Friction Good or bad?

Frictional forces resist or oppose motion. (Left) This is a scanning electron microscope close up of an insect’s leg. To our naked eye, it seems smooth, but close-up is another view. (Right) This an SEM of an egg shell. An egg shell feels smooth, yet close- up you can see it is not.

What is friction? As seen on the previous slide, all surfaces have microscopic hills and valleys. When two surfaces are in contact, and slide against (or past) one another, these hills and valleys interfere with each other. Lots of hills and valleys = high friction Frictional forces resist or oppose motion.

Types of Friction There are five types of friction that we will look at in this course. –Static –Sliding –Rolling –Air –Viscous

Static Friction When an object is at rest, you must apply a force large enough to overcome its inertia AND the interference caused by the two surfaces touching one another (friction). Static means “still” or no motion.

Sliding Friction When two surfaces are sliding past one another, the interference experienced by the surfaces interacting is called sliding friction. This is also a type of kinetic friction. Most types of kinetic friction are not as strong as static friction.

Rolling Friction As implied by the name, this type of friction is experienced when one object rolls over another. This is also a type of kinetic friction. Rolling friction is usually weaker than sliding friction.

Air Friction When air moves around an object, the object’s motion is slowed by air resistance. We looked at this during our study of falling objects. This is also a type of kinetic friction. The strength of air resistance can vary depending on pressure, speed, temperature, and make-up of the air.

Viscous Friction Much like air friction, when an object moves through water, or any fluid, the resistance of the fluid to move out of the way causes viscous friction. This is also a type of kinetic friction. The strength of viscous friction can vary depending on the make-up of the fluid, speed, temperature, and pressure. Compare the viscous friction of honey to water…

So, is friction good or bad? Neither. Sometimes we use friction to our advantage, and it helps us accomplish a task. Ex. Walking, flying, parachuting, stopping while driving Sometimes friction makes it more difficult to accomplish a task. Ex. Sliding a heavy box, turning a tight bolt, walking through mud

Coefficient of Friction Friction depends on the surfaces in contact. Example: It is easier to push a desk across a tile floor than across the same floor covered in carpet.

The coefficient of friction is a ratio of forces. The coefficient of kinetic friction is the ratio of the force of kinetic friction to the normal force. μ k = F k /F n μ = the coefficient of friction

Coefficient of friction: static friction The coeffiecient of static friction is the ratio of the maximum value of the force of static friction to the normal force. μ s = F s /F N

Force with friction accounted for: To calculate the magnitude of the force of friction: F fr = μF N

Example A 24kg crate at rest on the floor requires a 75N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor.

Importance of Static Friction