FrictionFriction

Lesson 4Lesson 4

I. DefinitionI. Definition

Friction Friction is a force that always exists between is a force that always exists between any two surfaces in contact with each otherany two surfaces in contact with each other

It is a force that opposes motionIt is a force that opposes motion

II. Two Theories on FrictionII. Two Theories on Friction

Nobody is exactly sure why friction acts the way it Nobody is exactly sure why friction acts the way it does…does…

a. minute (tiny) imperfections in the surfaces grinding a. minute (tiny) imperfections in the surfaces grinding against each other. (two pieces of sandpaper)against each other. (two pieces of sandpaper)

- all surfaces have tiny bumps which will grind past - all surfaces have tiny bumps which will grind past other bumps on other surfaces and cause friction.other bumps on other surfaces and cause friction.

b. small electrostatic attractions between atoms of b. small electrostatic attractions between atoms of the two surfaces, pulling on each other.the two surfaces, pulling on each other.

- electrons in one of the surfaces attract to the - electrons in one of the surfaces attract to the protons in the other surface. Billions of these protons in the other surface. Billions of these attractions between the electrons and protons of attractions between the electrons and protons of the two objects cause them to stick to each other the two objects cause them to stick to each other somewhat causing frictionsomewhat causing friction

III. Two kinds of frictionIII. Two kinds of friction

1. 1. Static frictionStatic friction - exists between two surfaces - exists between two surfaces

that are not moving relative to that are not moving relative to each other.each other.

2. 2. Kinetic frictionKinetic friction - exists between two surfaces - exists between two surfaces

that are moving relative to each that are moving relative to each other.other.

Static FrictionStatic Friction

In any situation, the In any situation, the static friction static friction is greater than the kinetic friction.is greater than the kinetic friction.

Did you notice that you were Did you notice that you were pushing harder, and harder, and pushing harder, and harder, and HARDER, until suddenly it was HARDER, until suddenly it was like glue that was holding it to the like glue that was holding it to the floor snapped? Then, it felt easier floor snapped? Then, it felt easier to push the object than it did just to push the object than it did just to get it started.to get it started.

Kinetic FrictionKinetic Friction

● ● When it was still, you were When it was still, you were trying to overcome the trying to overcome the static static friction friction (bigger force).(bigger force).

● ● When it finally started to When it finally started to move, you were now pushing move, you were now pushing against the against the kinetic friction kinetic friction (smaller force)(smaller force)

IV. Two factors that affect IV. Two factors that affect frictional forcefrictional force

the magnitude of the normal the magnitude of the normal force acting on the object by the force acting on the object by the surfacesurface

the nature of the interaction the nature of the interaction between the atoms and electrons between the atoms and electrons of the object and the atoms and of the object and the atoms and electrons of the surfaceelectrons of the surface

V. Coefficient of FrictionV. Coefficient of Friction The interaction between the surfaces is The interaction between the surfaces is

represented by a number μ (pronounced represented by a number μ (pronounced “mu”)“mu”)

called the called the coefficient of frictioncoefficient of friction (an indicator (an indicator of the “stickiness” of the interaction between of the “stickiness” of the interaction between two surfaces)two surfaces)

Properties:Properties: the higher the coefficient, the stickier the the higher the coefficient, the stickier the

interaction, the greater the frictional force.interaction, the greater the frictional force. For frictionless interactions, which are only For frictionless interactions, which are only

theoretical, μ = 0. theoretical, μ = 0. coefficient of friction is unitless.coefficient of friction is unitless.

Friction CoefficientsFriction Coefficients

Surfaces Surfaces μs μs μkμk steel on steel steel on steel 0.74 0.74 0.570.57 aluminum on steel aluminum on steel 0.61 0.61 0.470.47 copper on steel copper on steel 0.53 0.53 0.360.36 rubber on concrete rubber on concrete 1.0 1.0 0.80.8 wood on wood wood on wood 0.25 – 0.5 0.25 – 0.5 0.20.2 glass on glass glass on glass 0.94 0.94 0.40.4 waxed wood on wet snow waxed wood on wet snow 0.14 0.14 0.10.1 waxed wood on dry snow waxed wood on dry snow - - 0.040.04 metal on metal (lubricated) 0.15 metal on metal (lubricated) 0.15 0.060.06 ice on ice ice on ice 0.1 0.1 0.030.03 teflon on teflon teflon on teflon 0.04 0.04 0.040.04 synovial joints in humans synovial joints in humans 0.01 0.01 0.0030.003

The The coefficient of static friction coefficient of static friction (μs) (μs) is used to calculate the frictional is used to calculate the frictional force which must be overcome to force which must be overcome to start start the object moving.the object moving.

The The kinetic coefficient of friction kinetic coefficient of friction (μk) (μk) is used to calculate the frictional is used to calculate the frictional force when an object is force when an object is already already in in motion.motion.

VI. Calculating Friction:VI. Calculating Friction:

Friction always acts in the direction Friction always acts in the direction opposite to the motion of the object.opposite to the motion of the object.

- Just look at the direction the object - Just look at the direction the object is traveling. The direction of the is traveling. The direction of the force due to friction will be exactly force due to friction will be exactly 180° opposite.180° opposite.

- Friction is also proportional to the - Friction is also proportional to the normal force, which is how we'll be normal force, which is how we'll be able to calculate it.able to calculate it.

f NF F

Example 1Example 1

What friction force must be What friction force must be overcome to start a 50 kg object overcome to start a 50 kg object sliding across a horizontal sliding across a horizontal surface when the static surface when the static coefficient of friction is 0.35?coefficient of friction is 0.35?

Example 2Example 2 A 400 N object is pulled over a horizontal A 400 N object is pulled over a horizontal

surface. If the coefficient of kinetic friction surface. If the coefficient of kinetic friction is 0.23 and the applied force is 120 N, is 0.23 and the applied force is 120 N, what is its acceleration?what is its acceleration?

Example 3Example 3

A 10 kg box is dragged over a A 10 kg box is dragged over a horizontal surface by a force of horizontal surface by a force of 40 N. If the box moves with a 40 N. If the box moves with a constant speed of 0.5 m/s, what constant speed of 0.5 m/s, what is the coefficient of kinetic is the coefficient of kinetic friction for the surface?friction for the surface?

Example 4Example 4

A 10 kg box is dragged across a A 10 kg box is dragged across a level floor with a force of 60 N. level floor with a force of 60 N. The force is applied at an angle The force is applied at an angle of 30° above the horizontal. If of 30° above the horizontal. If the coefficient of friction is 0.20, the coefficient of friction is 0.20, what is the acceleration of the what is the acceleration of the box?box?