A body of mass 2 kg slides down with an acceleration of 3 m/s2 on a rough inclined plane having a slope of 30°. The external force required to take the same body up the plane with the same acceleration will be: (g = 10 m/s2) (a) 14 N (b) 20 N (c) 6 N (d) 4 N

Answer: (b). Sliding down: mgsin 30° - f = ma ⇒ 10 - f = 2 × 3 ⇒ f = 4 N. Going up, friction acts down the slope: F - mgsin 30° - f = ma. F = 10 + 4 + 6 = 20 N So the answer is (b) 20 N.

A body of mass 2 kg slides down with an acceleration of 3 m/s2 on a… | JEE Main 2018 (15 Apr, Shift 2)

Q23JEE Main 2018 · 15 APR · SHIFT 2Medium

Friction on an Incline

A body of mass 2 kg slides down with an acceleration of 3 m/s2 on a rough inclined plane having a slope of $30^\circ$ . The external force required to take the same body up the plane with the same acceleration will be: (g = 10 m/s2)

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Answer: (b)

Sliding down: $mg\sin 30^\circ - f = ma \Rightarrow 10 - f = 2 \times 3 \Rightarrow f = 4\text{ N}$ .
Going up, friction acts down the slope: $F - mg\sin 30^\circ - f = ma$ .
$F = 10 + 4 + 6 = 20\text{ N}$
So the answer is (b) 20 N.

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