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Ex planation of R ole of Friction in Starting R olling Motion by R on Kurtus - Succe e d in Unde rstanding Physics. Key words: physics,
whe e l, tire , static friction, traction, torque , tre ads, sliding, m ud, snow, School for C ham pions. C opyright © R e strictions
Role of Friction in Starting Rolling
Motion
by Ron Kurtus (revised 22 May 2013)
When a force is applied to a ball or wheel, it will start rolling, provided there is a resistive force preventing
the wheel from simply sliding along the other surface. The applied force may be a torque on the axle of the
wheel or a linear push or pull on the wheel. The resistance of static friction causes the rolling motion. This
friction force is called the traction of the wheel. If the wheel or tire has treads and the other surface is soft,
then those treads provide extra friction or traction.
Note that the friction required to start rolling motion is not rolling friction, which is defined as
the friction slowing down rolling motion.
Questions you may have include:
How does friction cause rolling when torque is applied?
How does friction rolling when a push is applied?
What role do treads have in starting rolling in soft materials?
This lesson will answer those questions.
Friction topics
Resistive Force of Friction
Basics
Uses of Friction
Causes of Friction
Standard Friction Equation
Changing Sliding Friction on Hard Surfaces
Coefficient of friction
Coefficient of Friction for Clean Surfaces
Coefficient of Friction for Greased Surfaces
Rolling friction
Role of Friction in Starting Rolling Motion
Rolling Friction
Useful tool: Metric-English Conversion
Applying torque to the wheel
Applying a torque on the axle of a wheel will cause the wheel to roll, provided there is friction resistance at
the point of contact with the other surface. That friction is called the traction of the wheel.
Rolling Friction and Automobile Tires
Traction Friction of Tires
Fluid friction
Changing Fluid Friction
Torque and friction cause rolling
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A torque is a rotational force applied to the axle of a wheel, causing it to rotate. The relationship between
torque and force is:
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τ = rF
or
F = τ/r
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where
τ (Greek letter tau) is the torque in foot-pounds or newton-meters
r is the radius or distance from the center in feet or meters
F is the force in pounds or newtons at the edge of the wheel
rF is r times F
The force, F, is resisted by the static friction at the point of contact of the wheel and the ground or other
surface. This resistance causes the wheel to roll forward.
Torque resisted by friction results in rolling forward
Friction and spinning
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When the static friction at the point of contact is greater than the force caused by the torque, the wheel
will simply roll forward. But if the force from the torque is greater than the friction, the wheel will start
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spinning.
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Once the wheel starts spinning, the static friction becomes kinetic friction, which has a lower coefficient of
friction. Although the wheel will roll forward, since it is also spinning, the forward speed will be less than if it
was not spinning.
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Pushing or pulling the wheel
You can also cause a wheel to turn by pushing on it. Again, the wheel is prevented from sliding forward by
the static friction at the point of contact. In this case, the static friction is preventing the wheel from sliding
forward and is in the opposite direction.
Pushing on a wheel starts it rolling
If the wheel is on an incline, the force of gravity will pull on the wheel, the same principles apply and the
static friction will causing the wheel to roll as it is pulled down down the slope.
Treads and soft surface
If the wheel or tire has treads and is being turned by a torque on its axle, a new factor comes into play
when the other surface is soft. In such a situation, the treads will dig into a soft surface and dramatically
increase the traction. Typically, a soft surface material—such as mud or snow—has a much lower coefficient
with a tire or wheel. Thus, the treads are necessary to create sufficient traction to move the wheel (and its
vehicle) forward.
Treads dig in and help the wheel roll forward
Snow tires and tires made for driving in mud have treads that will dig into the soft surface and allow the tire
to roll forward. Most everyday automobile tires have a majority of their treads aligned to prevent water from
creating a lubricating layer that would cause sliding during a rainstorm.
Summary
A wheel will start rolling when a force is applied and there is a resistive force or friction at the point of
contact with the ground. The force may be a torque or a linear push on the wheel. Static friction causes the
rolling motion. It is also called the traction of the wheel. If the ground surface is soft, then treads are often
used to provide sufficient traction to cause the wheel or tire to roll forward.
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Resources and references
Ron Kurtus' Credentials
Websites
Rolling friction and rolling resistance - includes coefficients - Engineering Toolbox
Rolling Friction - simple explanation - Davidson College
Rolling Resistance - mathematical approach - MathWorks
Tire-Road Interaction - equations - MathWorks
Rolling Resistance Equations - derivations - Real World Physics Problems
Rolling Resistance - Wikipedia
Friction Resources - Extensive list
Books
Top-rated books on Friction Physics
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Top-rated books on Friction Experiments
Friction Science and Technology (Mechanical Engineering Series) by Peter J. Blau; Marcel Dekker Pub.
(1995) $89.95
Tribology: Friction and Wear of Engineering Materials by I. M. Hutchings; CRC Press (1992) $68.88
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$19.95.
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The Kindle version is available in the US, UK or DE, as well as
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