Friction

All forces act at a distance. This is seen more obviously in the case of gravity. There is gravity affecting the attachment of the moon to the earth even though they are not connected. Friction is an electromagnetic force between atoms that do not literally touch. This brings on the concept of a force field that surrounds an object that creates the force. The field or force field becomes the thing that carries the force from one object to another. It depends on the object creating it and not on the object being acted on. This is why the force of gravity is the same for everything on the earth. A force field is also seen with electrical charges.

FRICTION

Friction is a force that opposes relative motion between systems that also allow for movement as it would be difficult to walk, for example, without friction against the surface we walk on. It is always a force in parallel with the contact surface. If there are two systems in contact with one another and moving, the friction is called kinetic friction; if the object is stationary, the friction is called static friction. In addition, if an object is stationary, the static friction must be greater than kinetic friction.

There is a certain magnitude of static friction, which is equal to the coefficient of static friction multiplied by N, which is the magnitude of the normal force (the opposite of the weight of an object). In reality, the static friction can be less than this product but it can be no greater than the product of mu-s (the coefficient of static friction) and N. If the force applied exceeds the static friction, the object will move. Once moving, the magnitude of kinetic friction is mu-k multiplied by the normal force, in which mu-k is the coefficient of kinetic friction.

What are the coefficient of static friction and the coefficient of kinetic friction? These are arbitrary and unitless numbers that, of course, depend on the substance itself and the surface it resides on. There are tables that exist which will list the coefficient of friction for things like rubber on cement that can be used by engineers who study friction and the effects of friction. Friction on an incline will still be based on the coefficient of friction but, because the person is on an incline (such as a wagon on an

incline), the weight of the wagon is not perpendicular to the slope but is perpendicular to the earth as is seen in figure 18:

Figure 18.

If the force of friction is less than the parallel weight of the wagon, then there will be acceleration down the slope. The normal weight will be mass times the force of gravity times cosine of the angle of the incline. Kinetic friction is defined as mu-k (the kinetic coefficient of friction) times the mass of the wagon times the force of gravity times the cosine of the angle of the incline. Figure 19 looks at these equations:

Figure 19.