Friction: Friction: from book, ever present resistance to motion whenever two materials are in contact with each other. Friction: (ME) Two surfaces rubbing

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 stickinessCauses Frictional Forces

It’s time to experience friction so rub your hands

together.

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• 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)

Static Friction, ƒs

• Just enough force to keep object at rest.

• s is coefficient of static friction

• N is the normal force

f

F

Kinetic Friction, ƒk

• k is coefficient of kinetic friction

• Friction force opposes direction of motion

• N is the normal force

F

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.

F = Push by fingers = 90 NFReaction = Push Back = -90 NNo exception, F always = -FR

Note: Reaction force is sometimes all friction and sometimes not.

Newton’s 3rd Law: Reaction Force Diagram

F FR

fk = Kinetic Friction = k FNF = Push Net Force = ΣF = ma ΣF = Fpush + fk = ma Can also find acceleration: a = ΣF/ m

Newton’s 2rd Law: ΣF = maFree Body Diagram in X direction

(Box is in motion)

FΣF = ma

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 accelerationa = ΣF/ m gives a = 60N / 10 kg = 6 m/s2

Newton’s 2rd Law: ΣF = maFree Body Diagram in X direction

(Box is in motion)

FΣF = ma

fk

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?


Known Variables k = 0.0500vo = 4.00 m/svf = 0.00 m/s

Unknown Variables ax = x =

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=vo

2+2axx. Find x


Known Variables k = 0.0500vo = 4.00 m/svf = 0.00 m/s

Unknown Variables ax = 0.49 m/s2

x =16.3m



3) ΣF= max = k FN

4) max = k mg

5) (/m=>) ax= k g

6) ax = .05 (9.80m/s2)

7)vf2=vo

2+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.



3) ΣF= max = k FN

4) max = k mg

5) (/m=>) ax= k g

6) ax = .05 (9.80m/s2)

7)vf2=vo

2+2axx. Find x

Static and Kinetic Frictional ForcesStatic 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.


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 ForcesStatic 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 ForcesStatic Frictional Force Breaks at a certain value:

fs = s FN



FN = Normal force

s is a given value. It depends on the object and

the surface.




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)

Documents

Friction: Friction: from book, ever present resistance to motion whenever two materials are in contact with each other. Friction: (ME) Two surfaces rubbing