London Dispersion Forces and Dipole-Dipole forces together? [closed]

How do London Dispersion Forces and Dipole-Dipole forces exist at the same time between polar molecules?

closed as off-topic by M.A.R. ಠ_ಠ, airhuff, Gaurang Tandon, Jannis Andreska, aventurinMar 18 '18 at 15:56

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Let's take a polar, covalently bonded molecule, like $\ce{HCl}$.