JEE Advanced 2014 Physics question
A planet of radius (radius of Earth) has the same mass density as Earth. Scientists dig a well of depth on it and lower a wire of the same length and of linear mass density kg m⁻¹ into it. If the wire is not touching anywhere, the force applied at the top of the wire by a person holding it in place is (take the radius of Earth = m and the acceleration due to gravity on Earth is 10 m s⁻²)
Answer: (B)
Solution
For the same density, surface gravity ∝ radius: m s⁻², with m. Inside the planet . The wire extends from r = R down to , so the force needed is
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