Journal Entry 11 THE FORCE OF FRICTION

Frictional force causes resistance to motion. It is present when any two surfaces are in contact. Friction is always opposite to the motion of an object.

There are two categories of friction, and both oppose motion. Consider this: You slide your book across the table. It soon comes to a stop. Friction causes this behavior, of course. Kinetic friction exists between objects that are sliding against each other. (Only one has to be in motion) Kinetic friction attempts to stop the motion of objects.

Static Friction exists between surface that are at rest ONLY when an outside force is attempting to set one of the objects in motion. In order to cause an object on a surface to BEGIN SLIDING , enough force must be applied to overcome static friction. Once SLIDING begins, kinetic friction takes over. Fk FA

Consider this: You’re trying to push a heavy crate across concrete. You push with enough force to match the weight of the box (or Fg), but it doesn’t move. Again, friction causes this behavior. An opposite force responds to your push…the force of static friction. Static friction increases as the applied force increases, but it has a limit. FA Fs

Pronounced “mew” or “myou”. How is Friction determined? Definition: coefficient of friction – a non-unit, scalar value that determines the amount of friction present. It is represented with “μ”. The higher the coefficient, the larger the force of friction can be. For example, Rubber on Pavement has a higher μ than Steel on Ice. Pronounced “mew” or “myou”.

Force of static friction: Fs = μsFn Fs equals the amount of force needed to set an object in motion. There is NO force of static friction if no force is attempting to move the object. Fk equals the applied force when an object is sliding at a constant speed. Generally, Fk < Fs. Once an object begins moving, friction becomes weaker in most cases. Force of static friction: Fs = μsFn Force of kinetic friction: Fk = μkFn

DO NOT LEAVE THIS SLIDE BLANK!