mechanical · 26 minutes · Official OETT Aligned
Wheel & Axle Speed
Same number of turns, different distance traveled
What to know
- RPM: turns per minute
- radius: center-to-edge distance
- speed at the rim
- wheels fixed to the same axle
Two points can make the same number of turns but travel different distances.
The core rule
On one solid wheel, a point farther from the center travels faster, even though all points turn together.
An axle is the shaft through the center of a wheel. Wheels fixed to the same axle complete each turn together. A point on a larger rim travels around a bigger circle, so it covers more distance in the same time. Turning speed is measured in RPM; travel speed can be measured in meters per second.
Worked examples
Example 1: easy
Two points on one rigid disk share RPM.
- Both marked points are on the same solid wheel.
- They complete each turn together, so their RPM is the same.
The outer point travels a greater distance in the same time.
Example 2: medium
At equal RPM, a 12-inch radius rim moves twice as fast linearly as a 6-inch radius rim.
- The 12-inch radius is twice the 6-inch radius. Both have the same RPM.
- The outer point travels twice as far per turn, so its travel speed is twice as great.
A bigger circle has a longer distance around it.
Example 3: hard
For a stepped wheel on one shaft, rank rim speeds from radius even though every step completes each turn together.
- All the steps of the wheel are fixed to one shaft and turn together.
- Order their travel speeds by radius: the largest rim travels fastest.
Equal turns per minute do not mean equal distance traveled per minute.
Common mistakes
Equal RPM means every point travels at the same speed.
The points complete turns together, but a point farther from the center covers more distance per turn.
Put it into practice
Use guided explanations, check your answers, and try a new variation.