how is static friction measured?

Answer 1

The simple way is an inclined plane. Since it is a function of the two materials involved the inclined plane must be made of one of the materials.

As the angle of the incline is increased the object will start to slide down the incline. You can do the math but it works out like this. Just before the object slides the gravitational acceleration and the static friction are just balanced so that sin (angle) x g = cos (angle) x friction factor. So the friction factor is tan (angle).
At the point just before the item starts to it is the static friction factor.

At some smaller angle the item will continue to slide if it is pushed (started). That would be the sliding friction which is usually lower and more useful than the static friction.

Answer 2

The force of static friction is the minimum force required to be applied to an object before the object will begin to move from rest with respect to the surface.

The force of static friction is dependent on the applied force on the object. If the applied force on the object is zero, the force of static friction is zero. As the applied force increase, so does the force of static friction up until the point where the static frictional force reaches its maximum...after this point the object will being to accelerate from rest.

The maximum force of static friction is given by,
F_f = coefficient of static friction * normal force
Coefficients of friction are usually symbolized with the Greek character mu, µ.

Normal force is given as,
F_n = mg*cos(theta)
where theta is the angle the surface makes with respect to the ground.
You can see that if the object is resting on a horizontal place, cos(theta) = 1 and the normal force equals the object's weight. Or if the object is resting on a vertical plane, cos(theta) = 0 and the normal force = 0. If the plane is at any arbitrary reference angle between horizontal and vertical then the normal force is somewhere between the object's weight and zero.

One the object begins to move, the force of static friction is replaced by the force of kinetic friction which has a lower value.

Answer 3

All friction is measured by the coefficient of friction.

The formula is µ = f / N, where µ is the coefficient of friction, f is the amount of force that resists motion, and N is the normal force.

Normal force is the force at which one surface is being pushed into another.

If a rock that weighs 50 newtons is lying on the ground, then the normal force is that 50 newtons of force.

The higher µ is, the more force resists motion if two objects are sliding past each other.