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Gravitation question

2020 · 7 Jan · Shift 2 · Q43
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Gravitation question

2020 · 7 Jan · Shift 2 · Q43

JEE MainPhysicsGravitationMCQ+4 / −1
A box weight 196 N on a spring balance at the north pole. Its weight recorded on the same balance if it is shifted to the equator is close to (Take g = 10 ms–2 at the north pole and the radius of the earth = 6400 km) :
  1. A
    194.32 N
  2. B
    195.66 N
  3. C
    195.32 N
  4. D
    194.66 N
View written solutionFree

Correct answer: C

  1. Weight at the north pole

At the north pole, there is no reduction in apparent weight due to Earth's rotation.

Given: Wp=196 NW_p = 196\,\text{N}Wp​=196N gp=10 m s−2g_p = 10\,\text{m s}^{-2}gp​=10m s−2

So the mass of the box is m=Wpgp=19610=19.6 kgm = \frac{W_p}{g_p} = \frac{196}{10} = 19.6\,\text{kg}m=gp​Wp​​=10196​=19.6kg

  1. Effect at the equator

At the equator, the spring balance reads the apparent weight: We=m(g−ω2R)W_e = m(g - \omega^2 R)We​=m(g−ω2R)

Since the problem gives g=10 m s−2g = 10\,\text{m s}^{-2}g=10m s−2 at the north pole, we take gravitational acceleration approximately same, and subtract centrifugal acceleration at the equator.

  1. Compute centrifugal acceleration

Angular speed of Earth: ω=2πT\omega = \frac{2\pi}{T}ω=T2π​ with T=24×3600=86400 sT = 24 \times 3600 = 86400\,\text{s}T=24×3600=86400s

Thus, ω=2π86400 rad s−1\omega = \frac{2\pi}{86400}\,\text{rad s}^{-1}ω=864002π​rad s−1

Radius of Earth: R=6400 km=6.4×106 mR = 6400\,\text{km} = 6.4 \times 10^6\,\text{m}R=6400km=6.4×106m

Now, ω2R=(2π86400)2(6.4×106)\omega^2 R = \left(\frac{2\pi}{86400}\right)^2 (6.4\times 10^6)ω2R=(864002π​)2(6.4×106)

Using the standard value, ω2R≈0.0339 m s−2\omega^2 R \approx 0.0339\,\text{m s}^{-2}ω2R≈0.0339m s−2

  1. Apparent acceleration due to gravity at equator

ge=10−0.0339=9.9661 m s−2g_e = 10 - 0.0339 = 9.9661\,\text{m s}^{-2}ge​=10−0.0339=9.9661m s−2

  1. Apparent weight at equator

We=mge=19.6×9.9661W_e = m g_e = 19.6 \times 9.9661We​=mge​=19.6×9.9661

We≈195.34 NW_e \approx 195.34\,\text{N}We​≈195.34N

This is closest to 195.32 N\boxed{195.32\,\text{N}}195.32N​

  1. Option check
  • A: 194.32 N194.32\,\text{N}194.32N — too low
  • B: 195.66 N195.66\,\text{N}195.66N — too high
  • C: 195.32 N195.32\,\text{N}195.32N — correct
  • D: 194.66 N194.66\,\text{N}194.66N — too low

Therefore, the correct option is C.

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