Understanding Force

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Coefficient of Kinetic Friction 


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Newton's First Law of Motion
Newton's Second Law of Motion
Newton's Third Law of Motion
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The coefficient of kinetic friction between two surfaces can be determined in the laboratory using the same experiment as that of kinetic friction:

    coefficient of kinetic friction experiment1

By the addition of the right weights that makes the block to move at constant velocity, the net force acting on the body becomes zero.

Then, considering all forces acting on the block, horizontally and vertically as shown below:

coefficient of kinetic friction experiment2

Resolving horizontal forces:

Kinetic frictional force, Fk minus the Tension on the string equals zero. That is,

Fk – T = 0

Fk - T = 0

Fk = T

  From the formula of kinetic friction, Fk  = µk N

Since Fk = T, then

T = µk N

Since the Tension on the string equals the attached weights, Mg, then,

  Mg =  µk N   --- equation 1

Resolving vertical forces:

Normal force N acting on the block minus weight of the block mg equals zero. That is,

N – mg = 0

N = mg

Substituting mg for N in equation 1, we have,

Mg = µk mg

Therefore  µk = Mg/mg

This means that the coefficient of kinetic friction between the two surfaces is the ratio of the attached weights and the weight of the block.

See kinetic friction here
See static friction here
See coefficient of static friction here
See friction here
See calculating friction here