TOO . PRESSURE OF LIGHT 



The tangential velocity gives a force along BA 



/"* R - o . 



/ r~\j2 S r 27ra Sln 



Adding them together and integrating, the total force 

 along BA is 



4 _ a 2 Rjp 

 3 V 2 



We can now find the diminution of the orbit of a 

 spherical absorbing particle moving in a nearly circular 

 orbit round the sun. 



If the solar energy absorbed per second per sq. cm, is 

 S, this is taken in through a cylinder of cross section 

 ira 2 . It is emitted at rate R over the whole surface 



Then 47m 2 R = 7ra 2 S 



or R = iS 



The resisting force is therefore 



If the density of the sphere is p its mass is ^ ira 3 p, and 

 the negative acceleration is 



i i S# 

 4 ' ap ' v 2 



. If m is the mass of the particle, the energy taken out 

 per second is 



rf i . m Sz> 2 



Force x velocity = - 



4<tp V 2 



Let us suppose that the orbit is so nearly circular that 

 we may put 



= acceleration to Sun 



r 



9 GM 

 or z/ 2 = 



y 



