WHAT IS PHYSICS?
As we discussed in Chapter 10, physics includes the study of rotation.
Arguably, the most important application of that physics is in the rolling motion
of wheels and wheel-like objects.This applied physics has long been used. For ex-
ample, when the prehistoric people of Easter Island moved their gigantic stone
statues from the quarry and across the island, they dragged them over logs acting
as rollers. Much later, when settlers moved westward across America in the 1800s,
they rolled their possessions ±rst by wagon and then later by train. Today, like it
or not, the world is ±lled with cars, trucks, motorcycles, bicycles, and other rolling
The physics and engineering of rolling have been around for so long that you
might think no fresh ideas remain to be developed. However, skateboards and in-
line skates were invented and engineered fairly recently, to become huge ±nan-
cial successes. Street luge is now catching on, and the self-righting Segway (Fig.
11-1) may change the way people move around in large cities. Applying the
physics of rolling can still lead to surprises and rewards. Our starting point in
exploring that physics is to simplify rolling motion.
Rolling as Translation and Rotation Combined
Here we consider only objects that
along a surface; that is, the objects
roll without slipping or bouncing on the surface. Figure 11-2 shows how complicated
smooth rolling motion can be:Although the center of the object moves in a straight
line parallel to the surface, a point on the rim certainly does not. However, we can
study this motion by treating it as a combination of translation of the center of mass
and rotation of the rest of the object around that center.
The self-righting Segway
Images News and Sport Services)
A time-exposure photograph of a rolling disk. Small lights have been at-
tached to the disk, one at its center and one at its edge.The latter traces out a curve called
cycloid. (Richard Megna/Fundamental Photographs)