Although radiation pressures are very small (about 5 $ 10
)
6
N/m
2
for direct
sunlight), they have been measured with torsion balances such as the one shown in
Figure 34.8. A mirror (a perfect reflector) and a black disk (a perfect absorber) are
connected by a horizontal rod suspended from a fine fiber. Normal-incidence light
striking the black disk is completely absorbed, so all of the momentum of the light is
transferred to the disk. Normal-incidence light striking the mirror is totally reflected,
and hence the momentum transferred to the mirror is twice as great as that transferred
to the disk. The radiation pressure is determined by measuring the angle through
which the horizontal connecting rod rotates. The apparatus must be placed in a high
vacuum to eliminate the effects of air currents.
NASA is exploring the possibility of solar sailing as a low-cost means of sending
spacecraft to the planets. Large sheets would experience radiation pressure from sunlight
and would be used in much the way canvas sheets are used on earthbound sailboats. In
1973 NASA engineers took advantage of the momentum of the sunlight striking the solar
panels of Mariner 10 (Fig. 34.9) to make small course corrections when the spacecraft’s
fuel supply was running low. (This procedure was carried out when the spacecraft was in
the vicinity of the planet Mercury. Would it have worked as well near Pluto?)
S E C T I O N 3 4 . 4 • Momentum and Radiation Pressure
1077
Light
Black
disk
Mirror
Figure 34.8 An apparatus for measuring
the pressure exerted by light. In practice,
the system is contained in a high vacuum.
Figure 34.9 Mariner 10 used its solar panels to
“sail on sunlight.”
Quick Quiz 34.4
To maximize the radiation pressure on the sails of a
spacecraft using solar sailing, should the sheets be (a) very black to absorb as much
sunlight as possible or (b) very shiny, to reflect as much sunlight as possible?
Quick Quiz 34.5
In an apparatus such as that in Figure 34.8, the disks are
illuminated uniformly over their areas. Suppose the black disk is replaced by one with
half the radius. Which of the following are different after the disk is replaced? (a) radi-
ation pressure on the disk, (b) radiation force on the disk, (c) radiation momentum
delivered to the disk in a given time interval.
Courtesy of NASA
Conceptual Example 34.3 Sweeping the Solar System
cube of the radius of a spherical dust particle because it is
proportional to the mass and therefore to the volume 4,r
3
/3
of the particle. The radiation pressure is proportional to the
square of the radius because it depends on the planar cross
section of the particle. For large particles, the gravitational
force is greater than the force from radiation pressure. For
particles having radii less than about 0.2 &m, the radiation-
pressure force is greater than the gravitational force, and as a
result these particles are swept out of the Solar System.
A great amount of dust exists in interplanetary space.
Although in theory these dust particles can vary in size from
molecular size to much larger, very little of the dust in our
solar system is smaller than about 0.2 &m. Why?
Solution The dust particles are subject to two significant
forces—the gravitational force that draws them toward the
Sun and the radiation-pressure force that pushes them away
from the Sun. The gravitational force is proportional to the