11.5 Friction - 1

A 20 kg metal block is placed on horizontal surface. A block is just begin to slide when the horizontal force of 100 N is applied to it. Calculate the coefficient of static friction. If the coefficient of kinetic friction is 0.4, find the minimum force to maintain its uniform motion.

 

Solution:

Given :

Mass of Block                                 =       m      =       20 kg

Force                                               =       F_s      =       100 N

Coefficient of kinetic friction         =       µ_k     =      0.4

Theory à Friction between two bodies in contact when one body is just move or tends to move over the other is called static friction

F_s α N

F_s = µ_s N

µ_s = F_s  / N

where, µ_s is a coefficient of static friction, which can  be defined as the ratio of limiting force of static friction to the normal reaction.

µ_s = F_s  / N

µ_s = F_s  / m g

µ_s = 100  / ( 20 X 9.8 )

µ_s = 0.5101

Kinetic  Friction:- 

The friction between two solid bodies in contact when  when one body is sliding over other body is called kinetic friction  

F_k α N

F_k = µ_k N

µ_k = F_k  / N

where, µ_k is a coefficient of kinetic friction, which can  be defined as the ratio of limiting force of kinetic  friction to the normal reaction.

F_k = µ_k N

F_k = µ_k m g

F_k = 0.4 X 20 X 9.8

F_k = 78.4 N