Welcome to the Wonderful World of Friction

What is friction?

Friction is:

• Friction: A force that resists the relative motion or tendency to such motion of two bodies in contact.

•Source: Dictionary.com

Or why this won’t work very well

Places where friction is good

• The soles of shoes

• Car tires

• The feet of table legs

• Rock climbing holds

• Brakes

• etc

Places where friction is bad

• In engines

• In transmissions

• On the bottoms of skis and snowboards (To a point)

• When you are trying to move a box by pushing it along the floor

• etc

What causes friction?

Source of Friction

• Picture of extremely smooth mica with a scanned probe microscope

At the microscopic level even the smoothest of surfaces is dotted with little “mountain peaks”.

The tips of the peaks are the only parts that touch the other material.

Only a very small portion of the apparent surface area is in contact with the other surface

This causes extremely high pressures to form on the parts that touch. This causes the two surfaces to become “welded” almost at the points of contact

Source of friction (continued)

• The true surface contact area is proportional to the normal force because the peaks will deform plastically when force is applied increasing the contact area

• Plastic deformation: to change shape permanently without fracturing

• Other lesser reasons for friction

• Surface adhesion between pure metals

• Ploughing of one surface by the other harder one

• Elastic deformation

Experiment: Does the friction force depend on surface area?

• We stick a 1kg mass on a piece of plexiglass on top of the clean surface and, using a Newton meter, pull the weight across the table with the string provided at constant speed.

• We put the same weight on a different piece of plexiglass with drastically different surface area and repeat

• What do you notice?

Friction doesn’t depend on apparent surface area (much)

• You may have noticed a small dependence. This is partially due to the greater weight of the larger piece of plexiglass, and also partially due to the fact that there is a dependence of friction on surface area, just one that is small enough to be ignored under most circumstances.

• Why is the friction force approximately independent of surface area? Because as discussed previously the materials plastically deform at the peaks depending on the pressure increasing real surface area.

• RSA = real surface area, F = Normal Force, SA = Surface area, F/SA = P Pressure, C = constant depending on the materials

CF

CSA

FSA

CPSARSA

What is k ands?

• The coefficient of friction is different depending on if the objects are in motion relative to each other or not as in the graph. s is the coefficient of static friction and k is the coefficient of kinetic friction.

Why?

• The slightly larger value for static friction results from irregularities and contaminants on the surfaces and is less accurate in general than the coefficient of kinetic friction

• Between (very) carefully cleaned surfaces the difference between the two coefficients disappears

Finally something useful..

The only formula you need for friction forces:

or

Ff = Fnk

force normalfriction kinetic oft coefficien ForceFriction

What is the normal force?

Newton’s Second and Third Laws

• Newton’s Second Law: If there is an unbalanced force on an object it will accelerate according to FUnbalanced =

• Newton’s Third Law: If you exert a force on an object it exerts an equal and opposite force on you

am

Block at Rest on a Table

Normal Force

• From Newton’s third law we know that if gravity or some other force pushes an object (like a block) into a second object (like a table) that second object will be exerting an

equal force back on the first. • Normal force is the force the table exerts back on the

block• Normal force is always exerted perpendicular to the

surface• Friction Force is always parallel to the surface• So if the table is horizontal and gravity is the only force

on the block Fn = -Fg

Normal force on a hill

• Normal force is exerted perpendicular to the surface in accordance with Newton’s Third law

• No unbalanced force so the block is stationary or at least not accelerating

Typical Physics question

Ancient Egyptians are pushing a stone block up the side of a pyramid of angle 20 degrees. What stone should they be using so they can take a break and not have it slide down, but have it still be not much over the coefficient of friction needed so they don’t have to push so hard?

Stone A .2

Stone B .3

Stone C .4

Stone D .5

Solution

)20tan(

)20cos(

)20sin(

)20sin()20cos(

)20sin()20cos(

)20sin(

)20cos(

)20cos(

)(

)(

k

k

k

ksurfaceparalleltogf

surfaceparalleltog

nkf

n

gn

gmgmFF

gmF

FF

gmF

FF

Experiment

• With the Newton meter find the coefficient of kinetic and static friction for the plexiglass and the plastic track.

Wormbot?

• Wormbot is a crawling machine divided into three sections.

• Each section weights 3.67 kg

• Each section has a plexiglass bottom layer so they each have the same coefficient of friction that we found earlier for plexiglass on the surface.

Can Wormbot gain a net movement in a direction?

• Why is this possible/impossible?

If you have two weights with the coefficient of friction connected by

something that can expand and contract can you gain a net

movement?What if you have 4 weights with connections

in between?

What if you have three weights but one is heavier than the other two. How much heavier can it be?

How much weight can it pull on flat ground with the corner pulley?

Does it matter where the additional weight

is attached?

How does putting the weight on plexiglass on the table and getting it to tow it that

way change things?

If you were to put Wormbot on a

table at an angle what is the

steepest angle he could climb?