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Asked in JEE Main 6th April 1st Shift 2023 · Centripetal force and uniform circular motion
Idea: F=mω²r, and the period is chosen so that ω is a whole number.
ω=(2π)/T=(2π)/(3.14)≈2 rad/s, since 3.14≈π.
F=mω²r=5×4×2=40 N.
The 3.14 is there precisely so that ω=2 exactly; spotting that saves all the arithmetic.
'Centrifugal' here means the outward force felt in the child's own rotating frame; its magnitude is the same mω²r as the centripetal force an observer on the ground would measure.
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