a horizontal plane will suffer no friction, in principle. At every instant, there is just one point of contact between the body and the plane and this point has no motion relative to the plane. In this ideal situation, kinetic or static friction is zero and the body should continue to roll with constant velocity. But in practice, this will not happen and some resistance to motion (rolling friction) does occur, i.e. to keep the body rolling, some applied force is needed. For the same weight, rolling friction is much smaller (even by 2 or 3 orders of magnitude) than static or sliding friction.

During rolling, the surfaces in contact get momentarily deformed a little, and this results in a finite area (not a point) of the body being in contact with the surface. The net effect is that the component of the contact force parallel to the surface opposes motion.

We often regard friction as something undesirable. In many situations, like in a machine with different moving parts, friction does have a negative role. It opposes relative motion and thereby dissipates power in the form of heat, etc. Lubricants are a way of reducing kinetic friction in a machine. Another way is to use ball bearings between two moving parts of a machine. (Fig. a) .

Since the rolling friction between ball bearings and the surfaces in contact is very small, power dissipation is reduced. A thin cushion of air maintained between solid surfaces in relative motion is another effective way of reducing friction (Fig. a).

In many practical situations, however, friction is critically needed. Kinetic friction that dissipates power is nevertheless important for quickly stopping relative motion. It is made use of by brakes in machines and automobiles. Similarly, static friction is important in daily life. We are able to walk because of friction. It is impossible for a car to move on a very slippery road. On an ordinary road, the friction between the tyres and the road provides the necessary external force to accelerate the car.
That is why friction is being called as necessary evil.

Related posts :

Friction introduction
Newton's First law of motion
Newton's second law of motion
Newton's third law of motion
Common Forces in mechanics

Determination of Specific Charge of Electron J J Thomson Experiment 

Millikan Oil Drop Method to determine Charge of Electron 

Photo Electric Effect Experimental Observations

Einstein’s Photo Electric Equation and Applications

Photo Electric Effect Problems and Solutions 

Problems and Solutions on Einstein's Photo Electric Equation

De Broglie Hypothesis and Wave Nature of Particle

Problems and solutions on de Broglie's Hypothesis

Friction Introduction

The force acting between two bodies in contact which opposed the reletive moiton is called frictional force.

It comes because of interlocking of molecules at the surface of contact of two bodies. It is a microscopic phenomenon. It is because of all surfaces are irregular and when two bodies are in contact the irregular surfaces get interlocked with each other and further opposes the relative motion. This opposition is called friction.

We can polish the surfaces and reduce the friction and it can not be reduced to zero. It is because when you over polish a surface the molecules comes into contact and again there is a attractive force between them which also opposes relative motion.

The frictional force between two bodies in the state of rest is called static and when they are in motion it is called kinetic friction.

Let us consider a example of a body of mass m at rest on a horizontal table. The force of gravity (mg) is cancelled by the normal reaction force (N) of the table. Now suppose a force F is applied horizontally to the body. We know from experience that a small applied force may not be enough to move the body. But if the applied force F were the only external force on the body, it must move with acceleration F/m, however small. Clearly, the body remains at rest because some other force comes into play in the horizontal direction and opposes the applied force F, resulting in zero net force on the body.

This force fs parallel to the surface of the body in contact with the table is known as frictional force, or simply friction (Fig.a below). The subscript stands for static friction to distinguish it from kinetic friction fk that we consider later (Fig. b given below). Note that static friction does not exist by itself. When there is no applied force, there is no static friction. It comes into play the moment there is an applied force.

As the applied force F increases, fs also increases, remaining equal and opposite to the applied force (up to a certain limit), keeping the body at rest. Hence, it is called static friction. Static friction opposes impending motion. The term impending motion means motion that would take place (but does not actually take place) under the applied force, if friction were absent.

When the applied force exceeds a certain limit, the body begins to move. It is found experimentally that the limiting value of static friction ( )max s f is independent of the area of contact and aries with the normal force(N) approximately as

static frictional force maximum = u_s N .

where μs is a constant of proportionality depending only on the nature of the surfaces in contact. The constant μs is called the coefficient of static friction. The law of static friction thus be written as fs ≤ μs N .

If the applied force F exceeds (f_s ) the body begins to slide on the surface. It is found experimentally that when relative motion has started, the frictional force decreases from the static maximum value (f_s ) . Frictional force that opposes relative motion between surfaces in contact is called kinetic or sliding friction and is denoted by fk . Kinetic friction, like static friction, is found to be independent of the area of contact. Further, it is nearly independent of the velocity. It satisfies a law similar to that for static friction fk = μs N.

where μk the coefficient of kinetic friction, depends only on the surfaces in contact. μk is less than μs. When relative motion has begun, the acceleration of the body according to the Second Law is ( F – fk )/m. For a body moving with constant velocity, F = fk. If the applied force on the body is removed, its acceleration is – fk /m and it eventually comes to a stop.

When two bodies are in contact, each experiences a contact force by the other. Friction, by definition, is the component of the contact force parallel to the surfaces in contact, which opposes impending or actual relative motion between the two surfaces. Note that it is not motion, but relative motion that the frictional force opposes .

Related posts :

Newton's First law of motion
Newton's second law of motion
Newton's third law of motionDetermination of Specific Charge of Electron J J Thomson Experiment 

Millikan Oil Drop Method to determine Charge of Electron 

Photo Electric Effect Experimental Observations

Einstein’s Photo Electric Equation and Applications

Photo Electric Effect Problems and Solutions 

Problems and Solutions on Einstein's Photo Electric Equation

De Broglie Hypothesis and Wave Nature of Particle

Problems and solutions on de Broglie's Hypothesis

Concept of momentum
conservation of momentum
Common Forces in mechanics