# Velocity of particles on a disc

According to this website, particles of the disc that are located at the point of contact between the ground and the disc have a velocity of zero, and the velocity of each of the particles increases as you move from the bottom of the disc to the top of the disc. How is this possible? I always thought the particles on the rim of a wheel had constant velocity, no matter where they're located.
http://cnx.org/content/m14311/latest/

## Answers and Replies

I think I've figured this out.
If you think of the ground as a fulcrum about which a wheel is rotating then the point of contact between the ground and the wheel is going to have zero velocity.

The particles on the rim have constant speed relative to the center of mass (e.g. the reference frame where the axle stays in one place). The velocity is different for each point because they are moving in different directions (at the common speed).

Draw vectors for the top, bottom, and a few other points. Now add a vector where the whole wheel is also moving, thus adding the same overall vector to each of the sample rim vectors. You'll see that it cancels out on one side, doubles on the opposite side, and so on.