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e_r and e_t are the radial and tangential unit vectors directed relative to the vector from M to m. Use this to simplify the formula for work of gravity to,

This calculation uses the fact that

Potential energy for electrostatic forces between two bodies

The electrostatic force exerted by a charge Q on another charge q separated by a distance r is given by Coulomb's law

where is a vector of length 1 pointing from Q to q and ?₀ is

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