Friction

There are several responsive forces which:

Act to resist movement

When you move move an object across a surface:

The rubbing of the two surfaces result in kinetic friction

Kinetic Friction:

Always acts in a direction opposite to the motion of the object

Because it always opposes motion:

Kinetic Friction will eventually slow any moving object to a stop

Ice is polished to minimize friction, but unless it’s constantly being pushed:

A hockey puck will eventually slide to a stop in even the most slippery of rinks

When an object is not moving, a force that attempts to slide it across a surface:

Will encounter a different type of friction

Attempting to push a stationary box across a concrete floor will:

Require a person to overcome the static friction that opposes any attempted movement

Two kinds of friction are based on two factors:

The coefficient of friction(mu), and normal friction(FN)

What does the coefficient of friction(mu) represent?

How much two materials resist sliding against each other

The normal force(FN) represents?

The equal and opposite force a surface exerts when an object presses against it

What is the formula for kinetic friction?

Ff=muk x Fn

What is the formula for static friction?

Ff=mus x Fn

The coefficients of friction are constant:

For any two surfaces in contact

The coefficients of friction are:

Dimensionless quantities, with no units of their own

Therefore, whatever force unit is used to measure Fn(N or lbs, for example):

Will also be used to measure Ff

Slippery surfaces like ice or wet asphalt:

Have low coefficients of both kinetic and static friction with most objects

Why is it important for motorists to understand that weather conditions can make braking to a stop less effective?

Because friction is directly proportional to these coefficients during weather conditions

Another responsive force comes from air resistance, sometimes known as:

Air Friction but more commonly known as a drag

Drag opposes movement:

Not across a solid surface but through a fluid, like water or air

The two main variables that affect drag are:

Cross sectional area and speed

Why are High performance cars, boats, and aircraft are designed to be sleek and pointy?

To minimize the area of the side that is plowing through the air or water

Parachutes work by intentionally increasing area for a diver so:

That drag is increased and the person can be slowed down to a safe speed before hitting the ground