Reducing Friction

A common way to reduce friction is by using a lubricant, such as oil, that is placed between the two surfaces, often dramatically lessening the coefficient of friction. The science of friction and lubrication is called tribology. Superlubricity, a recently-discovered effect, has been observed in graphite: it is the substantial decrease of friction between two sliding objects, approaching zero levels - a very small amount of frictional energy would be dissipated due to electronic and/or atomic vibrations.

Lubricants to overcome friction need not always be thin, turbulent fluids or powdery solids such as graphite and talc; acoustic lubrication actually uses sound as a lubricant.

Causes of friction

Friction is caused by the roughness of the materials rubbing against each other, deformations in the materials, and a molecular attraction between molecules of two surfaces.

1. Surfaces not completely smooth:

Most friction results because the surfaces of materials being rubbed together are not completely smooth. If you looked at what seems to be a smooth surface under a microscope, you would see bumps, hills and valleys that would interfere with sliding motion. Of course, the rougher the surface, the more is the friction.

If both surfaces become ultra-smooth and flat, the friction from surface roughness becomes negligible, but then friction from molecular attraction comes into play, often becoming greater than the normal friction.

2. Deformations:

Soft materials will deform when under pressure. This also increased the resistance to motion. For example, when you stand on a rug, you sink in slightly, which causes resistance when you try to drag your feet along the rug's surface. Another example is how rubber tires flatten out at the area on contact with the road.

When materials deform, you must "plow" through to move, thus creating a resistive force.

3. Molecular attraction:

There is another factor in friction, and that is stickiness caused by molecular attraction. This was mentioned above where surfaces are so smooth that the materials stick together due to molecular forces.

Soft rubber is an example of a material that can have this type of friction. This factor is usually seen in rolling friction. The stickiness will create a resistance to any motion. Although this force is the smallest, it still can be a factor when the other causes of friction are low.

The fundamental forces in nature

All the different forces observed in nature can be explained in terms of four basic interactions that occur between elementary particles:

1.       The gravitational force

2.       The electromagnetic force

3.       The strong nuclear force

4.       The weak nuclear force

The everyday forces that we observe between macroscopic objects are due to either the gravitational force or the electromagnetic force.

Forces may be placed into two broad categories, based on whether the force resulted from the contact or non-contact of the two interacting objects.

Action at a distance:

The fundamental forces of gravity and electromagnetism act between particles that are separated in space. This creates a philosophical problem referred to as action at a distance.

Contact forces:

Many forces we encounter are exerted by objects in direct contact. These forces are electromagnetic in origin and are exerted between the molecules of each object.

Normal force:

Consider a book on a table. The weight of the book pulls it downward, pressing it against the molecules in the table’s surface, which resist compression and exert a force upward on the book. Such a force, perpendicular to the surface, is called a normal force.

Frictional force:

Objects in contact can also exert forces on each other that are parallel to the surfaces in contact. The parallel component of a contact force is called a frictional force.

Static friction:

Friction is a complicated, incompletely understood phenomenon that arises due to the bonding of molecules between two surfaces that are in close contact. This bonding is the same as the molecular bonding that holds an object together. When you apply a small horizontal force to a large box resting on the floor, the box may not move because of the force of static friction,exerted by the floor on the box, balances the force you are applying. The force of static friction, which opposes the applied force, can adjust from zero to some maximum force f _{s, max} depending on how hard you push. You might expect f _{s, max} to be proportional to the area of contact between the two surfaces, but this is not the case. To a good approximation, f _{s, max} is independent of the area of contact and is simply proportional to the normal force exerted by one surface on the other:

f _{s, max} = m _s F_n

where, m_s is called the coefficient of static friction, a dimensionless quantity that depends on the nature of the surfaces in contact. If you exert a horizontal force smaller than f _{s, max} on the box, the frictional force will just balance this horizontal force. In general, we can write

f _s £ m _s F_n

 Kinetic friction:

If you push the box hard enough, it will slide across the floor. When the box is sliding, molecular bonds are continually being formed and ruptured, and small pieces of the surfaces are being broken off. The result is a force of kinetic friction,that opposes the motion. To keep the box sliding with constant velocity, you must exert a force on the box that is equal in magnitude and opposite in direction to the force of kinetic friction exerted by the floor.

