Coefficient of Static Fraction

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Static friction is the frictional force generated when an object placed on a rough surface is given some force, to make it move.So the characteristic of static friction which prevents motion till a certain limit is called the coefficient of static friction represented by ‘µs’.The coefficient of static friction is usually denoted just by a number and has no units. Because of the interlocking of the rough surfaces, the static friction prevents any relative motion but at a certain limit of applied force, the object resists friction and gets into motion. 

Example 1: The force applied on a box of mass 5kg to push it on the ground is 65N. If the box does not move with the applied force, then what is the coefficient of friction in this situation?

Here the applied force = Frictional force since the box is not moving = 65N

Static friction, fs = μs * N

Here, μs= coefficient of static friction

N = Normal force= Weight of the object= (mass) * (gravity) = 5kg * 9.8m/sec2 = 49N

So, 65 = μs * (49)==>μs = (65)/ (49) ==>μs = 1.33

Therefore the coefficient of friction, μs = 1.33

Example 2: If the coefficient of maximum static friction of the surface on which an object is placed is 0.86, then what is the exact amount of force needed to move the object of mass 10kg from rest?

Static friction, fs = μs * N

Here, μs= coefficient of static friction = 0.86

N= Normal force= Weight of the object = (mass) * (gravity) = 10kg * 9.8m/sec2 = 98N

So, fs = 0.86 * (98) ==>fs = 84.3N

Therefore the maximum static frictional force required, fs = 84.3N
 


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