# Fiction

## (4) Rolling Friction

• Consider a situation of the ring or a sphere rolling without slipping over a horizontal plane.In this case there is only one point of contact between the body and the plane
• The frictional forces developed between two surfaces in case described above is called rolling friction
• Rolling friction developes between two surfaces when one body rolls over the surface of another body
• We know that it is very difficult to pull a heavy metal box on a rough surface and if we attah four metal wheel to the box it becomes easiar to move the
• Thus resistance offered by the surface during rolling is relatively less than offered during sliding friction
• This is because while rolling surfaces in contact do not rub each other
• Rolling friction is negligible in comparision to the kinetic and static friction which are present simlutanously
• In many parts of the machine where this type of friction is undesirable ball bearings(small steel balls) are generally kept between the rotating parts of

## (5) Angle of friction

• It is the angle which the normal force N makes with the contact force when the equlibrium is limiting i.e when the condition of maximum static friction
• Consider a block of mass m resting on a horizontal surface .Weight W=mg of this block is balanced by the normal force (N) of reaction as shown below in the figure.

Mathematically
W=mg=-N
• if F is the contact force which each body exerts on the another body then this contact force can be resolved into two components
• Perpendicular Components which is normal force N
• Components parallel to the contact surface known as friction which is usually denoted by the fs,fk or fms
• Angle BCD respresented by λ is angle of friction
Since N and fms are components of the contact force F,so we can write
Fcosλ=N
and Fsinλ=fms
or we can say
tanλ=fms/N
Also from the defination of static friction we know that
fmssN So tan λ=μs
Thus we can conclude that coefficient of static friction is equal to the tangent of the angle of friction

## (6) Methods to Reduce Friction

• Friction can be reduced by the numbers of methods.Some of them are listed below
(a)Lubrication: when a lubricant is applied between the two surfaces in contact then a thin layer of lubricant is formed between the two surfaces resulting in reduction in friction.Example of Lubricant Grease oil,graphite,compressed air
(b)Polishing : Irregularities of a surface can be reduced by polishing the surface smooth which results in reduction in friction
(c) Friction between the two surfaces can be reduced by using ball bearing between the two surfaces.Also sliding friction can be converted into rolling friction which is much less then sliding friction