A small iron ball falls through a viscous liquid at a constant speed of 20cm/sec. If the steel ball is pulled upwards with a force equal to twice its effective weight, how will it move upward?

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with constantly increasing speed

with constantly decreasing speed

with the same speed

will be at rest without any movement

answer is C.

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Detailed Solution

For the first case, B+6πηrv=mgFor the second case, F+B=mg+6πηrvt2mg−2B+B=mg+6πηrvt⇒mg−B=6πηrvtSolving we get, v= 20 cm/sec

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