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Explanation of Role of Friction in Starting Rolling Motion - Succeed in Understanding Physics. Also refer to physics, wheel, tire, static friction, traction, torque, treads, sliding, mud, snow, Ron Kurtus, School for Champions. Copyright © Restrictions
Role of Friction in Starting Rolling Motion
by Ron Kurtus (revised 28 March 2008)
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. There is a mini-quiz near the end of the lesson.
Useful tools: Metric-English Conversion | Scientific Calculator.
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.
Torque and friction cause rolling
A torque is a rotational force applied to the axle of a wheel, causing it to rotate. The relationship between torque and force is:
τ = rF
or
F = τ/r
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
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 spinning.
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.
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
The following resources provide information on this subject:
Websites
Friction Concepts - HyperPhysics
Friction - Wolfram Research Science World
Books
The following books are available from Amazon.com.
Basic
Complete Idiot's Guide To Physics by Johnnie T. Dennis; Alpha (2003) $18.95
What Is Friction? (Ages 4-8) by Lisa Trumbauer; Children's Press (CT) (2004) $4.95
Advanced
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
Mini-quiz to check your understanding
If you got all three correct, you are on your way to becoming a Champion in Physics. If you had problems, you had better look over the material again.
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Role of Friction in Starting Rolling Motion