Resource Lesson
Properties of Friction
Printer Friendly Version
Friction is a force and is measured in newtons. Friction comes in various types, for example: static, kinetic, and rolling. We will begin our study by listing some general properties. Friction
is essentially an electrostatic force between two surfaces
never initiates motion; it only responds to motion
depends on the types of materials that are in contact (µ - the coefficient of friction)
depends on the net force normal pressing the two surfaces in contact (N)
acts parallel to the surfaces that are (or might have the potential to be) moving with respect to each other
opposes the direction of motion
is independent of the area of the surfaces in contact
Some additional facts about frictional forces include:
static friction > kinetic friction > rolling friction for the same combinations of surfaces
when two surfaces are slipping across each other in the presence of kinetic friction, heat is generated and mechanical energy is not conserved
when a ball rolls (static friction) without slipping across a surface, mechanical energy is conserved and no heat is generated
Kinetic friction
When two surfaces are sliding across each other, the amount of kinetic friction present can be calculated using the equation
f
_{k}
= µ
_{k}
N
where µ
_{k}
is the coefficient of kinetic friction and N is the net force normal.
The magnitude of kinetic friction does not vary as the object moves. The coefficient of friction, µ, depends on the materials that are in contact with each other - it is a value usually looked up in a table. This dimensionless ratio of the force required to overcome friction to the net force normal does not have a unit of measurement. The magnitude of the coefficient of friction is usually less than 1, although some combinations can be greater than one.
approximate values of some frictional coefficients
table courtesy of New York Science Regents Examinations
Static friction
The
maximum
amount of static friction can be calculated using the equation
f
_{s max}
= µ
_{s}
N
where µ
_{s}
is the coefficient of static friction and N is the net force normal.
However, according to Newton's Third Law, the amount of static friction required to maintain static equilibrium, when an object is resisting a push or pull, depends directly on the amount of external force applied to it. For example, consider the 10 kg box initially just resting on a table.
Suppose that the coefficient of static friction between the box and the table's surface is 0.4. This would mean that the maximum amount of static friction present between the two surfaces would be
f
_{s max}
= µ
_{s}
N
f
_{s max}
= (0.4)(10)(9.8)
f
_{s max}
= (0.4)(98)
f
_{s max}
= 39.2 N
Now, suppose that you only push on the box with a horizontal force of 15 N.
Since the applied force, F = 15 N, is smaller than the maximum allowed static friction, f
_{s max}
= 39.2 N, it would initially appear as if the box should begin to accelerate towards the left. That is, static friction would not only initiate motion, but would cause the box to have a non-zero acceleration!
That scenario would obviously be physically impossible! Thus, according to Newton's 3rd Law, the only amount of static friction required would be 15 N to keep the box in equilibrium. Notice that the friction vector is now equal in length to that of the applied force.
Once the applied force equals the maximum value for static friction, the object will slip and begin to slide. At that time, the friction between the two surfaces will be kinetic friction, no longer static friction.
Sometimes it is convenient to think of the static friction between two surfaces as being like a "saving account." You only use (withdraw) what you need at any given time to keep the system in equilibrium until you reach your maximum, or critical, amount available.
To read a discussion of static friction and rolling spheres see the lesson on rotational dynamics.
