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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