friction. a world without friction… watch?v=vufqjseezng watch?v=vufqjseezng

Friction

A World Without Friction…

•https://www.youtube.com/watch?v=VUfqjSeeZng

Friction

•A force that resists motion

•Occurs due to the attraction between particles▫To walk you have to break the attraction

between the particles in your foot and the particles in the floor

▫To continue sliding you have to resist the particles in your foot being attracted to the particles in the floor

Two Types of Friction•Kinetic friction

▫Occurs when the object is in motion▫Acts against motion

•Static friction▫Occurs when the object is stationary▫Acts to resist being put in motion

Which is bigger? Static or Kinetic Friction•Think of Newton’s first law!

•Force to get something moving (static) is greater than the force to keep something moving (kinetic)

•The object at rest wants to stay at rest so the static force is greater because the object in motion wants to stay in motion

Special Value of Static Friction

Limiting Static Friction•the amount of force that must be

overcome to start a stationary object moving

•The object will not move unless you apply more than this amount of force

The Difference!**** (Hilarious)- www.youtube.com/watch?v=JS4ODWG7Els*** (Bill Nye) - www.youtube.com/watch?v=-pZbM17-zzk** (Guy talking friction)-

www.youtube.com/watch?v=CTLXubXOTUQ* - https://www.youtube.com/watch?v=L3if3_pOVl0

How do we measure friction?•Weigh an object with a force scale•Pull an object with a force scale•Divide the two values•We call the result the coefficient of friction

•Let’s give it a try! Materials: Mass, Newton Scale▫Measure:

The force needed to start the motion The force needed to keep the motion constant Weight (Fg) of the mass

What surface is your desk?

•http://www.engineershandbook.com/Tables/frictioncoefficients.htm

•Masses are steel!

Coefficient of Friction ()•Ratio of the magnitude of the frictional force to the normal force

•In other words… Divide how much friction there is by the force between the two surface

•Depends on the two surfaces in contact

The Math

N

kfrk

N

sfrs

F

F

F

F

Nkfr

Nsfr

FF

FF

k

s

m- Coefficient of Friction (no units) Ffr – Force of Friction (N) FN – Normal Force (N)

Example # 1

The coefficient of static friction between glass and glass is 0.9. Find the minimum force required to start an 8.0 kg mass moving.

Example # 2

•An 8.0kg teflon pot is on a steel table. If it takes 9.4N of force to keep the pot moving at a constant speed. What is the coefficient of friction between the pot and table?

Example # 3

FN = Fg = ma = (1.810 kg)(9.81 m/s2)

= N µS = FS

FN = 12 N 17.8 N = 0.67  

A textbook has a mass of 1810 g. A force of 12 N is needed to just start the book moving across the table.a.) Calculate the coefficient of static frictionb.) If a second identical book is placed on top of the first book when it is at rest, what horizontal force would be needed to just start the book sliding?

FN = (1.810 X 2 kg)(9.81 m/s2) = 35.5 NFs = µS FN

= (0.67)(35.5 N) = 24N

Need help?

Khan Academyhttps://www.youtube.com/watch?v=ZA_D4O6l1lo

Controlling Friction

•How can we reduce friction?

•Why do we need to reduce friction?

Keep in Mind!

•Friction wastes Energy•Turns useful energy into heat

Controlling Friction

What is used to increase friction?

Why do we need to increase friction?

Industrial Applications

•Inertial Friction Welding▫ https://www.youtube.com/watch?v=5JbnDXw-0pM

•Creating a Friction Fire▫ http://www.youtube.com/watch?v=VOmwLxgI06c