Let the body S radiate isotropically. If the particle P does not move relatively to S then the light pressure force acts in the direction of the line SP and decreases as 1/r2 (r is the distance between the S and P). Therefore, the ratio of the gravitational force F®1 to the S to the light pressure force FPr is constant. It is convenient to introduce the The coefficient q depends on the light intensity and the particle cross section and for the black round particle with density 5 and radius p is given by the formula:
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