Find expressions for the acceleration due to gravity a and the tidal acceleration gradient da dr at distance r from a point mass M. Hence calculate the value of da/dr at the Earth's surface due to its own gravity.(3) (b) The equivalence principle mentions local experiments, but what constitutes "local" depends on the context. If an experiment at the Earth's surface can measure acceleration to an accuracy of 1 um s, estimate how big the experiment can be while still being "local".(2)

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