A particle moves with uniform acceleration along a straight line from

A particle moves with uniform acceleration along a straight line from rest. The percentage increase in displacement during sixth second compared to that in fifth second is about

11%
22%
33%
44%
This question was previously asked in
UPSC CDS-2 – 2018
For a particle starting from rest (initial velocity u = 0) and moving with uniform acceleration ‘a’, the displacement in the n-th second (S_n) is given by the formula S_n = u + a(n – 1/2). Since u = 0, S_n = a(n – 1/2).
Displacement in the fifth second (n=5): S₅ = a(5 – 1/2) = a(4.5).
Displacement in the sixth second (n=6): S₆ = a(6 – 1/2) = a(5.5).
The percentage increase in displacement during the sixth second compared to the fifth second is calculated as [(S₆ – S₅) / S₅] * 100%.
Percentage Increase = [(a(5.5) – a(4.5)) / a(4.5)] * 100% = [a(5.5 – 4.5) / a(4.5)] * 100% = [a(1) / a(4.5)] * 100% = (1 / 4.5) * 100% = (10 / 45) * 100% = (2 / 9) * 100% ≈ 0.2222 * 100% ≈ 22.22%.
The closest option is 22%.
– Displacement in the n-th second formula: S_n = u + a(n – 1/2).
– Uniform acceleration from rest means u=0.
– Percentage increase calculation involves finding the difference, dividing by the original value, and multiplying by 100.
The total displacement after ‘n’ seconds is given by S = ut + (1/2)at². For a body starting from rest (u=0), S = (1/2)at². The displacement in the n-th second is the difference between the total displacement after n seconds and the total displacement after (n-1) seconds: S_n = S(n) – S(n-1) = (1/2)an² – (1/2)a(n-1)² = (1/2)a [n² – (n² – 2n + 1)] = (1/2)a [n² – n² + 2n – 1] = (1/2)a (2n – 1) = a(n – 1/2). This confirms the formula used.
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