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Friction is the force that opposes movement.

The 3 Laws of Friction

1. The limiting frictional force (F_L) is directly proportional to the normal contact force (N)

note: the limiting frictional force is the maximum frictional force

2. The ratio of the limiting frictional force ( F_L ) to the normal contact force( N ) is called the coefficient of friction (μ) ... the Greek letter 'mu'

3. When there is no motion, but the object is on the point of moving,

applied force = frictional force (limiting friction)

and when there is motion,

applied force > frictional force(limiting friction)

then this equality applies:

F_L = μ N

Up to this point, when the frictional force is less than limiting friction(maximum)*, then the inequality below applies.

F_L< μ N

*object is static and not on the point of moving

Example #1

A flat stone is thrown horizontally across a frozen lake.

If the stone decelerates at 2.5 ms^-2 , what is the coefficient of friction between the stone and the ice? (take g=10 ms^-2)

Example #2

A 2 kg mass in limiting equilibrium rests on a rough plane inclined at an angle of 30 deg. to the horizontal.

Show that the coefficient of friction between the mass and the plane is √3 / 3 .

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The Angle of Friction

If we examine the normal reaction force( N ) and the frictional force( F_L ) when it is limiting, then the equation F_L = μ N applies.

If the resultant between N and F_L is R , and it is inclined at an angle α(alpha) to the normal N, then we can write equations for F_L and N in terms of R .

In Example #2(above) the angle of friction = tan ^-1(√3 / 3)

= tan ^-1(0.5773) = 30^o (the angle of the plane)

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