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A simple pendulum with a string 30 cm long performs 20 oscillations in 10 s. The length of string required for the pendulum to perform 40 oscillations in the same time is ______ cm. (The mass of the bob is unchanged.)

Asked in JEE Main 23rd Jan 1st Shift 2026 · Time period, length and g

• (1) 0.75
• (2) 15
• (3) 120
• (4) 7.5

Step-by-step solution

Why the other options are wrong

• (1) Divides the length by 40 instead of by 4.
• (2) Halves the length, as though T∝ L.
• (3) Multiplies by 4; doubling the number of oscillations needs a shorter string, not a longer one.

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