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**Suppose a nickel sits on a flat turntable that spins at 78 rpm (revolutions per minute) while sitting on a horizontal surface. The nickel is 15 cm from the center of the turntable. What is the minimum coefficient of friction that keeps the nickel on the turntable?**

**Relevant equations: I am not sure which equations to use, to be honest. I know it involves calculating centripetal force and frictional force.**

**This problem has stumped me all day. I believe it is unsolvable without knowing the mass of the nickel. I know a US nickel weighs 5 grams, but I do not know if we're supposed to assume that.**