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Suppose that the man in the chest fixes a rope to the inner side

of the lid, and that he attaches a body to the free end of the

rope. The result of this will be to stretch the rope so that it

will hang "vertically" downwards. If we ask for an opinion of the

cause of tension in the rope, the man in the chest will say: "The

suspended body experiences a downward force in the gravitational

field, and this is neutralised by the tension of the rope; what

determines the magnitude of the tension of the rope is the

gravitational mass of the suspended body." On the other hand, an

observer who is poised freely in space will interpret the

condition of things thus: "The rope must perforce take part in

the accelerated motion of the chest, and it transmits this motion

to the body attached to it. The tension of the rope is just large

enough to effect the acceleration of the body.