Mention two simple methods for reducing friction

Question 1 Why is the surface of slide polished to make it smooth?

Question 2 Name 2 lubricants?

Question 3 Why do we sprinkle fine powder on carrom board?

Question 4 Which force gets reduced when two surfaces in contact are polished to make them smooth?

Question 5 State one way in which the friction between wheel and axle can be reduced?

Question 6 Why does oiling the axles of a bicycle make the bicycle move more easily?

Question 7 Name one example from every day life where wheels are used to reduce friction?

Question 8 Name device which is attached to heavy luggage to move it easily by pulling?

Question 9 What is lubrication?

Question 10 Why are lubricants applied to rubbing surfaces of machines?

Question 11 Explain why wheels are so useful?

Question 12 Why is lubrication important?

Question 13 How will you reduce friction between those machine parts which rub against each other?

Question 14 What is ball bearing?

(1) Friction can be reduced by making the surface smooth by polishing

The friction is due to the roughness of surface. If we make the surfaces smooth by polishing, then friction will be reduced. For example a slide in the park is polished to make its surface smooth and reduce friction. Due to reduced friction of a polished, smooth slide, children can slide down easily.

(2) Friction can be reduced by applying lubricants (like oil or grease) to the rubbing surface

When oil or grease is applied between the moving parts of a machine, a thin layer of oil or grease is formed between the two rubbing surfaces. This layer of oil separates the two rubbing surfaces a little bit due to which their interlocking is reduced to a large extent. This friction is reduced and movement becomes smooth. When oil or  grease is applied to the moving parts of a machine, then their surfaces do not rub directly against each other, they rub through a layer of oil and grease .

The substance which reduce friction are called lubricants

Oil, grease, graphite and fine powder are lubricants.

The applying of lubricants to a machine is called lubrication.

Friction can be reduced by lubrication.

A well lubricated machine runs more smoothly and lasts longer.

(1) A bicycle mechanic and motor mechanic uses grease between the moving parts of these machines to reduce friction and increase efficiency.

Drops of oil are poured on the hinges of a door, the friction is reduced and the door smoothly.

(3) We Sprinkle fine powder as dry lubricant on carrom board to reduce friction.

(4) In some machines an air cushion between the moving parts is used to reduce friction. For example the frictional drag from the sea on hovercraft is reduced by a cushion of compressed air. (A vehicle or craft which travels over land or water on a cushion of air provided by a downward blast is called hovercraft)

(3) Friction can be reduced by using wheels to move objects

It is quite difficult to move a heavy suitcase by dragging it on the ground because the sliding friction between the heavy suitcase and the ground is very large. If this heavy suitcase is fitted with small wheels then it can be pulled very easily. Because when we attach wheels then sliding friction is converted into rolling friction. And rolling friction between the wheels of suitcase and ground is much less.

Friction can be reduced by attaching wheels to move the objects.

(4) Friction can be reduced by using ball bearing between the moving parts of machine

Ball bearing is a device which consists of a ring of small metal balls. The small metal balls of a ball bearing can roll freely. They are designed to make the moving parts of a machine to roll over each other rather than slide. The ball bearing is introduced between the two surfaces which have to rotate over each other. The axle is fixed on the inner side of a ball bearing and wheel is fixed to the outer side of the ball bearing the ball bearing. They  reduces friction by making the two surfaces (axle and wheel) to roll over each other. This happens due to the rolling action of small metal balls present inside the ball bearing.

Even a wheel produces some friction where its central hole rubs with the axle. To reduce friction still further,  wheels are mounted on ball bearing. The ball bearing is fixed between the hub of wheel and axle. When the wheel revolves the balls of ball bearing roll and reduce friction. The use of ball bearing makes the wheel roll smoothly over the axle. In most of the machine friction is reduced by using ball bearing. The Wheels of the bicycle turn on sets of ball bearing. These ball bearings reduce friction because they roll rather than slide.

Text Solution

Solution : (a) Lubricating machinery parts with oil, grease etc. reduces friction. <br> (b) Polishing and powdering the same reduces force of friction on dancing floors, billiard and snooker boards etc. <br> (c ) Aerodynamics shape of bodies produces less air friction and water friction in vehicles and aquatic animals. <br> (d) Ball bearings and roller bearings reduce rolling friction.

A microscopic view of these surfaces in relative motion reminds us that each surface contains tiny, jagged asperities (rough and uneven surfaces), no matter how closely these surfaces are machined.

Without some form of separation, such as that formed by a lubricant film, these surface asperities may seize upon contact. At the very least, some abrasion, adhesion and/or ploughing of these asperities will take place as the movement occurs.

Factors That Affect Friction

A number of factors affect the frictional conditions at the interface between these two surfaces in relative motion. These factors are:

• Surface Finish — The number, roughness and even the directional contact points of the asperities on the surfaces can dramatically affect the frictional coefficient.

• Temperature — Both ambient and operational temperature can affect friction. For example, temperature is a critical element in whether an anti-wear or extreme pressure additive will be effective in certain applications.

• Operational Load — Friction varies directly with load. A load exceeding the designed capacity will dramatically increase the frictional coefficient.

• Relative Speed — Increasing the speed beyond that which is safely specified will dramatically increase friction.

• Nature of the Relative Motion between the Surfaces — Sliding motion versus rolling motion can affect the coefficient of friction.

• Lubricant Characteristics — These characteristics are the base oil, the viscosity of the base oil and the additives combined with the base oil for the particular formulation.

The challenge is to reduce the coefficient of friction as much as possible by either eliminating the factors that may have an adverse effect on the surface in relative motion or at the very least controlling those factors.

Reducing Friction

There are several ways to reduce friction:

1. The use of bearing surfaces that are themselves sacrificial, such as low shear materials, of which lead/copper journal bearings are an example.

2. Replace sliding friction with rolling element friction, such as with the use of rolling element bearings.

3. Improve overall lubrication either by changing viscosity, using differing or improved additives or through the use of different lubricants themselves, i.e., synthetics, solids, etc.

Surface Interaction

It is important to understand how two metal surfaces within a machine interact with each other.  All metal surfaces have some degree of surface roughness.  Regardless of how smooth a surface may appear, each metal surface has high points and low valleys. 

The high points are called the surface asperities.  When the two surfaces move past each other, it is the asperities on one surface that come in contact with the asperities on the other surface.  

The number and height of the asperities on the surfaces dramatically affect the friction between the surfaces.  It is the job of the lubricant to keep these asperities apart and prevent them from contacting each other, thus lowering or eliminating metal-to-metal contact and friction. 

Without an adequate oil film to separate the metal surfaces, metal-to-metal contact occurs, asperities from the two surfaces weld together and then are ripped apart from each other by the motion of the surfaces sliding against each other, causing adhesion (smearing) and abrasion (cutting).  In severe cases, the two pieces of metal may weld and seize to each other.

If the oil film is sufficient to keep the surfaces separated but is too high a viscosity for the speed of the moving surfaces, then some drag or internal resistance within the fluid (fluid friction) will occur.  This can be thought of as friction caused by layers of oil being forced to slide past each other .