FrictionFriction is the term is used to

describe forces that result from the relative motion

between objects.

What that means is that when two objects come into contact with one another, a type

of resistance force is created.

• Frictional forces always work against the motion that produces them.– For example, when you watch a Hockey game

and the puck is sliding across the ice, the interaction between the ice and the puck creates the frictional forces that resist motion.

• This is why the puck will eventually slow and stop.– And this is also why we have that most wonderful

invention, the Zambonie. » It smoothes the ice in order to reduce the friction that

builds up when the ice is “Rough” or there is a lot of lose ice particles on it.

As mentioned earlier, Friction holds back the motion of an object.

• Friction comes from two surfaces moving against each other.– Where ever you have moving parts coming into

contact with one another, you want to have smooth surfaces to decrease the amount of friction that is incurred.

• Even the smoothest of surfaces have microscopic ridges, hills, bumps, valleys, etc. running across them.

• Wherever there is friction there are problems!

Where ever there is friction there is WEAR.

• Objects that rub against each other cause Wear.• Wear refers to how moving parts can erode each

other.– Erosion is the loss of surface area, and this can lead

to many different problems.• The least of which occurs when parts that used to fit snuggly

and securely now are loose and sloppy.• When parts begin to get loose they start hitting each other

with increasing force and this adds more friction to the overall effect.

– The more force between the parts in contact the more friction, and the more heat that is produced.

• Friction can also lead to problems associated with heat.

When you’re cold what is one of the things that you can do to warm your

hands?• Why, you rub them together!

– This contact between your hands and the motion of you rubbing them together creates the friction that you feel in the form of the warmth that you want and need.

• But this heat isn’t always wanted, especially in industry.– Engineers strive for extremely smooth surfaces.

• This is because they want to reduce the collisions between the surfaces and in turn reduce the friction that slows the parts.

• They can make these surfaces even smoother by introducing a lubricant such as oil into the interaction.– What oil or any other lubricant does is that it fills in the nooks

and crannies that are found in even the smoothest of surfaces.• This reduces the overall surface area that is effected by friction.

The main thing that Friction does however is that is slows the objects

overall speed.• Everything on the Planet is effected by

Friction.– The only place that isn’t effected by friction is

outer space.• But the things in outer space are effected by the

force of gravity.

• There are many different types of Friction.– Let’s take a look at a few of the more obvious

ones that can effect us everyday.

Air Friction

• The air moving around objects creates an opposing force.– This is why many modes of transportation have rounded, or

aerodynamic shapes.• These shapes allow the air to flow around them in a more efficient

manner, with less drag.

• This is sometimes called “Air Resistance.”

Sliding Friction

• When two surfaces rub against each other, we get sliding friction.– Sliding friction is caused by irregularities in the surfaces of the

interacting objects.

Viscous Friction

• This occurs with objects that move in water or some other type of fluid.– When we use oil as a lubricant, we are doing it to reduce the

sliding friction and change it into viscous friction.• That reduces wear.

Rolling Friction

• In this type of friction one object is rolling over another, like a wheel rolling over the road.– Ball bearings in axles and wheels are designed to reduce the

effect of rolling friction.

How does Friction effect Acceleration?

• Friction is a force that always opposes motion.– That means the force of friction is working opposite

whatever force is causing motion.• If you are rolling downhill due to gravity, then

friction is going to eventually slow you to a stop, because it is working in the opposite direction.– It’s actually pushing whatever is rolling downhill, up

the hill!• This means that friction reduces acceleration!

– We can overcome friction but only by adding an additional force to the equation, like adding an engine to overcome the friction caused by the interaction between the tires and the road.

We need to include Friction when we look at the Net Force on any

object!• When we look at the “F” in Newton’s 2nd Law, the

“F” stands for the Total Force present.– To find the total force, we need to subtract the friction

force from the acceleration of gravity.• What is left is called the “Net Force”, or the Total

Force.– Remember this frictional force will always be in a

direction opposite to that of gravity.• That means that when an object is dropped say from an

airplane, the gravity is accelerating it downward, but the air resistance is a type of friction and it is pushing upward on the object trying to slow it.

– We need to subtract the air resistance (friction) from the gravitational force!

No matter how much you want to deny it….

Friction is a part of every interaction on the surface of

Earth!

All true machines have Friction.

• That means there are always forces that tend to oppose motion.

• Unless you continually supply force, eventually, friction will slow everything to a stop.

• Even the smoothest running, most efficient cars, trains, planes, etc. will eventually come to a standstill.

Throughout history the Human Race has tried to invent a machine that will run forever once started

with no additional force to keep it going.

• This type of machine would have to run with absolutely NO effect of friction slowing it!– This is known as the “Perpetual Motion Machine.”

• And since friction opposes motion there is no way that this can happen.– If anyone ever says that they have invented one of

these machines, be suspicious!• They probably have a hidden power source somewhere!

• These machines will never work because there is no escaping Friction!

Here are some of the FAILED Perpetual Motion Machines that

people have tried to pass off!

Most of these machine use water to provide the initial force.

• The Museum of Unworkable Devices

• While others used magnets.