Problem 1 (5 points)
What is the radiation pressure exerted on a light-absorbing surface by a laser beam whose
intensity is 180 W/cm2.
Solution:
P
F
1 dU S

  6  10 3 Pa
A Ac dt
c
Problem 2 (5 points)
The average intensity of noonday sunlight is about 1 kW/m2.
What is the radiation force on a solar collector measuring 60 cm by 2.5 m if it is oriented
at right angles to the incident light and absorbs all the light?
Solution:
F  PA,
P
S
c
F 
SA
 (0.6  2.5)  103 (3  108 )  5  10 6 N
c
Problem 3 (10 points)

A 65-kg astronaut is floating in empty space. If the astronaut shines a 1.0-W flashlight in
a fixed direction, how long will it take the astronaut to accelerate to a speed of 10 m/s?
Solution:
F  dp dt  (dU dt) c ,
dU dt 1.0W .
where
v  at  t  v a  mv F 

 t  (65kg)(10 m s)(3 10 8 m s) (1W)  1.95 1011 s  6 10 3 y

Problem 4 (10 points)

A laser beam shines vertically upward. What laser power is necessary for this beam to
support a flat piece of aluminum foil with mass 30 mg and diameter equal to that of the
beam? Assume that the foil reflects all the light.
Solution:
F foil  mg  (30 10 6 )(9.8)  2.94 10 4 N ;
Frad  2
1 dU
c dt
dU
 (1 2)F foil c  (1 2)(2.94 10 4 N )(3 10 8 m s)  4.26 10 4 W
dt
