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We calculate the friction force between two metallic bodies with flat surfaces separated by a vacuum slab of thickness $d,$ and moving with a relative velocity $v.$ The separation d is assumed to be so large that the only interaction between the bodies is via the Coulomb field. The friction force depends linearly on the velocity v (for small v) and decays rapidly with increasing $d.$ In most practical cases, the Coulomb drag makes a negligible contribution to the friction force, but it may make an important contribution in some special cases.
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Persson et al. (1998) studied this question.
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