SECTION 6.4 • Motion in the Presence of Resistive Forces
165
so that,
(6.9)
Using this expression, we can determine how the terminal speed depends on the di-
mensions of the object. Suppose the object is a sphere of radius r. In this case, A . r
2
(from A ! /r
2
) and m . r
3
(because the mass is proportional to the volume of the
sphere, which is
). Therefore,
.
Table 6.1 lists the terminal speeds for several objects falling through air.
v
T
.
√
r
V !
4
3
/
r
3
v
T
!
√
2mg
D-A
Object
Mass (kg)
Cross-Sectional Area (m
2
)
v
T
(m/s)
Sky diver
75
0.70
60
Baseball (radius 3.7 cm)
0.145
4.2 ) 10
'
3
43
Golf ball (radius 2.1 cm)
0.046
1.4 ) 10
'
3
44
Hailstone (radius 0.50 cm)
4.8 ) 10
'
4
7.9 ) 10
'
5
14
Raindrop (radius 0.20 cm)
3.4 ) 10
'
5
1.3 ) 10
'
5
9.0
Terminal Speed for Various Objects Falling Through Air
Table 6.1
Conceptual Example 6.11 The Sky Surfer
Consider a sky surfer (Fig. 6.17) who jumps from a plane
with her feet attached firmly to her surfboard, does some
tricks, and then opens her parachute. Describe the forces
acting on her during these maneuvers.
Solution When the surfer first steps out of the plane, she has
no vertical velocity. The downward gravitational force causes
her to accelerate toward the ground. As her downward speed
increases, so does the upward resistive force exerted by the air
on her body and the board. This upward force reduces their
acceleration, and so their speed increases more slowly. Eventu-
ally, they are going so fast that the upward resistive force
matches the downward gravitational force. Now the net force
is zero and they no longer accelerate, but reach their terminal
speed. At some point after reaching terminal speed, she opens
her parachute, resulting in a drastic increase in the upward re-
sistive force. The net force (and thus the acceleration) is now
upward, in the direction opposite the direction of the velocity.
This causes the downward velocity to decrease rapidly; this
means the resistive force on the chute also decreases. Eventu-
ally the upward resistive force and the downward gravitational
force balance each other and a much smaller terminal speed
is reached, permitting a safe landing.
(Contrary to popular belief, the velocity vector of a sky
diver never points upward. You may have seen a videotape in
which a sky diver appears to “rocket” upward once the chute
opens. In fact, what happens is that the diver slows down
while the person holding the camera continues falling at
high speed.)
Figure 6.17 (Conceptual Example 6.11) A sky surfer.
Jump Run Productions / Getty Images
Quick Quiz 6.7
A baseball and a basketball, having the same mass, are
dropped through air from rest such that their bottoms are initially at the same height
above the ground, on the order of 1 m or more. Which one strikes the ground first?
(a) the baseball (b) the basketball (c) both strike the ground at the same time.