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A Closer Look

A Bicycle Racer’s Edge
alileo was one of the first to recognize likely to have the lower-pressure-producing
Gthe role of friction in opposing motion. air turbulence behind (and resulting greater
As shown in Figure 2.9, friction with the sur- pressure in front) because it smoothes, or
face and air friction combine to produce a streamlines, the air flow.
net force that works against anything that is The frictional drag of air is similar to
moving on the surface. This article is about the frictional drag that occurs when you
air friction and some techniques that bike push a book across a rough tabletop. You
riders use to reduce that opposing force— know that smoothing the rough tabletop
perhaps giving them an edge in a close race. will reduce the frictional drag on the book.
The bike riders in Box Figure 2.1 are Likewise, the smoothing of a surface ex-
forming a single-file line, called a pace- BOX FIGURE 2.1 The object of the race posed to moving air will reduce air fric-
line, because the slipstream reduces the air is to be in the front, to finish first. If this is tion. Cyclists accomplish this “smoothing”
re sistance for a closely trailing rider. Cyclists true, why are these racers forming a single- by wearing smooth Lycra clothing and by
file line?
say that riding in the slipstream of another shaving hair from arm and leg surfaces that
cyclist will save much of their energy. They are exposed to moving air. Each hair con-
can move 8 km/h faster than they would ex- tributes to the overall frictional drag, and
pending the same energy riding alone. turbulent versus a smooth flow of air and removal of the arm and leg hair can thus
In a sense, riding in a slipstream means (2) the problem of frictional drag. A turbu- result in seconds saved. This might provide
that you do not have to push as much air out lent flow of air contributes to air resistance enough of an edge to win a close race. Shav-
of your way. It has been estimated that at because it causes the air to separate slightly ing legs and arms and the wearing of Lycra
32 km/h, a cyclist must move a little less than on the back side, which increases the pres- or some other tight, smooth-fitting gar-
one-half a ton of air out of the way every sure on the front of the moving object. This ments are just a few of the things a cyclist
minute. Along with the problem of moving is why racing cars, airplanes, boats, and can do to gain an edge. Perhaps you will be
air out of the way, there are two basic fac- other racing vehicles are streamlined to able to think of more ways to reduce the
tors related to air resistance. These are (1) a a teardroplike shape. This shape is not as forces that oppose motion.

Galileo checked this calculation by rolling balls on an
inclined board with a smooth groove in it. He used the in- 9.8 m/s in 1 s
clined board to slow the motion of descent in order to measure
the distance and time relationships, a necessary requirement
since he lacked the accurate timing devices that exist today. 19.6 m/s in 2 s
He found, as predicted, that the falling balls moved through a
distance proportional to the square of the time of falling. This
also means that the velocity of the falling object increased at a
constant rate, as shown in Figure 2.13. Recall that a change 29.4 m/s in 3 s
of velocity during some time period is called acceleration. In
other words, a falling object accelerates toward the surface of
Earth.
Since the velocity of a falling object increases at a constant
rate, this must mean that falling objects are uniformly acceler-
ated by the force of gravity. All objects in free fall experience a
39.2 m/s in 4 s
constant acceleration. During each second of fall, the object on
Earth gains 9.8 m/s (32 ft/s) in velocity. This gain is the accelera-
2
2
tion of the falling object, 9.8 m/s (32 ft/s ).
The acceleration of objects falling toward Earth varies
slightly from place to place on the surface because of Earth’s
shape and spin. The acceleration of falling objects decreases
from the poles to the equator and also varies from place to place
because Earth’s mass is not distributed equally. The value of FIGURE 2.13 The velocity of a falling object increases at a
2
2
2
9.8 m/s (32 ft/s ) is an approximation that is fairly close to, but constant rate, 9.8 m/s .
2-13 CHAPTER 2 Motion 37