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A flywheel, accelerated uniformly from rest, rotates through 5 rad in the first 4 s. The angle rotated during the next 4 s is

Asked in JEE Main 12th April 1st Shift 2019 · Equations of rotational motion

(1) 5 rad
(2) 10 rad
(3) 15 rad
(4) 20 rad

Answer: (3) 15 rad

Step-by-step solution

From rest, θ₁=1/2α(4)²=8α=5, so α=5/8.

In 8 s: θ=1/2α(8)²=32α=20 rad.

Next 4 s: 20-5=15 rad.

Why the other options are wrong

(1) Comes from assuming equal angles in equal intervals.
(2) Comes from doubling the first interval's angle.
(4) Comes from reporting the total angle in 8 s instead of the next interval.

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