# Static Friction

## (2) Static Friction

• We already know that frictional forces can also act between the bodies in contact with each other even if they are not moving and such type frictional force is known as static friction
• Consider a heavy metal block kept on the floor and you are trying very hard to push it to another location and you are not able to slide it even by a centimetre
• Since the block is at rest resultant force on it should be zero.To counter balance the force applied by you ,floor exerts a frictional force on the block s
• Now if you begin to increase the magnitude of force gradually then block does not start moving until force applied is greater then a minimum value of force
• This force of static friction must be overcome by the applied force before the body at rest begin to move
• Force of friction is always equal and opposite to the external applied force as long as the body is at rest
• This means that static friction force is a self adjusting force.It adjust its value accordingly with the increase in magnitude of applied force
• This frictional force cannot be unlimited and its value cannot go beyond a maximum value fms
• This maximum value of static friction between the two surfaces in contact is known as limiting friction
• Thus magnitude of static friction can not go beyond the magnitude of the limiting friction i.e, fs <= fms
• This limiting friction is proportional to the normal contact force (N) between the bodies i.e,
fms is proportional to N
fmssN
Where N =Normal contact force
μs is the proportionality constant known as coefficient of static friction
• Value of coefficient of static friction depends on the material and roughness of the surfaces of bodies in contact
• fms is the maximum value that force of static friction acting between two bodies can reach
• The actual force of static friction can be equal to zero or less then fms and its value depends on the force applied on the body thus
fs <= fmssN