Question
Mon September 17, 2012 By:
 

If the mass of an unknown planet is half the mass of earth and its radius is twice that of earth then find the weight of an object on its surface which weighs 400N at the surface of the earth.

Expert Reply
Mon September 17, 2012
we know that the weight of the object on the earth's surface=400 =mg=mGME/RE2 .......(1)as g=GME/RE2
               weight of the object on the other planet's surface=mg'=mGMp/Rp2 .........(2)
                                             where, mass of the other planet=Mp
                                                    radius of the other planet=Rp
                                                    according to the question:Mp=ME/2
                                                                                       Rp=2RE
putting these in eq 2 and on dividing 2 by eq 1:
weight of object on that planet /400 =mGMp/Rp2÷mGME/RE2
    weight of object on that planet=400/8=50 N   
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