Wheel: Encyclopedia II - Wheel - Mechanics

Wheel - Mechanics

Wheels are used in conjunction with an axle, either the wheel turns on the axle or the wheel is rigidly attached to the axle which then turns in bearings in the body of the vehicle. The mechanics are the same in either case.

The low density of friction (compared to dragging) is explained as follows:

• the sliding distance is reduced, because the sliding takes place between the wheels and the axles or between the axles and the bearings
• the coefficient of kinetic friction μ for the sliding friction is less

Example:

• If dragging a 100 kg object for 10 m along a surface with μ = 0.5, the normal force on Earth is 980 N and the work done (required energy) is 980 × 10 × 0.5 = 4900 joules.
• Now give the object 4 wheels. The normal force between the 4 wheels and axles is the same (in total) 980 N, assume μ = 0.1, finally the most important factor is the wheel diameter (1000 mm) and axle diameter (50 mm). Now while the object still moves 10 m the sliding frictional surfaces only slide over each other a distance of 0.5 m. So work done is 980 x 0.5 x 0.1 = 49 joules.

Additional energy is lost at the wheel to road interface (rolling friction), but it is deformation loss which can be very small. An example would be train wheels on rail tracks). The tradeoff is that a wheeled object in motion carries more momentum than dragging, and thus require an external force in the opposite direction in order to stop the object or change its direction, for example, such as brakes.

Adapted from the Wikipedia article "Mechanics", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki