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A simple pendulum is suspended from the roof of a trolley which moves in a horizontal direction with an acceleration a. Then the time period is given by T=2π√ℓ/g, where g is equal to

Asked in CBSE AIPMT 1991 · Pendulum with changed g

Answer: (4) √g²+a²

Step-by-step solution

Idea: work in the trolley's frame, where the bob feels gravity plus a pseudo-force.

Weight: mg vertically downward.

Pseudo-force: ma horizontally, opposite to the trolley's acceleration.

The two are perpendicular, so the resultant has magnitude m√g²+a².

The bob hangs along that resultant, and it plays the part of gravity for the swing.

g_eff=√g²+a²

Why the other options are wrong

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