Friction: Friction: from book, ever present resistance to motion whenever two materials are in contact with each other. Friction: (ME) Two surfaces rubbing together and their stickiness Causes Frictional Forces It’s time to experience friction so rub your hands together. 

Friction: Two surfaces rubbing together and their stickiness Friction: Two surfaces rubbing together and their stickiness. Always direction opposite of motion. frictional force: A force that slows motion, or prevents motion from starting. Friction: frictional forces arise at the contact points between the molecules of the different bodies. Contact force that is resistance to Motion. ALWAYS OPPOSES MOTION, i.e, in opposite direction of motion. Friction increases as contact pressure increases, i.e., more weight means more friction. Usually dissipated as heat or usually generates heat. SURFACE AREA DOES NOT CHANGE FRICTION. * Friction Force is Usually lowercase “f ” usually in italics.

Frictional Forces Static friction – REST occurs during Motion. frictional force: A force that slows motion, or prevents motion from starting. 2 kinds – Kinetic friction – MOTION slows objects down. occurs during Motion. Static friction – REST prevents motion from starting. occurs before motion, when still, not moving, or at REST. Which is bigger?????

Frictional Forces Which is bigger????? Question: Does a car stop faster when the tires are locked and sliding or when rolling? 2 kinds – Kinetic friction – MOTION Sliding tires Static friction – REST Rolling tires

New Symbol μ – greek letter “Mu” Coefficient of friction. μs – coefficient of static friction μk – coefficient of kinetic friction Coefficient of friction: Decimal between 0.0 and 1.0, unitless. Decimal percent of force in the normal direction. (Mu is percent of weight that turns into friction force.) Coefficient of friction: μ where f = μ FN Usually just given in the problem, depends on materials used and their surface conditions.

Static and Kinetic Frictional Forces Kinetic Frictional Force: friction force that occurs while sliding. The magnitude fk of the kinetic frictional force is given by: fk = k FN where  k is the coefficient of Kinetic friction, and FN is the magnitude of the normal force. Normal Force: Force that is 90 degrees to the surfaces. (Normal is 90) degrees. Kinetic frictional forces occur when the object is moving; it acts to slow down the motion! Frictional forces are independent of the area of contact between the surfaces! (talk about sides of block then race tires)

f F Static Friction, ƒs Just enough force to keep object at rest. ms is coefficient of static friction N is the normal force f F

f F Kinetic Friction, ƒk mk is coefficient of kinetic friction Friction force opposes direction of motion N is the normal force F

Coefficients of Friction

Instructions for demo on next slide: Place light mole box on table and slide. Place brick on table and slide. List mu static = .4, mu k = .3 Mass of light box is 2kg. Mass of brick is 10 kg. Find Friction resistance force when static and when in motion on board. Find both friction resistance and Reaction Force. Same for Static but Not the same thing when in accelerating.

Newton’s 3rd Law: Reaction Force Diagram F FR F = Push by fingers = 90 N FReaction = Push Back = -90 N No exception, F always = -FR Note: Reaction force is sometimes all friction and sometimes not.

Free Body Diagram in X direction Newton’s 2rd Law: ΣF = ma Free Body Diagram in X direction (Box is in motion) ΣF = ma F fk fk = Kinetic Friction = k FN F = Push Net Force = ΣF = ma ΣF = Fpush + fk = ma Can also find acceleration: a = ΣF/ m

Free Body Diagram in X direction Newton’s 2rd Law: ΣF = ma Free Body Diagram in X direction (Box is in motion) ΣF = ma F fk fk = Kinetic Friction = k FN = -30N F = Push = 90N Net Force = ΣF = ma ΣF = Fpush + fk = ma ΣF = 90N - 30N = 60N Can also find acceleration a = ΣF/ m gives a = 60N / 10 kg = 6 m/s2

Kinetic Friction Force Static Friction Force μs = 0.4 fs = μsFN Box fs = 0.4 x 20 N = 8 N Brick fs = 0.4 x 100 N = 40 N Also known as breaking force: = the force that will begin movement = maximum Static Friction Force = μsFN Kinetic Friction Force μk = 0.3 fk = μkFN Box fk = 0.3 x 20 N = 6 N Brick fk = 0.3 x 100 N = 30 N

Static and Kinetic Frictional Force Example A sled is traveling 4.00m/s along a horizontal stretch of snow. The coefficient of kinetic friction is k = 0.0500. How far does the sled go before stopping?

Static and Kinetic Frictional Force Formula 1) FN= mg = weight 2) ΣF= fk , fk = k FN 3) ΣF= max = k FN 4) max = k mg 5) (/m=>) ax= k g 6) ax = .05 (9.80m/s2) 7)vf2=vo2+2axx. Find x Known Variables k = 0.0500 vo = 4.00 m/s vf = 0.00 m/s Unknown Variables ax = x =

Static and Kinetic Frictional Force Formula 1) FN= mg = weight 2) ΣF= fk , fk = k FN 3) ΣF= max = k FN 4) max = k mg 5) (/m=>) ax= k g 6) ax = .05 (9.80m/s2) 7)vf2=vo2+2axx. Find x Known Variables k = 0.0500 vo = 4.00 m/s vf = 0.00 m/s Unknown Variables ax = 0.49 m/s2 x =16.3m

Static and Kinetic Frictional Force Formula 1) FN= mg = weight 2) ΣF= fk , fk = k FN 3) ΣF= max = k FN 4) max = k mg 5) (/m=>) ax= k g 6) ax = .05 (9.80m/s2) 7)vf2=vo2+2axx. Find x Did we need to know the mass of the sleder? No. Why? It cancels out in the ax equation. Real Life Ap: This applies to car tires in accidents. By measuring the length of skid marks, they can calculate the speed a car was going before an accident. k of a tire is the same for all cars since it does not depend on car mass or surface area of the tires.

Static and Kinetic Frictional Forces Static Frictional Force: Reaction force to anything trying to start motion. Equal and opposite to applied force. DOES NOT EXCEED THE APPLIED FORCE, but is equal to it.

Static and Kinetic Frictional Forces Static Frictional Force: Reaction force to anything trying to start motion. Equal and opposite to applied force, until reaches maximum value and motion starts. friction “breaks” when F is great enough and motion begins.

Static and Kinetic Frictional Forces Static Frictional Force Breaks at a certain value: fs = s FN fs = force of static friction s = coefficient of static friction FN = Normal force

Static and Kinetic Frictional Forces Static Frictional Force Breaks at a certain value: fs = s FN fs = force of static friction s = coefficient of static friction FN = Normal force s is a given value. It depends on the object and the surface.

Static and Kinetic Frictional Forces Static Frictional Force: fs = force of static friction s = coefficient of static friction FN = Normal force (usually weight) Normal force is usually just the weight of the object. FN = Mass* 9.80 m/s2 IMPORTANT!!!!! If the surface is not horizontal use trig. Multiply by cos of the angle of incline.

Notes on friction Almost always: μs > μk It is easier to keep an object moving than it is to start from rest. Think about pushing a car. Both are almost always less than 1. If it was greater than one, it would be easier to pick the object up and carry it than it would be to push it across the flat surface (something like velcro)