Aim: How can we calculate static Aim: How can we calculate static and kinetic friction?and kinetic friction?

A 3.5 kilogram box is being pulled to the A 3.5 kilogram box is being pulled to the right with a force of 15 newtons. The box right with a force of 15 newtons. The box is moving at constant velocity. is moving at constant velocity. A) Sketch a free body diagram.A) Sketch a free body diagram.B) Identify the net force.B) Identify the net force.C) Determine the frictional force.C) Determine the frictional force.

ActivityActivity

Slide block without text book on table.Slide block without text book on table.Slide block with text book on table.Slide block with text book on table.Slide block on sand paper with text book.Slide block on sand paper with text book.

FrictionFrictionA force that causes surfaces to STICK TOGETHER

At the MICROSCOPIC level, most surfaces are very BUMPY and IRREGULAR

Ways to minimize friction

SMOOTH SURFACESLUBRICATION

FrictionFriction

How can we describe frictional How can we describe frictional force?force?

FRICTIONFRICTIONHow can we determine the direction of How can we determine the direction of

the frictional force?the frictional force?Acts opposite to direction of motion.Acts opposite to direction of motion.Two types of friction:Two types of friction:1. Static friction (force needed to put an 1. Static friction (force needed to put an

object in motion)object in motion)2. Kinetic friction (force needed to keep 2. Kinetic friction (force needed to keep

an object moving at constant speed).an object moving at constant speed).

Coefficient of Friction (Coefficient of Friction (μμ))

How much materials How much materials STICK TOGETHERSTICK TOGETHER

DIMENSIONLESS (no units)DIMENSIONLESS (no units)

The The HIGHERHIGHER the coefficient, the the coefficient, the MOREMORE two two surfaces surfaces STICK TOGETHERSTICK TOGETHER

LOWERLOWER if surfaces are if surfaces are SLIDINGSLIDING past each past each otherother

What else does frictional force What else does frictional force depend on……depend on……

1. Coefficient of friction. 1. Coefficient of friction. 2. The normal force of the object 2. The normal force of the object

to be moved.to be moved.

Coefficients of FrictionCoefficients of Friction

[Examine table of coefficients of friction in [Examine table of coefficients of friction in reference table]reference table]

Coefficient of friction relates to the Coefficient of friction relates to the roughness of the two surfaces in contact.roughness of the two surfaces in contact.

Note: Coefficient of friction has no units.Note: Coefficient of friction has no units.Note: Note: Static friction is always greater than Static friction is always greater than

kinetic friction between two surfaces – it is kinetic friction between two surfaces – it is harder to start something moving than to harder to start something moving than to keep it moving.keep it moving.

Friction formula:Friction formula:FFff = = µFµFNN, where, where

FFff = force of friction = force of frictionµ = “coefficient of friction” andµ = “coefficient of friction” andFFNN = “normal force” acting on the = “normal force” acting on the

object to be moved.object to be moved.

Fg=mg

FN=mg

Only for Horizontal surfaces Fg = FN

Does friction depend on area?Does friction depend on area?

Slide the block on your table. Read the Slide the block on your table. Read the scale (Newtons).scale (Newtons).

The force of friction does not depend The force of friction does not depend on the surface area of the surfaces.on the surface area of the surfaces.

Example:Example:

How much force must be applied to How much force must be applied to begin moving a stationary 5 kg begin moving a stationary 5 kg wooden box on a wooden floor?wooden box on a wooden floor?

FFf f = = µFµFNN = µmg= µmg= (0.42) (5 kg) (9.8 m/s= (0.42) (5 kg) (9.8 m/s22))= 20.6 Newtons= 20.6 Newtons

Aim: How can we solve for Aim: How can we solve for kinetic and static friction?kinetic and static friction?

1) Explain the difference between static and 1) Explain the difference between static and kinetic friction.kinetic friction.

2) A student pushes a stationary 500 newton 2) A student pushes a stationary 500 newton steel box on a level steel surface. The student steel box on a level steel surface. The student exerts a 15 newton horizontal force on the exerts a 15 newton horizontal force on the box.box.A) Does the box move? What is the static frictional A) Does the box move? What is the static frictional

force?force?B) Determine the minimum amount of force B) Determine the minimum amount of force

needed to move the box. needed to move the box.

Example 2:Example 2:

How much force does it take to keep How much force does it take to keep a 1000 kg car moving at constant a 1000 kg car moving at constant speed on a dry asphalt road?speed on a dry asphalt road?

FFff = = µFµFNN = µmg = µmg= (0.67) (1000 kg) (9.8 m/s= (0.67) (1000 kg) (9.8 m/s22))= 6,566 Newtons= 6,566 Newtons

The coefficient of kinetic friction between a 640.-newton crate and a level warehouse floor is 0.25. Calculate the magnitude of the horizontal force required to move the crate across the floor at constant speed.

Example 3:Example 3:

Calculate the coefficient of friction Calculate the coefficient of friction between a 60 newton block of wood and between a 60 newton block of wood and sandpaper if it takes 12 Newtons of force sandpaper if it takes 12 Newtons of force to move the wood on the sandpaper?to move the wood on the sandpaper?

FFff = = µFµFN N

Example 4:Example 4:

What is the coefficient of friction of a tire What is the coefficient of friction of a tire on a level road if an 1,100kg car on a level road if an 1,100kg car decelerates at a rate of 3 m/sdecelerates at a rate of 3 m/s22 once the once the driver takes her foot off the gas pedal?driver takes her foot off the gas pedal?

SummarySummary

Describe frictionDescribe frictionHow are static and kinetic friction How are static and kinetic friction

similar?...how are they different?similar?...how are they different?Describe what frictional force Describe what frictional force

depends on.depends on.