Related Documents
Lab:
Labs -
Coefficient of Friction
Labs -
Coefficient of Friction
Labs -
Coefficient of Kinetic Friction (pulley, incline, block)
Labs -
Conservation of Momentum in Two-Dimensions
Labs -
Falling Coffee Filters
Labs -
Force Table - Force Vectors in Equilibrium
Labs -
Inelastic Collision - Velocity of a Softball
Labs -
Inertial Mass
Labs -
LabPro: Newton's 2nd Law
Labs -
Loop-the-Loop
Labs -
Mass of a Rolling Cart
Labs -
Moment of Inertia of a Bicycle Wheel
Labs -
Relationship Between Tension in a String and Wave Speed
Labs -
Relationship Between Tension in a String and Wave Speed Along the String
Labs -
Static Equilibrium Lab
Labs -
Static Springs: Hooke's Law
Labs -
Static Springs: Hooke's Law
Labs -
Static Springs: LabPro Data for Hooke's Law
Labs -
Terminal Velocity
Labs -
Video LAB: A Gravitron
Labs -
Video LAB: Ball Re-Bounding From a Wall
Labs -
Video Lab: Falling Coffee Filters
Resource Lesson:
RL -
Advanced Gravitational Forces
RL -
Air Resistance
RL -
Air Resistance: Terminal Velocity
RL -
Forces Acting at an Angle
RL -
Freebody Diagrams
RL -
Gravitational Energy Wells
RL -
Inclined Planes
RL -
Inertial vs Gravitational Mass
RL -
Newton's Laws of Motion
RL -
Non-constant Resistance Forces
RL -
Springs and Blocks
RL -
Springs: Hooke's Law
RL -
Static Equilibrium
RL -
Systems of Bodies
RL -
Tension Cases: Four Special Situations
RL -
The Law of Universal Gravitation
RL -
Universal Gravitation and Satellites
RL -
Universal Gravitation and Weight
RL -
What is Mass?
RL -
Work and Energy
Worksheet:
APP -
Big Fist
APP -
Family Reunion
APP -
The Antelope
APP -
The Box Seat
APP -
The Jogger
CP -
Action-Reaction #1
CP -
Action-Reaction #2
CP -
Equilibrium on an Inclined Plane
CP -
Falling and Air Resistance
CP -
Force and Acceleration
CP -
Force and Weight
CP -
Force Vectors and the Parallelogram Rule
CP -
Freebody Diagrams
CP -
Gravitational Interactions
CP -
Incline Places: Force Vector Resultants
CP -
Incline Planes - Force Vector Components
CP -
Inertia
CP -
Mobiles: Rotational Equilibrium
CP -
Net Force
CP -
Newton's Law of Motion: Friction
CP -
Static Equilibrium
CP -
Tensions and Equilibrium
NT -
Acceleration
NT -
Air Resistance #1
NT -
An Apple on a Table
NT -
Apex #1
NT -
Apex #2
NT -
Falling Rock
NT -
Falling Spheres
NT -
Friction
NT -
Frictionless Pulley
NT -
Gravitation #1
NT -
Head-on Collisions #1
NT -
Head-on Collisions #2
NT -
Ice Boat
NT -
Rotating Disk
NT -
Sailboats #1
NT -
Sailboats #2
NT -
Scale Reading
NT -
Settling
NT -
Skidding Distances
NT -
Spiral Tube
NT -
Tensile Strength
NT -
Terminal Velocity
NT -
Tug of War #1
NT -
Tug of War #2
NT -
Two-block Systems
WS -
Advanced Properties of Freely Falling Bodies #1
WS -
Advanced Properties of Freely Falling Bodies #2
WS -
Calculating Force Components
WS -
Charged Projectiles in Uniform Electric Fields
WS -
Combining Kinematics and Dynamics
WS -
Distinguishing 2nd and 3rd Law Forces
WS -
Force vs Displacement Graphs
WS -
Freebody Diagrams #1
WS -
Freebody Diagrams #2
WS -
Freebody Diagrams #3
WS -
Freebody Diagrams #4
WS -
Introduction to Springs
WS -
Kinematics Along With Work/Energy
WS -
Lab Discussion: Gravitational Field Strength and the Acceleration Due to Gravity
WS -
Lab Discussion: Inertial and Gravitational Mass
WS -
net F = ma
WS -
Practice: Vertical Circular Motion
WS -
Ropes and Pulleys in Static Equilibrium
WS -
Standard Model: Particles and Forces
WS -
Static Springs: The Basics
WS -
Vocabulary for Newton's Laws
WS -
Work and Energy Practice: Forces at Angles
TB -
Systems of Bodies (including pulleys)
TB -
Work, Power, Kinetic Energy
PhysicsLAB
Copyright © 1997-2022
Catharine H. Colwell
All rights reserved.
Application Programmer
Mark Acton