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In a rotor (a hollow vertical cylinder rotating about its vertical axis), a person stands pressed against the inner wall. Above a certain rotational speed the floor is removed and the person stays pinned to the wall by friction. If the rotor radius is 2 m and the coefficient of static friction between wall and person is 0.2, find the minimum speed at which the floor can be safely removed. Take g = 10 m/s².

1. 10 m/s
2. 20 m/s
3. 50 m/s
4. 100 m/s

Correct answer: 10 m/s

Solution

The minimum speed satisfies mu*N = mg with N = m v²/r, giving v = sqrt(r g / mu) = sqrt(2*10/0.2) = 10 m/s.

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