The coefficient of kinetic friction m _k is defined as the ratio of magnitudes of the kinetic frictional force f _k and the normal force F_n:

f _k =m _k F_n

where m _k depends on the nature of the surfaces in contact. Experimentally, it is found that m _k is less than m _s and is approximately constant for speeds ranging from about 1 cm/s to several meters per second.

The plot of the frictional force vs. the applied force illustrates some of the features of the frictional force. Note that the frictional force equals the applied force (in magnitude) until it reaches the maximum possible value µ_sN. Then the object begins to move as the applied force exceeds the maximum frictional force. When the object is moving the frictional force is kinetic and roughly constant at the value µ_kN which is below the maximum static friction force.

Examples of kinetic friction:

¨         Sliding friction is when two objects are rubbing against each other. Putting a book flat on a desk and moving it around is an example of sliding friction.

¨         Rolling friction occurs when the two objects are moving relative to each other and one "rolls" on the other (like a car's wheels on the ground). The coefficient of rolling friction is typically denoted as μ _r.

¨         Fluid friction is the friction between a solid object as it moves through a liquid or a gas. The drag of air on an airplane or of water on a swimmer are two examples of fluid friction.

Contact Forces	Action-at-a-Distance Forces
Frictional Force	Gravitational Force
Tensional Force	Electrical Force
Normal Force	Magnetic Force
Air Resistance Force
Applied Force
Spring Force

Force & Friction

The science of mechanics is based on three natural laws relating force and motion. These were clearly stated for the first time by Sir Isaac Newton [1642 – 1727] and were published in 1686 in his Philosophiae Naturalis Principia Mathematica. Newton’s three laws relate an object’s acceleration to its mass and the forces acting on it. A modern wording of Newton’s laws follows:

I.  Newton's First Law of Motion: Every object continues to be at rest or in a state of uniform motion unless acted on by an external force.

This we recognize as essentially Galileo's concept of inertia, and this is often termed simply the "Law of Inertia".

To say that something is moving always implies a specific frame of reference. An inertial frame of reference is one in which Newton’s first law of motion holds.

II. Newton's Second Law of Motion:  Newton's second law of motion explains how an object will change velocity if it is pushed or pulled upon.

The rate of change of momentum of a body is directly proportional to the applied force acting on the body.

Firstly, this law states that if you do place a force on an object, it will accelerate, i.e., change its velocity, and it will change its velocity in the direction of the force.

It accelerates in the direction…………..

That you push it.

Secondly, this acceleration is directly proportional to the force. For example, if you are pushing on an object, causing it to accelerate, and then you push, say, three times harder, the acceleration will be three times greater.

If you push twice as hard…………..

It accelerates twice as much.

Thirdly, this acceleration is inversely proportional to the mass of the object. For example, if you are pushing equally on two objects, and one of the objects has five times more mass than the other, it will accelerate at one fifth the acceleration of the other.

If it gets twice the mass……………..

It accelerates half as much.

III. Newton's Third Law of Motion: 

The word force is used to describe the interaction between two objects. When two objects interact, they exert force on each other. Newton’s third law states that these forces are equal in magnitude and opposite in direction.

For example, if you push on a wall, it will push back on you as hard as you are pushing on it.

If you push on it…………………

It pushes on you.

The full power of Newton’s second law emerges when it is combined with the force laws that describe the interactions of objects. For example, Newton’s law for gravitation, gives the gravitational force exerted by one object on another in terms of the distance between the objects and the masses of each. This, combined with Newton’s second law, enables us to calculate the orbits of planets around the sun, the motion of the moon, and variations with altitude of g, the acceleration due to gravity