Question

right answer will mark as brainlist ​

Answer 1

Answer:

28 seconds.

Explanation:

ω₁ = 90 rpm = 90×2π (radians/minute) = 90×2π = $\frac{90*2\pi }{60}$ $\frac{rad.}{sec.}$ = $3\pi \frac{rad.}{sec.}$

ω₂ = 0

θ = 21 revolutions = 21 × 2π radians = 42π rad.

∵ ω₂² = ω₁² + 2αθ

∴ α =  $\frac{-9}{84\pi }$ $\frac{rad.}{sec.^{2} }$

Now,

∵ ω₂ = ω₁ + αt

∴ t = 28 seconds.

Thanks.