Given,
r = 10 cm = 0.1 m
radius at which it steps,
Centrifugal force = Remaining static friction
⇒ μsmg = mrω2
ω = 90rpm
= \(\frac{90\times 2\pi}{60}\) rad/s
= 1.5 x 2π
= 3π rad/s
⇒ μs x m x 9.8 = m x 0.1 x 9π2
⇒ μs = \(\frac{0.1\times 9 \times 9.8}{9.8}\)
= 0.9