FRICTION: A FORCE THAT OPPOSES

MOTION

Chapter 5, Section 3

FRICTION: A FORCE THAT OPPOSES MOTION

Friction is a force that opposes motion between

two surfaces that are in contact.

Friction can cause a moving object to slow down

and eventually stop moving.

I. THE SOURCE OF FRICTION

Friction occurs because the surface of any

object is rough.

Even surfaces that feel smooth are covered with

microscopic hills and valleys.

When two surfaces are in contact,

the hills and valleys of one surface

stick to the hills and valleys of the

other surface, which causes friction.

Two factors affect friction:

Amount of force pushing the surfaces together

Roughness (texture) of the surfaces

A. THE EFFECT OF FORCE ON FRICTION

If the amount of force pushing two objects together increases, the hills and valleys of the surfaces come into closer contact – this closer contact increases the friction between the surfaces.

Objects that weigh less exert less downward force than objects that weigh more do.

However, changing how much of the

surfaces come in contact does not

change the amount

of friction.

(changing surface

area does NOT

change the amount

of friction)

B. THE EFFECT OF ROUGHER SURFACES ON FRICTION

Rough surfaces have more microscopic hills and

valleys than smooth surfaces do.

The rougher the surface is, the greater the

friction. http://my.hrw.com/sh2/sh07_10/student/flash/visual_concepts/70316.htm

II. TYPES OF FRICTION

Friction

Kinetic Static

Sliding Rolling

Fluid http://my.hrw.com/sh2/sh07_10/student/flash/visual_concepts/70732.htm

A. KINETIC FRICTION

Kinetic means to move, so kinetic friction is the

friction between moving surfaces.

The amount of kinetic friction depends on how

the surfaces move.

Sliding Kinetic Friction:

When two objects slide past one another

Pushing a textbook across the table

Pushing a couch across the

hardwood floor.

Rolling Kinetic Friction:

When one object rolls past another

Skate board on a hilly sidewalk

Pushing a computer cart down the hallway

Usually the force of sliding friction is greater than the

force of rolling friction so it is easier to move objects on

wheels.

KINETIC

Fluid Kinetic Friction

Fluid friction occurs when a fluid is next to an object’s

surface. (A fluid is any substance in the liquid or gas

states of matter.)

The fluid (water, oil, wax, sometimes air) fills in the

hills and valleys which allows the object to move very

easily.

B. STATIC FRICTION

When a force is applied to an object but does not

cause the object to move, static friction occurs.

The object does not move because the force of

static friction balances the force applied by the

object to another surface.

Static friction can be overcome by applying a large

enough force.

Static friction disappears as soon as an object

starts moving – switching to kinetic friction.

III. FRICTION: HARMFUL AND HELPFUL

Friction is both harmful and helpful.

Friction causes erosion, holes in socks, wearing down of

car engine parts.

Friction causes your pen to be able to write on paper, you

to walk without falling (most of the time), your car to stop

when the brakes are applied.

A. REDUCING FRICTION One way to reduce friction is to use a lubricant – a

fluid applied between solid surfaces. Motor oil, wax, grease

Another way is to switch from sliding to rolling friction.

Another way is to make surfaces smoother before they slide against one another.

B. INCREASE FRICTION

One way to increase friction is to make surfaces

rougher.

Sand on icy sidewalks

Another way is to increase the force that is

pushing the surfaces together.

Increasing the force when scrubbing dirty dishes

http://my.hrw.com/sh2/sh07_10/student/flash/visual_concepts/70734.htm